Tips for using Oracle(mostly Oracle 8 but also 7.3)
After years of hammering on poor Ilustra, I decided that I needed a place to vent my rage against Oracle as well... It ends up that I have lots of experience to share but not much rage. Why not? Oracle lets me sleep at night. With Illustra I used to have to manually unwedge my deadlocked database (and therefore Web server) at 3 am, 11 am, 1 pm,... you get the idea. In Oracle, readers never wait for writers or vice versa. I can serve my 18,000 daily users. I can EXPORT a consistent backup dump. I can update a 20,000 row table. I can rebuild an index. All simultaneously.
So there are lots of things about Oracle that I wish were different. There are lots of things that have wasted my time. There are lots of things that were painful to set up. However, once set up properly, Oracle seems to be very reliable and well-behaved.
Inspiration: http://www.scorecard.org. This is a server that gets 30 requests/second at peak hours (it was a top story on ABC News, in Newsweek, in the New York Times, etc.). Each request does between 1 and 5 Oracle queries. It just about uses up an old pizza-box size Unix machine (Sun Ultra 2 with dual 167 MHz CPUs). If you use Oracle 8 intelligently (and connect to Oracle 8 intelligently; we use AOLserver), there is no scalability problem for Internet sites (though you can create one and increase your job security by purchasing an application server).
With an ANSI-standard database, you have
TIMESTAMP data types and a whole bunch of interval types.
With an ANSI RDBMS, you can store time with precision down to
microseconds (or optionally even finer, but the standard mandates that
implementations hold 6 significant digits after the second). ANSI
timestamps look like "1963-09-28 23:05:14" (September 28, 1963 at 11:05
pm and 14 seconds).
With Oracle, you have only one datetime datatype:
But it is actually the same granularity of information as an
TIMESTAMP(0), i.e., it stores the time down
to single-second precision. That wouldn't be so bad except that Oracle
SQL does not implement any of the ANSI syntax or functions. By default,
Oracle would say "28-SEP-63". Note that this is an invitation to Year
2000 bugs as well as a nightmare for people porting systems from other
RDBMS products. Note further that Oracle dates will not sort
lexicographically. If you start naming files with an Oracle date in the
filename, they won't sort in any meaningful order.
A good general rule with Oracle is that every parameter in the default Oracle installation is wrong. Date format is no exception. Here's what you should have for starters in your $ORACLE_HOME/dbs/initora8.ora:
Alternatively, you can decide that your entire database will really behave like TIMESTAMP(0) and use "YYYY-MM-DD HH24:MI:SS".# Use ANSI date format by default! nls_date_format = "YYYY-MM-DD"
Remember that Oracle's "hidden precision" can make queries fail in unexpected ways. For example, if you ask Oracle to print out the date, you'll get precision down to the day:
However, the internal precision will make naive comparisons fail:SQL> select sysdate from dual; SYSDATE ---------- 1998-07-12
We didn't get any rows because none matched down to the second. Are we stuck? No. We can rely on the fact that Oracle stores dates as numbers:create table test_dates ( the_value varchar(20), the_date date ); insert into test_dates values ('happy',sysdate); insert into test_dates values ('happy',sysdate); insert into test_dates values ('joy',sysdate); insert into test_dates values ('joy',sysdate); *** brief pause *** SQL> select * from test_dates where the_date = sysdate; no rows selected
An arguably cleaner approach is to useSQL> select * from test_dates where trunc(the_date) = trunc(sysdate); THE_VALUE THE_DATE -------------------- ---------- happy 1998-07-12 happy 1998-07-12 joy 1998-07-12 joy 1998-07-12
select * from test_dates where to_char(the_date,'YYYY-MM-DD') = to_char(sysdate,'YYYY-MM-DD');
(a statement that works nicely in some other RDBMSes), then you get a very unpleasant error message. It turns out that the easiest (only) way to rename a column isalter table phone_directory rename column name to full_name;
This is fixed to some extent for those who have ascended in Oracle 8.1 heaven.drop table phone_directory; edit SQL file create table phone_directory ...
The traditional RDBMS approach is to use a LONG column. This should not be used in Oracle 8, which has Character Large Objects (CLOBs). They are an industrial-strength implementation of an idea that was floating around in research RDBMSes and in the old Illustra system. A CLOB can be up to 4 GB in size and can sit in a separate tablespace from the table that references it. But if the character string is less than 4000 bytes, by default it gets stored "inline" (with the rest of the stuff in the row).
create table foobar ( mykey integer, moby clob ); insert into foobar values ( 1, 'foo');
The first time I used CLOBs was for an Oracle 8 port of my Q&A forum software (what you see running at photo.net). I use it for the MESSAGE column. In my Illustra implementation, it turned out that only 46 out of the 12,000 rows in the table had messages longer than 4000 bytes (the VARCHAR limit in Oracle 8). But those 46 messages were very interesting and sometimes contained quoted material from other works. So it seemed desireable to allow users to post extremely long messages if they want.
alter table bboard modify message clob;does not work (even with no rows in the table). If you want to use CLOBs, you apparently have to say so at table creation time.
My partner Cotton was custom writing me an Oracle 8 driver for AOLserver (my preferred RDBMS client). So he decided to use C bind variables instead. It turned out that these didn't work either for strings longer than 4000 chars. There is some special CLOB type for C that you could use if you knew in advance that the column was CLOB. But of course Cotton's C code was just taking queries from my AOLserver Tcl code. Without querying the database before every INSERT or SELECT, he had no way of knowing which columns were going to be CLOB.
One of the most heavily touted features of Oracle 8 is that you can partition tables, e.g., say "every row with an order_date column less than January 1, 1997 goes in tablespace A; every row with order_date less than January 1, 1998 goes in tablespace B; all the rest go in tablespace C." Assuming that these tablespaces are all on separate disk drives, this means that when you do a GROUP BY on orders during the last month or two, Oracle isn't sifting through many years worth of data; it only has to scan the partition of the table that resides in tablespace C. Partitioning seems to be a good idea, or at least the Informix customers who've been using it over the years seem to think so. But if you are a CLOB Achiever, you won't be using partitioning.
Right now, my take on CLOBs is that they are so unpleasant to use as to be almost not worth it. It has taken Cotton longer to get this one feature working than everything else he did with his driver. Informix Universal Server lets you have 32,000-character long VARCHARs and I'm looking longingly at that system right now...
Here's how much fun life is if you're as stupid as I am...
"create table bboard_copy as select * from bboard;"
"alter table bboard add (scratch varchar(4000));"
Note the elapsed time: 14 minutes for 20,000 rows, about 23 rows/second.SQL> update bboard set scratch = sort_key; 20453 rows updated. Elapsed: 00:14:20.59
The 21 MB system global area didn't sound so bad. But the 409K database buffer size didn't look right to me. Perhaps it can grow dynamically? I poked around in $ORACLE_HOME/dbs/initora8.ora:SVRMGR> ORACLE instance started. Total System Global Area 21633944 bytes Fixed Size 44924 bytes Variable Size 20655132 bytes Database Buffers 409600 bytes Redo Buffers 524288 bytes Database mounted.
db_block_buffers = 200 # SMALL
Hmmm... default block size of 2048 bytes (see below) times 200 = 409600 bytes. So with 256 MB of RAM, my computer was only able to use 400 KB as a cache for Oracle data. Oracle SQL High-Performance Tuning says that "many applications get good performance from a buffer cache as small as 10 MB." Oops. The book goes on to note that "high performance applications may have buffer caches of 50 to 100 MB and caches of over 200 MB are not rare." Double oops. How often are we missing our monster 400 KB cache?
Doh! 44 million physical reads from the disk in a few hours! Oracle saysSQL> SELECT name, value FROM v$sysstat WHERE name IN ('db block gets', 'consistent gets','physical reads'); NAME VALUE ---------------------------------------------------------------- ---------- db block gets 62969 consistent gets 49707710 physical reads 44449762
In this case, our hit ratio is 11%. So 89% of the time, we had to go to the disk. If a disk can do 100 operations/second, doing 44 million operations would take 440,000 seconds, or about 5 days. We're doing a bit better than this partly because we have lots of disks on this machine but mostly because the Unix file system is obviously caching much of the information.Hit Ratio = 1 - ( physical reads / (db block gets + consistent gets) )
We need to increase db_block_buffers until we hit the cache at least 90% of the time according to Oracle. On a big machine with a light load, my personal preference would be to hit the cache at least 95% of the time. Memory is cheap.
If our problems weren't so pathetically obvious, we'd have to think about the fact that a hit in the Oracle cache might still be a miss because that part of the SGA had been paged out to disk by the operating system.
