Re: Continual Postgres headaches...

On Dec 6, 2007, at 1:22 PM, Weber, Geoffrey M. wrote:

> I've been trying for quite a while to get Postgresql tuned for use
> as an OLTP system. I have several PL/pgSQL functions that handle
> inserts and updates to the main table and several near-real-time
> daemons written that access the data and can take automated actions
> on it (email/page concerned people, get complimentary information
> from a different system, etc.). I started with Postgres 8.1 and am
> now using 8.2.4 (and have been since its release). I'll try to
> provide enough information for a decent response, but as I can't
> obviously put my entire schema and database out there, I'm hoping
> that I can get some decent guidelines beyond that what I've found
> though Google, etc. to get this thing tuned better.
>
>
>
> Most of the data centers in on a central table and has 23 columns,
> 1 constraint, and 9 indexes. 4 of the indexes are partial. The
> table usually contains about 3-4 million rows, but I've cut it down
> to 1.2 million (cut out 2/3 of the data) in an effort to migrate
> the database to a 2nd sever for more testing. The two partial
> indexes used the most: 242MB accessed nearly constantly, and 15MB
> accessed every 5 seconds - but also updated constantly via inserts
> using the 242MB index. Other than one other 25MB index, the others
> seem to average around 300MB each, but these aren't used quite as
> often (usually about every minute or so).
Do you really have that many query patterns that you need 9 different
indexes on the table? Without seeing the table layout I'm
suspicious that with 23 column table with 9 indexes it could either
do with some normalization, vertical partitioning or less indexes.
There is definitely such a thing as too many indexes, especially if
any of them share indexed columns.
> My problems really are with performance consistency. I have
> tweaked the execution so that everything should run with sub-second
> execution times, but even after everything is running well, I can
> get at most a week or two of steady running before things start to
> degrade.
>
>
>
> In my efforts to correct these consistency in execution problems, I
> have gone from vacuuming (with