In addition to caching user data, Oracle separately caches housekeeping items in the "shared pool". The person who installed this Oracle for some reason decided to not take the 3.5 MB default because initora8.ora also contains
Is this a reasonable size? Well, there doesn't seem to be too much free:shared_pool_size = 20000000
All but 667K of the 20 MB are in use for caching parsed queries and data dictionary information:SQL> select * from v$sgastat where name = 'free memory'; POOL NAME BYTES ----------- -------------------------- ---------- shared pool free memory 667596
I think this means that only 828 times out of 130,800 did Oracle have to reparse a query. I'm surprised that this is so high, actually, because I don't use bind variables in my queries and queries can't be cached if they don't match character for character. In other words, "select * from users where user_id = 67" won't match "select * from users where user_id = 156".SQL> SELECT SUM(pins) "Executions", SUM(reloads) "Cache Misses while Executing" FROM v$librarycache; Executions Cache Misses while Executing ---------- ---------------------------- 130800 828
The Oracle tuning guide suggests checking for data dictionary cache performance with the following query:
99.5% of the time, Oracle was able to find data dictionary information in its cache and did not have to go to the Unix file system to get data about the structure of tables or the Oracle system itself. Anyway, I guess it is nice to have 20 MB of shared pool, but probably this is not the best use of memory when you only have 400 KB to cache user data.SQL> select (sum(gets - getmisses - usage - fixed)) / sum(gets) "Row Cache" from v$rowcache; Row Cache ---------- .995257207
OK.. so I upped db_block_buffers to 2000 (from 200) and decided to try updating bboard again with an index for SORT_KEY . Here's the original problem, a query that I do on every bulletin board posting page:
Note that this is doing a full table scan and 13,742 physical reads. Ordinarily at least some of these blocks would be read from the cache except that I think Oracle tries to avoid using the cache when doing full table scans (unless you put in an optimizer hint instructing it to do so and analyze the table (see below)). This is because one sequential scanning loser dragging a 20 MB table through the cache would displace all the other users' cached blocks.SQL> select decode(email,'email@example.com','f','t') as not_maintainer_p, to_char(posting_time,'YYYY-MM-DD') as posting_date,bboard.* from bboard where sort_key like '0006XO%' and msg_id <> '0006XO' order by not_maintainer_p, sort_key; Elapsed: 00:00:03.80 Execution Plan ---------------------------------------------------------- 0 SELECT STATEMENT Optimizer=CHOOSE (Cost=1811 Card=1 Bytes=828) 1 0 SORT (ORDER BY) (Cost=1811 Card=1 Bytes=828) 2 1 TABLE ACCESS (FULL) OF 'BBOARD' (Cost=1808 Card=1 Bytes=828) Statistics ---------------------------------------------------------- 0 recursive calls 4 db block gets 16544 consistent gets 13742 physical reads 0 redo size 2163 bytes sent via SQL*Net to client 995 bytes received via SQL*Net from client 5 SQL*Net roundtrips to/from client 2 sorts (memory) 0 sorts (disk) 1 rows processed
Following a painful 15 minutes of downtime ...
We're down to 0 physical reads from 13,000+. Not so bad. I still wish I understood why tables could not be read while an index was being built or every row in the table was being updated.Execution Plan ---------------------------------------------------------- 0 SELECT STATEMENT Optimizer=CHOOSE (Cost=6 Card=1 Bytes=828) 1 0 SORT (ORDER BY) (Cost=6 Card=1 Bytes=828) 2 1 TABLE ACCESS (BY INDEX ROWID) OF 'BBOARD' (Cost=3 Card=1 Bytes=828) 3 2 INDEX (RANGE SCAN) OF 'BBOARD_BY_SORT_KEY' (NON-UNIQUE) (Cost=2 Card=1) Statistics ---------------------------------------------------------- 0 recursive calls 0 db block gets 6 consistent gets 0 physical reads 0 redo size 2170 bytes sent via SQL*Net to client 995 bytes received via SQL*Net from client 5 SQL*Net roundtrips to/from client 2 sorts (memory) 0 sorts (disk) 1 rows processed
The first thing that I've observed with SQL*Plus is that legal SQL often generates errors when typed into SQL*Plus. Here's what I do to make things palatable for SQL*Plus:
create table photo_cds ( photocd_id varchar(20) not null primary key, photographer_id integer not null references photographers, -- no orphans client_id integer references clients, -- may often be null -- do we have a 6-resolution pro disk? pro_disk_p char(1) default 'f' check (pro_disk_p in ('t','f')), sharpen_p char(1) check (sharpen_p in ('t','f')), add_borders_p char(1) check (add_borders_p in ('t','f')), copyright_label varchar(100), -- bit vectors done with ASCII 0 and 1; probably convert this to -- Oracle 8 abstract data type jpeg_resolutions char(6), -- on which resolutions to write copyright label copyright_resolutions char(6), -- how this will be published url_stub varchar(100), -- e.g., 'pcd3735/' images_public_p char(1) check (images_public_p in ('t','f')) ); unknown command beginning "_public_p ..." - rest of line ignored. SQL> unknown command ")" - rest of line ignored. SQL> SQL> select * from photo_cds;
In some ideal world you would have separate development and production Oracle installations and would never be typing arbitrary SQL at a SQL*Plus session on a production system. In reality, here's what happens:SQL> update bank_accounts set balance = 0; 13,456 rows updated. *** panic *** SQL> rollback; Rollback complete. SQL> update bank_accounts set balance = 0 where account_number = 3745; 1 row updated. SQL> commit; Commit complete.
Remember that you might have to restart Web servers and/or the Oracle ConText servers.# su oracle ora8@orchid/solaris$ svrmgrl SVRMGR> connect internal Connected. SVRMGR> shutdown immediate Database closed. Database dismounted. ORACLE instance shut down. SVRMGR> startup ORACLE instance started. ...
An alternative is to poke around among the DBA views and try to figure
out who is causing the problem. Let's artificially create the problem:
|SQL*Plus session 1||SQL*Plus session 2|
SQL> create table foobar ( the_key integer primary key ); SQL> insert into foobar (the_key) values (1); 1 row created.
SQL> insert into foobar (the_key) values (1); *** wait forever ***
If you log in with dba privileges, e.g., as SYSTEM, you can do the following:
I have no idea what the magiccolumn username format a15 column program format a20 column machine format a10 column n_seconds format a6 select username,v$session.sid,v$session.serial#, ltrim(v$session_wait.seconds_in_wait) as n_seconds, process, machine, terminal, program from v$session, v$session_wait where v$session.sid = v$session_wait.sid and v$session.sid in (select sid from v$lock where lmode=6); USERNAME SID SERIAL# N_SECO PROCESS MACHINE TERMINAL --------------- ---------- ---------- ------ --------- ---------- ---------- PROGRAM -------------------- 3 1 882 1338 homepage ? oracle@homepage (LGWR) SCOTT 8 33 1448 16351 homepage ttypa sqlplus@homepage (TNS V1-V3) SCOTT 11 24 1484 16344 homepage ttyp9 USERNAME SID SERIAL# N_SECO PROCESS MACHINE TERMINAL --------------- ---------- ---------- ------ --------- ---------- ---------- PROGRAM -------------------- sqlplus@homepage (TNS V1-V3)
lmode=6clause does, but apparently it gives you the correct processes. One of them is the Oracle log writer (LGWR) process. You definitely don't want to mess with that! The other two are sessions for user SCOTT that have been waiting for more than 1400 seconds. You can kill one of them (the one that has been waiting longer) by specifying the session id (SID) and serial number:
Here's what you seealter system kill session '11,24';
|SQL*Plus session 1||SQL*Plus session 2|
SQL> select sysdate from dual; select sysdate from dual * ERROR at line 1: ORA-00028: your session has been killed
1 row created.
select * from bboard where contains (indexed_column, '$(dogs,running,wild)', 10) > 0 order by score(10) desc
It is very difficult to simply feed a user query from a Web form into ConText. If you just put it directly through, ConText does precise phrase matching, probably not what Web users expect. If you blindly split on spaces and then join with commas to produce queries like the one above, then the user will get an error if his query contained any any ConText operators (e.g., "and") or any stop words. I've found with my Q&A forum that I get much better results if I try using the NEAR operator first (";") and then resort to the accumulate operator (",") as above.
Another bad feature of ConText is that it has a very heavy flavor of "I'm designed for indexing big documents, each of which resides in one column, presumably as a LONG or a CLOB." Most of my Web stuff has data spread among multiple columns, e.g., the name, email, subject, and body fields of a bboard posting. You can create a concatenating view with two columns, one for the primary key and one with all the subsidiary columns mushed together with "||". Unfortunately, this doesn't work if the total length of the mush is more than 4000 characters or if one of the underlying columns is a CLOB. In that mournful case, you're forced to maintain, either with database triggers or in your application code, a denormalized copy of all the indexed text mushed together into a CLOB.
ConText the speed demon: searching for "nikon zoom lens" in my bulletin board took 2.8 seconds in Illustra/PLS. ConText took about 10 seconds. Searching for one word that was not present in any row took 0.17 seconds in Illustra/PLS, 1.5 seconds in Oracle/ConText (but then I dropped and rebuilt the ConText index and the time fell to 0.4 seconds).
One of my clients couldn't get ConText to work at all. He'd configured it to use its own tablespace but it insisted on filling up the SYSTEM tablespace instead. Attempting to index 300 MB of text failed and left behind 100 MB of garbage in the SYSTEM tablespace, thus rendering the entire Oracle installation useless. After the two of us spent a few nights on the phone with Oracle support in Australia, I wrote this...
-- here's our ConText replacement -- we take the indexed_stuff string (presumably straight from the DB) -- and a space-separated list of query tokens (presumably from the user) -- we loop through the space_sep_list and compare to INDEXED_STUFF -- we return 0 if we didn't find any, plus 10 for every match -- this doesn't work though for views in which string concatenation will -- overflow (see next function) create or replace function pseudo_contains (indexed_stuff IN varchar, space_sep_list_untrimmed IN varchar) return integer IS space_sep_list varchar(32000); upper_indexed_stuff varchar(32000); -- if you call this var START you get hosed royally first_space integer; score integer; BEGIN space_sep_list := upper(ltrim(rtrim(space_sep_list_untrimmed))); upper_indexed_stuff := upper(indexed_stuff); score := 0; IF space_sep_list is null or indexed_stuff is null THEN RETURN score; END IF; LOOP first_space := instr(space_sep_list,' '); IF first_space = 0 THEN -- one token or maybe end of list IF instr(upper_indexed_stuff,space_sep_list) <> 0 THEN RETURN score+10; END IF; RETURN score; ELSE -- first_space <> 0 IF instr(upper_indexed_stuff,substr(space_sep_list,1,first_space-1)) <> 0 THEN score := score + 10; END IF; END IF; space_sep_list := substr(space_sep_list,first_space+1); END LOOP; END pseudo_contains;
September 13, 1998: the same thing happened to me behind the db.photo.net server. In response to us inserting 16 MB of stuff to be indexed (in a table that is part of the ArsDigita Community System) ConText filled up the TOOLS tablespace with a 2 GB "service queue" of stuff that it supposedly needed to do. Since we'd set our database files to autoextend, this resulted in a database file longer than 2 GB (and a full hard disk), which apparently makes Oracle unhappy in profound ways. This brought down the core Oracle server itself and we spent five solid days trying to extricate ConText from our installation. Here's what I've learned about ConText:
Here's an example. We define a table of names and phone numbers.
Suppose Philip Greenspun and Cotton Seed are using a Web page where they can type their phone numbers into a form. Cotton chooses to leave this field blank. So that information should be represented as '' (empty string).create table empty_string_test ( name varchar(100), phone_number varchar(100) );
Jin Choi's name is entered from a table of mail-order customers where no phone numbers are kept. So we insert a row for him with a NULL.insert into empty_string_test values ('Philip Greenspun','617 253-8574'); insert into empty_string_test values ('Cotton Seed','');
Now we want to know how many rows we entered from the mail-order customers table...insert into empty_string_test values ('Jin Choi',NULL);
Oops! Oracle tells us that there were 2 instead of 1. It gets worse. Try to figure out how many people refused to enter their phone number on the form...select count(*) from empty_string_test where phone_number is null; COUNT(*) ---------- 2
But you just inserted a row for Cotton where phone_number was ''! Let's say that you want to get a list of people to call. You aren't going to want rows for which there isn't a phone number, so you use an "is not null" clause:select count(*) from empty_string_test where phone_number = ''; COUNT(*) ---------- 0
Note the obtuseness of SQL*Plus in formatting here. Because these are VARCHAR(100), it allocates 100 characters for each column and can't display them on the same line. Anyway, you only get one row back. In a pure ANSI SQL you'd expect to get two rows, one with Cotton Seed and his empty string. Oracle's documentation seems to threaten a move in the direction of the standard. So you program defensely...select * from empty_string_test where phone_number is not null; NAME -------------------------------------------------------------------------------- PHONE_NUMBER -------------------------------------------------------------------------------- Philip Greenspun 617 253-8574
That extra condition ("<> ''") should have been redundant with the "is not null". But it seems to have wiped out all the rows. I guess this isn't too surprising givenselect * from empty_string_test where phone_number is not null and phone_number <> ''; no rows selected
As far as I can tell, there is no way to program this that does not lock you into Oracle's non-standard behavior.select * from empty_string_test where phone_number <> ''; no rows selected
Oracle does not like this: "ORA-00936: missing expression". So I flipped through the subquery examples in Oracle8: The Complete Reference. They all put the subquery after a column name. Swapping the 0 and the subquery...select msg_id, one_line, sort_key, email, name from bboard bbd1 where topic = 'photo.net' and (select count(*) from bboard bbd2 where bbd2.refers_to = bbd1.msg_id) = 0 and refers_to is null
... worked like a charm.select msg_id, one_line, sort_key, email, name from bboard bbd1 where topic = 'photo.net' and 0 = (select count(*) from bboard bbd2 where bbd2.refers_to = bbd1.msg_id) and refers_to is null
We've chosen to represent someone's desperation level with an integer, presumably so that we can add and subtract increments depending on how thoroughly they are surfing the site. Any row with a desperation_status greater than 100 gets put into the "active" partition in tablespace DATA01. Folks whose desperation_status is less than 100 are put into the "inactive" partition. Note that the primary index, which covers all the rows, is stuck into a third tablespace because of thecreate table lonelyhearts ( lonelyheart_id integer primary key using index tablespace index01, first_names varchar(100), last_name varchar(100), desperation_status integer, bad_acne_p char(1), had_plastic_surgery_p char(1), needs_plastic_surgery_p char(1), ... ) partition by range (desperation_status) (partition inactive values less than (100) tablespace data02, partition active values less than (maxvalue) tablespace data01 );
PRIMARY KEY USING INDEX TABLESPACE INDEX01annotation.
How does this work? Great! If you
SELECT ... WHERE
DESPERATION_STATUS >= 100 then Oracle very cleverly hits only the
Can you go home happy? Uh.... try changing a user from active to inactive or vice versa. Oracle raises an error. If an otherwise legal SQL UPDATE would cause a row to move from one partition to another, Oracle won't let you do it. Thus the db administrator's decision to partition a table to get higher performance must be examined by all the application programmers who use the table.
Is this so bad? No. Just write a PL/SQL program to update the desperation_status of a row. It will:
How have I worked around this limitation? With the awesome programming power of ... AOLserver Tcl. Because of the powerful API, it turns out to be only a few lines of Tcl code to do all the steps above in a transaction.
If you install AOLserver on an already-partitioned data model will your troubles be over? No. Most likely there are other tables that refer to the LONELYHEARTS table, e.g.,
You find that your transaction is aborted because there are rows in other tables referencing "lonelyheart_id 37" and you therefore can't delete the row containing this id. You can't do the insert before you do the delete because the LONELYHEARTS table has a primary key constraint. There is actually a way to tell Oracle to wait until a transaction closes before checking an integrity constraint:create table email_alerts ( email_alert_id integer primary key, lonelyheart_id integer not null references lonelyhearts, ... )
The "deferrable" tells Oracle that this constraint may be deferred. The "initially deferred" tells Oracle that this constraint should be set at the ends of transactions.create table email_alerts ( email_alert_id integer primary key, lonelyheart_id integer not null references lonelyhearts initially deferred deferrable, ... )
Your pain is at an end.
Note that Informix Universal Server gets this right. Fragmenting a table has performance/caching/tuning implications. However, no application code SQL has to be examined or changed. Informix will transparently migrate rows from one tablespace to another if necessary.
makes sure that a topic in my bulletin board system is either a Q&A forum or a traditional USENET-style threaded discussion. This was fine until I copied the software over to www.scorecard.org where we wanted to tag threads with state, county, or zip code. This would be a new kind of forum: "usgeospatial". I'd already defined the table and was running a few forums. So I couldn't just change thepresentation_type varchar(20) check(presentation_type in ('q_and_a','threads'))
CREATE TABLEstatement. Here's what you actually have to do:
Note that this time we gave it a name so that we'd not have to search the-- first find the constraint name (since we weren't clever enough -- to explicitly name it in the CREATE TABLE) select constraint_name, search_condition from user_constraints where table_name = 'BBOARD_TOPICS'; -- drop the check constraint alter table bboard_topics drop constraint SYS_C002703; -- add it back with the extra value alter table bboard_topics add (constraint check_presentation_type check(presentation_type in ('q_and_a','threads', 'usgeospatial')));
USER_CONSTRAINTStable next time.
Oops! It turns out that Oracle is unhappy about just writing to standard output. For each user that wants to trace queries, you need to feed sqlplus the file $ORACLE_HOME/rdbms/admin/utlxplan.sql which contains a single table definition:SQL> set autotrace on Unable to verify PLAN_TABLE format or existence Error enabling EXPLAIN report
create table PLAN_TABLE ( statement_id varchar2(30), timestamp date, remarks varchar2(80), operation varchar2(30), options varchar2(30), object_node varchar2(128), object_owner varchar2(30), object_name varchar2(30), object_instance numeric, object_type varchar2(30), optimizer varchar2(255), search_columns number, id numeric, parent_id numeric, position numeric, cost numeric, cardinality numeric, bytes numeric, other_tag varchar2(255), partition_start varchar2(255), partition_stop varchar2(255), partition_id numeric, other long);
Anyway, here's the SQL Plus session
The first thing to note here is "10,299 physical reads". Disk drives are very slow. You don't really want to be doing more than a handful of physical reads. Let's look at the heart of the query plan:SQL> select count(*) from bboard where topic = 'photo.net' and one_line = 'foo' and dbms_lob.instr(message,'bar') > 0 ; 2 3 4 COUNT(*) ---------- 0 Execution Plan ---------------------------------------------------------- 0 SELECT STATEMENT Optimizer=CHOOSE 1 0 SORT (AGGREGATE) 2 1 TABLE ACCESS (BY INDEX ROWID) OF 'BBOARD' 3 2 INDEX (RANGE SCAN) OF 'BBOARD_BY_TOPIC' (NON-UNIQUE) Statistics ---------------------------------------------------------- 0 recursive calls 0 db block gets 59967 consistent gets 10299 physical reads 0 redo size 570 bytes sent via SQL*Net to client 741 bytes received via SQL*Net from client 4 SQL*Net roundtrips to/from client 1 sorts (memory) 0 sorts (disk) 1 rows processed
Looks to me as though Oracle is hitting the "bboard_by_topic" index for the ROWIDs of "just the rows that have a topic of 'photo.net'". It is then using the ROWID, an internal Oracle pointer, to pull the actual rows from the BBOARD table. Presumably Oracle will then count up just those rows where the ONE_LINE and MESSAGE columns are appropriate. This might not actually be so bad in an installation where there were 500 different discussion groups. Hitting the index would eliminate 499/500 rows. But BBOARD_BY_TOPIC isn't a very selective index, as "select topic, count(*) from bboard group by topic order by count(*) desc" reveals..2 1 TABLE ACCESS (BY INDEX ROWID) OF 'BBOARD' 3 2 INDEX (RANGE SCAN) OF 'BBOARD_BY_TOPIC' (NON-UNIQUE)
The BBOARD table only has about 19,000 rows and the photo.net topic has 14,000 of them, about 75%. So the index didn't do us much good. In fact, you'd have expected Oracle not to use the index. A full table scan is generally faster than an index scan if more than 20% of the rows need be examined. Why didn't oracle do the full table scan? Because the table hadn't been "analyzed". There were no statistics for the cost-based optimizer so the older rule-based optimizer was employed. You have to periodically tell Oracle to build statistics on tables if you want the fancy optimizer:
topic count(*) photo.net 14159 Nature Photography 3289 Medium Format Digest 1639 Ask Philip 91 web/db 62The final numbers don't look much better. But at least the cost-based optimizer has figured out that the topic index won't be worth much. Now we're just scanning the full BBOARD table. While transferring 20,000 rows from Illustra to Oracle, I'd not created any indices. This speeded up loading but then I was so happy to have the system running deadlock-free that I forgot to recreate an index that I'd be using on the Illustra system expressly for the purpose of making this query fast.SQL> analyze table bboard compute statistics; Table analyzed. SQL> select count(*) from bboard where topic = 'photo.net' and one_line = 'foo' and dbms_lob.instr(message,'bar') > 0 ; 2 3 4 COUNT(*) ---------- 0 Execution Plan ---------------------------------------------------------- 0 SELECT STATEMENT Optimizer=CHOOSE (Cost=1808 Card=1 Bytes=828) 1 0 SORT (AGGREGATE) 2 1 TABLE ACCESS (FULL) OF 'BBOARD' (Cost=1808 Card=1 Bytes=828) Statistics ---------------------------------------------------------- 0 recursive calls 4 db block gets 74280 consistent gets 12266 physical reads 0 redo size 572 bytes sent via SQL*Net to client 741 bytes received via SQL*Net from client 4 SQL*Net roundtrips to/from client 1 sorts (memory) 0 sorts (disk) 1 rows processedBboard postings are now indexed by subject line. Note that the index is shoved off into a separate tablespace from the main table. This speeds up inserts and updates since data can be written on two separate disk drives in parallel. Let's try the query again:SQL> create index bboard_index_by_one_line on bboard ( one_line ) tablespace philgidx; Index created.We've brought physical reads down from 12266 to 3. Oracle is checking the ONE_LINE index and then poking at the main table using the ROWIDs retrieved from the index. It might actually be better to build a concatenated index on the name of the person posting and the subject line, but I'm not going to sweat because this is good enough for me.SQL> select count(*) from bboard where topic = 'photo.net' and one_line = 'foo' and dbms_lob.instr(message,'bar') > 0 ; 2 3 4 COUNT(*) ---------- 0 Execution Plan ---------------------------------------------------------- 0 SELECT STATEMENT Optimizer=CHOOSE (Cost=2 Card=1 Bytes=828) 1 0 SORT (AGGREGATE) 2 1 TABLE ACCESS (BY INDEX ROWID) OF 'BBOARD' (Cost=2 Card=1 Bytes=828) 3 2 INDEX (RANGE SCAN) OF 'BBOARD_INDEX_BY_ONE_LINE' (NON-UNIQUE) (Cost=1 Card=1) Statistics ---------------------------------------------------------- 0 recursive calls 0 db block gets 3 consistent gets 3 physical reads 0 redo size 573 bytes sent via SQL*Net to client 741 bytes received via SQL*Net from client 4 SQL*Net roundtrips to/from client 1 sorts (memory) 0 sorts (disk) 1 rows processed
Tracing/Tuning Case 2: new questionsAt the top of each forum page, e.g., http://www.photo.net/bboard/q-and-a.tcl?topic=photo.net, I like to show questions asked in the last 7 days. I programmed this in AOLserver Tcl so that the user would get a partial page before I attempted the query, but still there was a perceptible lag before any questions would appear.A full table scan and 12,071 physical reads just to get 61 rows! It was time to get medieval on this query. If the query's WHERE clause contained topic, refers_to, and posting_time, then I'd build a concatenated index on all three columns:SQL> select msg_id, one_line, sort_key, email, name from bboard where topic = 'photo.net' and refers_to is null and posting_time > (sysdate - 7) order by sort_key desc; ... 61 rows selected. Execution Plan ---------------------------------------------------------- 0 SELECT STATEMENT Optimizer=CHOOSE (Cost=1828 Card=33 Bytes=27324) 1 0 SORT (ORDER BY) (Cost=1828 Card=33 Bytes=27324) 2 1 TABLE ACCESS (FULL) OF 'BBOARD' (Cost=1808 Card=33 Bytes=27324) Statistics ---------------------------------------------------------- 0 recursive calls 4 db block gets 13188 consistent gets 12071 physical reads 0 redo size 7369 bytes sent via SQL*Net to client 1234 bytes received via SQL*Net from client 8 SQL*Net roundtrips to/from client 2 sorts (memory) 0 sorts (disk) 61 rows processed60 reads is better than 12,000. One bit of clean-up, though. There is no reason to have a BBOARD_BY_TOPIC index if we are going to keep this BBOARD_FOR_NEW_QUESTIONS index, whose first column is TOPIC. The query optimizer can use BBOARD_FOR_NEW_QUESTIONS even when the SQL only restricts based on the TOPIC column. The redundant index won't cause any services to fail, but it will slow down inserts.SQL> create index bboard_for_new_questions on bboard ( topic, refers_to, posting_time ) tablespace philgidx; Index created. SQL> select msg_id, one_line, sort_key, email, name from bboard where topic = 'photo.net' and refers_to is null and posting_time > (sysdate - 7) order by sort_key desc; ... 61 rows selected. Execution Plan ---------------------------------------------------------- 0 SELECT STATEMENT Optimizer=CHOOSE (Cost=23 Card=33 Bytes=27324) 1 0 SORT (ORDER BY) (Cost=23 Card=33 Bytes=27324) 2 1 TABLE ACCESS (BY INDEX ROWID) OF 'BBOARD' (Cost=3 Card=33 Bytes=27324) 3 2 INDEX (RANGE SCAN) OF 'BBOARD_FOR_NEW_QUESTIONS' (NON-UNIQUE) (Cost=2 Card=33) Statistics ---------------------------------------------------------- 0 recursive calls 0 db block gets 66 consistent gets 60 physical reads 0 redo size 7369 bytes sent via SQL*Net to client 1234 bytes received via SQL*Net from client 8 SQL*Net roundtrips to/from client 2 sorts (memory) 0 sorts (disk) 61 rows processedI was so pleased with myself that I decided to drop an index on bboard by the REFERS_TO column, reasoning that nobody ever queries REFERS_TO without also querying on TOPIC. Therefore they could just use the first two columns in the BBOARD_FOR_NEW_QUESTIONS index. Here's my query looking for unanswered questions (from the above section on whether "=" is commutative):SQL> drop index bboard_by_topic; Index dropped.Ouch! 497,938 physical reads. Let's try it with the index in place:SQL> select msg_id, one_line, sort_key, email, name from bboard bbd1 where topic = 'photo.net' and 0 = (select count(*) from bboard bbd2 where bbd2.refers_to = bbd1.msg_id) and refers_to is null order by sort_key desc; ... 57 rows selected. Execution Plan ---------------------------------------------------------- 0 SELECT STATEMENT Optimizer=CHOOSE (Cost=49 Card=33 Bytes=27324) 1 0 SORT (ORDER BY) (Cost=49 Card=33 Bytes=27324) 2 1 FILTER 3 2 TABLE ACCESS (BY INDEX ROWID) OF 'BBOARD' (Cost=29 Card=33 Bytes=27324) 4 3 INDEX (RANGE SCAN) OF 'BBOARD_FOR_NEW_QUESTIONS' (NON-UNIQUE) (Cost=2 Card=33) 5 2 INDEX (FULL SCAN) OF 'BBOARD_FOR_NEW_QUESTIONS' (NON-UNIQUE) (Cost=26 Card=7 Bytes=56) Statistics ---------------------------------------------------------- 0 recursive calls 0 db block gets 589843 consistent gets 497938 physical reads 0 redo size 6923 bytes sent via SQL*Net to client 1173 bytes received via SQL*Net from client 7 SQL*Net roundtrips to/from client 2 sorts (memory) 0 sorts (disk) 57 rows processedThis is still a fairly expensive query, but 200 times faster than before and it executes in a fraction of a second. That's probably fast enough considering that this is an infrequently requested page.SQL> create index bboard_index_by_refers_to on bboard ( refers_to ) tablespace philgidx; Index created. SQL> select msg_id, one_line, sort_key, email, name from bboard bbd1 where topic = 'photo.net' and 0 = (select count(*) from bboard bbd2 where bbd2.refers_to = bbd1.msg_id) and refers_to is null order by sort_key desc; ... 57 rows selected. Execution Plan ---------------------------------------------------------- 0 SELECT STATEMENT Optimizer=CHOOSE (Cost=49 Card=33 Bytes=27324) 1 0 SORT (ORDER BY) (Cost=49 Card=33 Bytes=27324) 2 1 FILTER 3 2 TABLE ACCESS (BY INDEX ROWID) OF 'BBOARD' (Cost=29 Card=33 Bytes=27324) 4 3 INDEX (RANGE SCAN) OF 'BBOARD_FOR_NEW_QUESTIONS' (NON-UNIQUE) (Cost=2 Card=33) 5 2 INDEX (RANGE SCAN) OF 'BBOARD_INDEX_BY_REFERS_TO' (NON-UNIQUE) (Cost=1 Card=7 Bytes=56) Statistics ---------------------------------------------------------- 0 recursive calls 0 db block gets 8752 consistent gets 2233 physical reads 0 redo size 6926 bytes sent via SQL*Net to client 1173 bytes received via SQL*Net from client 7 SQL*Net roundtrips to/from client 2 sorts (memory) 0 sorts (disk) 57 rows processed
Tracing/Tuning Case 3: forcing Oracle to cache a full table scanYou may have a Web site that is basically giving users access to a huge table. For maximum flexibility, it might be the case that this table needs to be sequentially scanned for every query. In general, Oracle won't cache blocks retrieved during a full table scan. The Oracle tuning guide helpfully suggests that you include the following cache hints in your SQL:You will find, however, that this doesn't work if your buffer cache (controlled by db_block_buffers; see above) isn't large enough to contain the table. Oracle is smart and ignores your hint. After you've reconfigured your Oracle installation to have a larger buffer cache, you'll probably find that Oracle is still ignoring your cache hint. That's because you also need toselect /*+ FULL (students) CACHE(students) */ count(*) from students;and then Oracle will work as advertised in the tuning guide. It makes sense when you think about it because Oracle can't realistically start stuffing things into the cache unless it knows roughly how large the table is.analyze table students compute statistics;
db_block_sizeEvery Oracle and third-party tuning publication says "you have to have your database block size at least as large as your operating system block size". This is because it might actually take longer to read 2K bytes (the Oracle default) from the Unix file system than a whole block (8K bytes on most systems). Supposedly you also want to make sure that your block size is larger than all of your rows. So if you are using CLOBs, which stay in the row until they are 4000 bytes long, you'll presumably want to use 8K blocks (most tuning books recommend 4K or 8K).
All of the Oracle and computer trade publications confidently tell you to just look in $ORACLE_HOME/dbs/init.ora and you'll find the db_block_size. This would be pretty easy if it were true. In fact, if you run the standard Oracle install scripts, db_block_size is never specified anywhere.
After 45 minutes on the phone with Oracle support, I learned that you need to run "svrmgrl", connect internal, and type "show parameters":In this case, it looks like the default for HP Unix is in fact 2048 bytes (I tried this on a SPARC Solaris machine also and the default installation also resulted in an Oracle with 2K blocks).bash-2.01$ svrmgrl Oracle Server Manager Release 184.108.40.206.0 - Production (c) Copyright 1997, Oracle Corporation. All Rights Reserved. Oracle8 Enterprise Edition Release 220.127.116.11.0 - Production With the Partitioning and Objects options PL/SQL Release 18.104.22.168.0 - Production SVRMGR> connect internal Connected. SVRMGR> show parameters NAME TYPE VALUE ----------------------------------- ------- ------------------------------ O7_DICTIONARY_ACCESSIBILITY boolean TRUE allow_partial_sn_results boolean FALSE always_anti_join string NESTED_LOOPS aq_tm_processes integer 0 arch_io_slaves integer 0 audit_file_dest string ?/rdbms/audit audit_trail string NONE b_tree_bitmap_plans boolean FALSE background_core_dump string full background_dump_dest string /ora8/m01/app/oracle/admin/ora backup_disk_io_slaves integer 0 backup_tape_io_slaves boolean FALSE bitmap_merge_area_size integer 1048576 blank_trimming boolean FALSE buffer_pool_keep string buffer_pool_recycle string cache_size_threshold integer 20 cleanup_rollback_entries integer 20 close_cached_open_cursors boolean FALSE commit_point_strength integer 1 compatible string 8.0.0 compatible_no_recovery string 8.0.0 control_file_record_keep_time integer 7 control_files string /ora8/m02/oradata/ora8/control core_dump_dest string /ora8/m01/app/oracle/admin/ora cpu_count integer 1 create_bitmap_area_size integer 8388608 cursor_space_for_time boolean FALSE db_block_buffers integer 200 db_block_checkpoint_batch integer 8 db_block_checksum boolean FALSE db_block_lru_extended_statistics integer 0 db_block_lru_latches integer 1 db_block_lru_statistics boolean FALSE db_block_max_dirty_target integer 4294967294 db_block_size integer 2048 db_domain string WORLD db_file_direct_io_count integer 64 db_file_multiblock_read_count integer 8 db_file_name_convert string db_file_simultaneous_writes integer 4 db_files integer 80 db_name string ora8 dblink_encrypt_login boolean FALSE dbwr_io_slaves integer 0 delayed_logging_block_cleanouts boolean TRUE discrete_transactions_enabled boolean FALSE disk_asynch_io boolean TRUE distributed_lock_timeout integer 60 distributed_recovery_connection_hol integer 200 distributed_transactions integer 16 dml_locks integer 100 enqueue_resources integer 155 event string fixed_date string freeze_DB_for_fast_instance_recover boolean FALSE gc_defer_time integer 10 gc_files_to_locks string gc_lck_procs integer 1 gc_releasable_locks integer 0 gc_rollback_locks string global_names boolean TRUE hash_area_size integer 0 hash_join_enabled boolean TRUE hash_multiblock_io_count integer 8 ifile file /ora8/m01/app/oracle/admin/ora instance_groups string instance_number integer 0 job_queue_interval integer 60 job_queue_keep_connections boolean FALSE job_queue_processes integer 0 large_pool_min_alloc string 16K large_pool_size string 0 lgwr_io_slaves integer 0 license_max_sessions integer 0 license_max_users integer 0 license_sessions_warning integer 0 lm_locks integer 12000 lm_procs integer 127 lm_ress integer 6000 local_listener string lock_name_space string lock_shared_memory boolean FALSE log_archive_buffer_size integer 64 log_archive_buffers integer 4 log_archive_dest string ?/dbs/arch log_archive_duplex_dest string log_archive_format string %t_%s.dbf log_archive_min_succeed_dest integer 1 log_archive_start boolean FALSE log_block_checksum boolean FALSE log_buffer integer 8192 log_checkpoint_interval integer 10000 log_checkpoint_timeout integer 0 log_checkpoints_to_alert boolean FALSE log_file_name_convert string log_files integer 255 log_simultaneous_copies integer 0 log_small_entry_max_size integer 80 max_commit_propagation_delay integer 90000 max_dump_file_size string 10240 max_enabled_roles integer 20 max_rollback_segments integer 30 max_transaction_branches integer 8 mts_dispatchers string mts_listener_address string mts_max_dispatchers integer 5 mts_max_servers integer 20 mts_multiple_listeners boolean FALSE mts_rate_log_size string mts_rate_scale string mts_servers integer 0 mts_service string ora8 nls_currency string nls_date_format string nls_date_language string nls_iso_currency string nls_language string AMERICAN nls_numeric_characters string nls_sort string nls_territory string AMERICA object_cache_max_size_percent integer 10 object_cache_optimal_size integer 102400 open_cursors integer 50 open_links integer 4 open_links_per_instance integer 4 ops_admin_group string optimizer_mode string CHOOSE optimizer_percent_parallel integer 0 optimizer_search_limit integer 5 oracle_trace_collection_name string oracle_trace_collection_path string ?/otrace/admin/cdf oracle_trace_collection_size integer 5242880 oracle_trace_enable boolean FALSE oracle_trace_facility_name string oracled oracle_trace_facility_path string ?/otrace/admin/fdf os_authent_prefix string ops$ os_roles boolean FALSE parallel_default_max_instances integer 0 parallel_instance_group string parallel_max_servers integer 5 parallel_min_message_pool integer 48420 parallel_min_percent integer 0 parallel_min_servers integer 0 parallel_server boolean FALSE parallel_server_idle_time integer 5 parallel_transaction_resource_timeo integer 300 partition_view_enabled boolean FALSE plsql_v2_compatibility boolean FALSE pre_page_sga boolean FALSE processes integer 50 recovery_parallelism integer 0 reduce_alarm boolean FALSE remote_dependencies_mode string TIMESTAMP remote_login_passwordfile string NONE remote_os_authent boolean FALSE remote_os_roles boolean FALSE replication_dependency_tracking boolean TRUE resource_limit boolean FALSE rollback_segments string r01, r02, r03, r04 row_cache_cursors integer 10 row_locking string always sequence_cache_entries integer 10 sequence_cache_hash_buckets integer 10 serial_reuse string DISABLE serializable boolean FALSE session_cached_cursors integer 0 session_max_open_files integer 0 sessions integer 60 shadow_core_dump string full shared_pool_reserved_min_alloc string 5K shared_pool_reserved_size string 175000 shared_pool_size string 3500000 snapshot_refresh_interval integer 60 snapshot_refresh_keep_connections boolean FALSE snapshot_refresh_processes integer 0 sort_area_retained_size integer 0 sort_area_size integer 65536 sort_direct_writes string AUTO sort_read_fac integer 5 sort_spacemap_size integer 512 sort_write_buffer_size integer 32768 sort_write_buffers integer 2 spin_count integer 1 spin_count integer 1 spread_extents boolean TRUE sql92_security boolean FALSE sql_trace boolean FALSE star_transformation_enabled boolean FALSE tape_asynch_io boolean TRUE temporary_table_locks integer 60 text_enable boolean FALSE thread integer 0 timed_os_statistics integer 0 timed_statistics boolean FALSE transaction_auditing boolean TRUE transactions integer 66 transactions_per_rollback_segment integer 11 user_dump_dest string /ora8/m01/app/oracle/admin/ora utl_file_dir string
What about Unix?All of the "Oracle for Dummies" books claim that the Unix file system defaults to 8K blocks, but they don't say how to determine what you've actually got in an already-built system, though. Fanatical in my devotion to bringing readers the best possible information, I read dozens of man pages, the source code for Linux, 10 volumes of Oracle documentation, and then... called the HP Unix technical support 800 number. Wayne Guerin set me straight in about two minutes...
You have to first find out the "block device name" of the file system using "df -k".Suppose we're interested in the file system we know as "/extra2". That means we need to feed a block device name of "/dev/dsk/c0t3d0" to the "tunefs" command:# df -k Filesystem 1024-blocks Used Available Capacity Mounted on /dev/vg00/lvol3 1228800 1201755 25049 98% / /dev/vg00/lvol1 47829 14205 28841 33% /stand /dev/vg00/lvol4 20480 19322 1104 95% /home /dev/dsk/c0t3d0 4103198 2541381 1151497 69% /extra2 /dev/dsk/c0t1d0 4103198 3203259 489619 87% /extra1Sure enough, this has the standard "bsize" of 8192 (8K). If tunefs doesn't have this kind of display option on your Unix box, or if you are an HP-UX Achiever running the VxFS journaled file system, then you need to use "mkfs -m", which gives you back the command that you would have had to type to create the file system:# /usr/sbin/tunefs -v /dev/dsk/c0t3d0 super block last mounted on: /extra2 magic 95014 clean FS_OK time Sat Dec 20 22:45:39 1997 sblkno 16 cblkno 24 iblkno 32 dblkno 232 sbsize 2048 cgsize 3072 cgoffset 40 cgmask 0xfffffff0 ncg 421 size 4194157 blocks 4103198 bsize 8192 bshift 13 bmask 0xffffe000 fsize 1024 fshift 10 fmask 0xfffffc00 frag 8 fragshift 3 fsbtodb 0 minfree 10% maxbpg 312 maxcontig 1 rotdelay 0ms rps 60 csaddr 232 cssize 7168 csshift 9 csmask 0xfffffe00 ntrak 16 nsect 39 spc 624 ncyl 6722 cpg 16 bpg 1248 fpg 9984 ipg 1600 nindir 2048 inopb 64 nspf 1 nbfree 194670 ndir 2168 nifree 631327 nffree 4457 cgrotor 304 fmod 0 ronly 0 fname fpack cylinders in last group 2 blocks in last group 156Note that a VxFS journaled file system defaults to a block size of 1024. Here's one of my already-created ones:# /usr/sbin/mkfs -m /dev/dsk/c0t3d0 mkfs -F hfs -L /dev/dsk/c0t3d0 4194157 39 16 8192 1024 16 10 60 6237What should you do if you find that your block size is smaller than your typical row lengths and/or your operating system block size? Here's my favorite quote from a "for Dummies"-type computer trade book (page 275 of the Oracle8 DBA Handbook):# /usr/sbin/mkfs -m /dev/vg00/lvol4 mkfs -F vxfs -o ninode=unlimited,bsize=1024,version=3,inosize=256,logsize=1024,nolargefiles /dev/vg00/lvol4 20480"The effect of increasing the database block size is stunning. In most environments, at least two block sizes are supported--for example, 2K and 4K. Most of the installation routines are set to use the lower of the two. However, using the next higher value for the block size may improve the performance of query-intensive operations by up to 50 percent."Well now, that doesn't sound too difficult does it? I often rebuild my entire database and delete all the tablespace files while my Web services are on-line, just for fun.
"This gain comes relatively free of charge. To increase the database block size, the entire database must be rebuilt, and all of the old database files have to be deleted."
When a tablespace fills upThe default roll back segment (RBS) tablespace seems to be much too small (maybe 15 MB) to do interesting transactions, e.g., rebuild a ConText index. Nor does it autoextend by default. So you'll often see big updates failing withThis is pretty humiliating if you have $500,000 worth of computer hardware and are just trying to work with some wimpy 20 MB table. Probably it means that the rest of your installation was badly done as well. Anyway, if you have dba privileges, you can investigate the problem withORA-01562: failed to extend rollback segment number 5 ORA-01650: unable to extend rollback segment R04 by 64 in tablespace RBSI think there might be a way to fix the existing rollback data file to make it autoextend. Alternatively, you can just add a datafile, possibly on a different disk:SELECT file_name, bytes, tablespace_name FROM sys.dba_data_files;The datafilename here is a complete Unix path. This one is going to start at 50 MB and grow up to 500 MB if necessary.ALTER TABLESPACE RBS ADD DATAFILE '/ora8/m03/oradata/ora8/rbs02.dbf' SIZE 50M AUTOEXTEND ON NEXT 5M MAXSIZE 500M;
I'm OK, you're dysfunctionalYou cannot define what Informix would call a "functional index" in Oracle. That is to say, you can't say "build me an index on UPPER(EMAIL) in the USERS table". This sounds trivial but it has horrifying implications. If you want to do a case-insensitive search for the user with an email address of "firstname.lastname@example.org" then you'll be sequentially scanning the entire table. If you want to enforce uniqueness of the EMAIL_ADDRESS column in a case insensitive manner, you'll have to write a PL/SQL trigger that sequentially scans the table before allowing an insert to proceed. If the USERS table has 1 million rows, this will take forever.
The only solution that I can think of is to use triggers to maintain a separate column called UPPER_EMAIL_ADDRESS that stores the uppercased version. Then declare it to be UNIQUE or define a UNIQUE index on that column. But the whole process is so ugly that I'm reluctant to admit to having done it.
Here's how it works in Informix Universal Server:Rumor: Oracle 8.1.create unique index users_by_email on users(upper(email));
The Oracle SQL Parser's Error Messages Won't Help YouIf you type an SQL statement that Oracle doesn't like, it will give you an error message. Unfortunately, the error message is almost never helpful. Here's a table definition that SQL*Plus doesn't like.So I fed stripped out all the comments and fed it to SQL*Plus again. Same error message. So I fed the same SQL to AOLserver and got back the following:create table photos ( photo_id integer not null primary key, photographer_id integer not null references photographers, film_roll_id integer references film_rolls, photocd_id integer references photo_cds, cd_image_number integer, -- will be null unless photocd_id is set frame_number integer, -- will be null unless film_roll_id is set -- oftentimes scans are not properly oriented and/or the film -- was done emulsion-side out instead of emulsion-side in; -- we need to store enough info to undo these required_rotation integer, -- in degrees to the right, e.g., 90 for CW; NULL or 0 means none requires_flip_p char(1) check (requires_flip_p in ('t','f')), -- image conversion stuff -- bit vectors done with ASCII 0 and 1; probably convert this to -- Oracle 8 abstract data type jpeg_resolutions char(6), -- only makes sense for images off PhotoCD -- is this image going to be searchable by the public? public_p char(1) check (public_p in ('t','f')), -- searchable? model_released_p char(1) default 'f' check (model_released_p in ('t','f')), exposure_date date, title varchar(100), -- this makes the most sense if we are delivering back to the user -- a collection of JPEGs for serving from his own site; for us, we'd -- probably rather store images in files tagged with photo_id filename_stub varchar(100), -- we may append frame number or cd image number caption varchar(4000), tech_details varchar(4000), -- f-stop, shutter speed, film used tutorial_info varchar(4000), copyright_label varchar(100), -- extra stuff for image retrieval search_keywords varchar(4000), -- can others else use this picture? rights_personal_web_p char(1) check (rights_personal_web_p in ('t','f')), rights_personal_print_p char(1) check (rights_personal_print_p in ('t','f')), rights_nonprofit_web_p char(1) check (rights_nonprofit_web_p in ('t','f')), rights_nonprofit_print_p char(1) check (rights_nonprofit_print_p in ('t','f')), rights_comm_web_p char(1) check (rights_comm_web_p in ('t','f')), rights_comm_print_p char(1) check (rights_comm_print_p in ('t','f')) ); unknown command beginning "eb_p in ('..." - rest of line ignored. For a list of known commands enter HELP and to leave enter EXIT. SQL> unknown command beginning "rights_per..." - rest of line ignored. SQL> unknown command beginning "rights_non..." - rest of line ignored. SQL> unknown command beginning "rights_non..." - rest of line ignored. SQL> unknown command beginning "rights_com..." - rest of line ignored. For a list of known commands enter HELP and to leave enter EXIT. SQL> unknown command beginning "rights_com..." - rest of line ignored. SQL> unknown command ")" - rest of line ignored. SQL> SQL>So I took out all the REFERENCES constraints and went back to AOLserver's SQL evaluator and ended up withORA-02267: column type incompatible with referenced column type SQL: [too long]I eventually figured out that PHOTOCD_ID should actually be VARCHAR(20) rather than INTEGER. My mistake but certainly Oracle was no help in finding it. So I fixed the problem and resubmitted the CREATE TABLE to SQL*Plus.ORA-00911: invalid character SQL: [too long]I took out all the comments and tried again:unknown command beginning "or image r..." - rest of line ignored. SQL> unknown command beginning "search_key..." - rest of line ignored.I fed the comment-free statement to AOLserver's SQL evaluator:invalid starting line number SQL> unknown command beginning "rights_per..." - rest of line ignored. SQL> unknown command beginning "rights_non..." - rest of line ignored.So I split up the table definition into a smaller CREATE and then a subsequent ALTER and fed it to SQL*Plus (still without comments):ORA-00911: invalid character SQL: [too long]Worked perfectly! Only took me an hour to find one bad column definition and define one table. I still hate how Illustra kept me up at night with its deadlocks, but the same process would have taken about 2 minutes with Illustra since its SQL parser would have said "I don't like PHOTOCD_ID".SQL> SQL> create table photos ( photo_id integer not null primary key, photographer_id integer not null references photographers, film_roll_id integer references film_rolls, photocd_id varchar(20) references photo_cds, cd_image_number integer, frame_number integer, required_rotation integer, requires_flip_p char(1) check (requires_flip_p in ('t','f')), jpeg_resolutions char(6), public_p char(1) check (public_p in ('t','f')), model_released_p char(1) default 'f' check (model_released_p in ('t','f')), exposure_date date, title varchar(100), filename_stub varchar(100), caption varchar(4000), tech_details varchar(4000), tutorial_info varchar(4000), copyright_label varchar(100), search_keywords varchar(4000)); SQL> SQL> SQL> 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Table created. SQL> alter table photos add ( rights_personal_web_p char(1) check (rights_personal_web_p in ('t','f')), rights_personal_print_p char(1) check (rights_personal_print_p in ('t','f')), rights_nonprofit_web_p char(1) check (rights_nonprofit_web_p in ('t','f')), rights_nonprofit_print_p char(1) check (rights_nonprofit_print_p in ('t','f')), rights_comm_web_p char(1) check (rights_comm_web_p in ('t','f')), rights_comm_print_p char(1) check (rights_comm_print_p in ('t','f')) ); 2 3 4 5 6 7 8 Table altered. SQL> SQL>
Bumping up the log file sizeMore fun fixing up the unusable defaults set by the Oracle Installer (world's greatest source of misery caused by a single program)... Here's how to get from measly 512k logfiles to something bigger, so you don't get a logfile switch every few seconds....REM find out where the logfiles are now and how big they are select v$logfile.member, v$log.status, v$log.bytes from v$log, v$logfile where v$log.group# = v$logfile.group# / REM add 3 new logfiles that are the right size... REM of course a production system would add 2 or more members REM per group on different devices... here we only have 1 member/group alter database add logfile '/user2/oracle73/dbs/oradata/oracle73/newredo1.log' size 10m; alter database add logfile '/user2/oracle73/dbs/oradata/oracle73/newredo2.log' size 10m; alter database add logfile '/user2/oracle73/dbs/oradata/oracle73/newredo3.log' size 10m; REM issue the following enough times until the CURRENT logfile is one of the REM NEW logfiles. The first query above shows you the current logfile.... alter system switch logfile; alter system switch logfile; REM Now, drop the little logfiles we don't need anymore. You need to erase or REM rm them from the filesystem yourself AFTER you drop them from the database. alter database drop logfile '/user2/oracle73/dbs/oradata/oracle73/redooracle7301.log'; alter database drop logfile '/user2/oracle73/dbs/oradata/oracle73/redooracle7302.log'; alter database drop logfile '/user2/oracle73/dbs/oradata/oracle73/redooracle7303.log';
PL/SQL KornerWriting PL/SQL is pretty easy. The hard part for me is always getting back from Oracle what I typed in. Here's the magic that you need:Note that you need dba privileges to execute this query.select line, text from dba_source where name = 'YOUR_PROC_NAME' order by line;
Everyday problem solvingIf Oracle rains on your parade withthen you need to go in as
ORA-02292: integrity constraint (PHOTONET.SYS_C003676) violated - child record found
select table_name from user_constraints where constraint_name = 'SYS_C003676;
If Oracle won't let you drop the table
user_groupsbecause some other table is referring to it, type(Remember that you need to use uppercase "USER_GROUPS" in your query.)
column table_name format a20 column constraint_name format a15 column r_constraint_name format a15 select uc1.table_name, uc1.constraint_name, uc1.R_constraint_name, uc2.table_name from user_constraints uc1, user_constraints uc2 where uc1.R_constraint_name = uc2.constraint_name and uc1.constraint_type = 'R' and uc2.table_name = 'USER_GROUPS'; TABLE_NAME CONSTRAINT_NAME R_CONSTRAINT_NA TABLE_NAME -------------------- --------------- --------------- -------------------- LYDIA_INFO SYS_C004316 SYS_C003225 USER_GROUPS MISC_INFO SYS_C004308 SYS_C003225 USER_GROUPS SITE_STAFF_INFO SYS_C004306 SYS_C003225 USER_GROUPS
You can infer from this query result that you'll have to drop
Reader's CommentsA great free tool for working with oracle is Toad. In particular it makes it very easy to edit PL/SQL procedures in the database.
Get it at: http://www.toadsoft.com
-- David Tauzell, August 29, 1998I'm not a DBA, I'm a sysadmin. But as a sysadmin, you have to work with DBAs and you learn little things about the RDBMS, like how to startup/shutdown, how you do a backup, how you find out which areas of the disk the RDBMS has glommed onto, etc.
From my perspective, Informix is a far better-designed product than Oracle. One of the DBAs at work says Informix is five years ahead of Oracle in technology, and I believe it.
Informix is also vastly easier to use and to learn; after a year of working at an Oracle site I had learned how to startup/shutdown. After a month of working at an Informix site, I had learned startup/shutdown, how to do backups, how to find out what areas of the disk the database has glommed onto, how to install the Perl DBI module for Informix, how to run it from an Intel/FreeBSD box to a HPPA/HP-UX box, etc.
Think of a computer-industry leader who is fantastically rich, fantastically driven by the art of the deal, who likes fast cars, who has near-total market share, and whose products have kinda crummy pragmatics (buggy, hard to work with, hard to troubleshoot, beloved of managerial types).
Not Bill Gates, it's Larry Ellison.
-- Daniel Boyd, September 22, 1998Oracle documentation The Oracle documentation became available around a year ago at http://technet.oracle.com; you have to register there before you can use it.
Locking The lmode=6 clause finds all the exclusive locks. A more direct way to find the blocking session and the sessions that are blocked would use the v$lock.block column.
col type heading "Lock Type" format a15 col lmode heading "Mode Held" col request heading "Mode Requested" col block heading "Blocking?"
select sid,type,lmode,request,block from v$lock where block != 0 or request != 0 order by id1,id2,block desc
ID Lock Type Mode Held Mode Requested Blocking? ---- --------------- ---------- -------------- ---------- 16 TX 6 0 1 31 TX 0 6 0 15 TX 0 6 0
From this you know that session 16 is blocking sessions 31 and 15. You can now query v$sqltext to find out what the sessions are doing.
select s.sid,s.serial#,a.sql_text from v$sqltext a,v$session s where a.hash_value = s.sql_hash_value and s.sid in &sidstring order by s.sid,a.hash_value,a.piece
SQL> / Enter value for sidstring: (16,31,15)
SID SERIAL# SQL_TEXT ---- ---------- -------------------------------------------------- -------------- 15 30 update x set the_data = 'this is not a test' where the_key=1 16 15 update x set the_data = 'this is a test' where the_key=1 31 185 update x set the_data = 'this is also a test' where the_key=1
'' is the same as Null For the next time around... you could default the phone_number column to 'n/a' when you create the table. If you load data behind the scenes you could use insert statements that omit the phone_number column. The loaded data will have pn='n/a'; the data entered on the form will have either null, or an actual phone number...
PL/SQL You can see the text of your own packages,procedures and functions without the select any table privilege: select line,text from user_source where name = 'YOUR_PROC_NAME' order by type,line
-- Chaim Katz, January 5, 1999
I would have to agree with the Informix vs. Oracle comments. As far as 99% of the user-base is concerned, both products supply about the same level of service, cost about the same amount of money, and run about as fast.
I should mention at this point that I'm an Informix DBA by profession who was forced to become an Oracle DBA about a year ago due to interesting management decisions. This probably gives me a skewed perspective on things, so keep that in mind.
Anyway, the one place that the two systems really diverge in my opinion is in basic entry-level administration. While both systems provide a complete set of 'system tables' that an advanced DBA can write queries on, Informix is light-years ahead as far as friendly common tasks go. A good example of this would be the amount of effort to do common administrative procedures. Some examples follow.
I was assured in official Oracle training that I wanted to rebuild my b-tree indexes periodically to reclaim the space lost when rows were deleted from the main table. This surprised me, as Informix takes care of this detail automatically as data is inserted and deleted. I don't actually recall the command to do it while the index is in place, and Oracle doesn't provide a 'how to do daily maintenance' user guide (at least, not in my opinion).
To determine the amount of space used in every data file in the system, the Informix command to use would be 'onstat -d'. In Oracle, I have to write a query in PL/SQL to accomplish the same task.
In Informix, I can get a complete point-in-time binary backup of a running system by typing 'ontape -c -l0' (Create a level 0 backup). This can go to a tape, or a single file on disk. Oracle, on the other hand, requires you to:
- Obtain a list of all current datafiles.
- For each file:
Manually suspend activity on the file.
Copy the file to a free area.
Restore activity on the file.
- Add the system configuration files to the free area.
- Tar the individual files together to a tape.
Admittedly, this is not difficult to do. It requires a fairly short shell script that can be cobbled together in an hour or so. However, when running an enterprise-level system this has to be tested and validated. You'd probably want to add some form of intelligence to the script to restore access to locked files if the script is killed in mid-backup as well.
Quite simply, Informix has either automated or provided utilities to cover 99% of standard maintenance issues (consolidation, backups, etc). They both give you sufficent information that you could write your own if you wished, but Informix assumes that you'll be wanting to make hot backups, etc. and provides tested, supported tools to make your life easier.
After a couple of years of dealing with Oracle, I am beginning to see why Oracle DBAs are more in demand than Informix DBAs. I can (and do) administrate over 100 live production OLTP Informix databases with more ease and confidence than I have when working with our 2-3 internal Oracle development systems.
-- Richard Stanford, February 24, 1999Another way to handle long strings is to break them up into segments. Instead of storing the text or a message directly in a message table, you add another table, say msg_text, with columns msg_id, seg_num, and seg_text. On insert the message is split into segments limited to the smaller of the query limit and database varchar limit, minus the overhead of the sql query (the "insert..." stuff). It makes for an ugly insert operation, though.
I had to do this to work with PostgreSQL, which has an 8K limit. If you split on a word boundary (a whitespace character) then a full text indexer might be able to make sense of this.
-- Ari Halberstadt, March 4, 1999ConText.. what ConText? Take a look at the Oracle Home Page.. Search engine licensed by Oracle is the Altavista Engine..... Or am I missing something here ???
-- Ravi Nagpal, May 20, 1999Re: How to rename a column in Oracle
You can do something like the following, though I agree it shouldn't be necessary:SQL> describe phone_directory Name Null? Type ------------------------------- -------- ---- NAME VARCHAR2(100) PHONE VARCHAR2(100) SQL> create table new_phone_directory (full_name varchar(100), phone varchar(100)); Table created. SQL> insert into new_phone_directory (full_name, phone) 2 select name, phone from phone_directory; 1 row created. SQL> commit; Commit complete. SQL> drop table phone_directory; Table dropped. SQL> rename new_phone_directory to phone_directory; Table renamed.
-- Larry Mulcahy, November 15, 1999Doctor Philip Greenspun,
About the Oracle Documentation you say "I can't for the life of me figure out why this isn't simply available at www.oracle.com but it isn't.".
However, Oracle has all of their documentation on its technical site: technet.oracle.com. You have to register yourself to access the technet but it's free. Besides, nowadays, everybody asks for registering. Even to add this comment! :))
For instance, the Oracle8i Server Documentation, Release 8.1.5 may be found at http://technet.oracle.com/doc/server815.htm.
Best regards and thank for your nice site, Pedro Bizarro
-- Pedro Bizarro, December 27, 1999This page is very helpful. Thanks.
One thing, i se that Informix is more appreciated than Oracle. I think there is one point where Oracle is however interesting, the use of SQL for all administration process of the database. This way will be the most useful, i believe, for tomorrow networked application.
Note that it's possible with Oracle to compile all database operation on SQL script, wich is very effective for migration or structural evolution of distributed architecture.
Nevertheless, Oracle suffer of this great problem, lot of important administration tasks could not be done outside SQL statements,even if the most common tasks are automated under entreprise tools.
-- C A, January 4, 2000To determine block size on Linux, use tune2fs.
According to the documentation I found, block size can be Linux distribution specific. On Red Hat 6.1, it appears to be 4096, and tune2fs can be found at: /sbin/tune2fs.
-- Jerry Asher, January 14, 2000More Linux filesystem stuff: On my (Debian) system, tune2fs will only change stuff; to see the blocksize use 'dumpe2fs -h' (without -h, you get a dump of the entire ondisk block structure, which can be interesting, but is probably not what you want).
The blocksize is determined when the filesystem is created (mke2fs). I've got two at 4k, probably the Debian default, and one at 1k that I created it later, and intended to put lots of small files on it (C code, sorry Phil). Now that Oracle is living on it, the choice seems less wise.
You'll need to be root to use this, as it directly accesses the raw disk device.
-- Steve Greenland, January 24, 2000I just came across a HOWTO for installing Oracle 8i on Linux that may be of help to anyone attempting such a project:
-- John Milazzo, February 18, 2000To get block size on RH Linux you can do a tune2fs -l /dev/disk1
-- Robert Bigin, March 1, 2000If you use Oracle server, you need a copy of Embarcadero's DB Artisan. It's an amazing product that allows you to change tables, indexes tables, tablespaces without having to learn the arcane syntax and strange table names of Oracle innards. It's a DBA in a box and hugely better than the obtuse GUI that comes with Oracle. Unfortunately I think it runs only under Windoze. Be careful though, it's a powerful tool and can powerfully ruin your production DB in seconds. No affiliation, blah, blah...
-- Rob Quinn, March 22, 2000Your page shows that Oracle REALLY IS worth the money spent on it.
In many cases single 800MHz P III 1 GB RAM with 2x2x18 GB SCSI LVD RAID is enough to run whole enterprise if developers know something about tuning PL/SQL queries AND Oracle DBA knows about setting startup parameters. You did great work showing people that sometimes changing a single line in init..ora speeds up database.
After some hard work on Oracle 8i I have some tips that help other DBAs:
- When Your developer forgets to compile all changed objects, You have to do it yourself. I know that DBA Studio allows this (yeah it's easier) , but via SQL*Plus it's much faster and even automatic. You do such a script:
SELECT 'ALTER ' || object_type || ' ' || object_name || ' compile;' from user_objects where status='INVALID';
and then You run
Nice, works fine for me :)
Guys from Oracle have prepared special procedure for compiling whole schema, but it does not work well in some cases. This script does the same in simplier way. In some circumstances I had to execute @recompile.sql twice. So I added two executions of @recompile.sql to a script.
- You can do refreshing optimizer's statistics this way:
SELECT 'ANALYZE TABLE ' || object_name || ' COMPUTE STATISTICS; ' from user_objects where object_type='TABLE';
and then run
I have installed these hints into two shell scripts that are executed by cron every night. I don't know whether it's foolproof but it works. There is also a procedure GATHER_DATABASE_STATS or so, but this script is faster.
- One of the ways to back up data from a single table is to create a backup one with same data in it. You do:
CREATE TABLE BACKUP_DATA AS SELECT * FROM DATA;
It's much faster than exporting table and works also when You have constrains between tables.
-- Marcin Marciniak, June 6, 2003
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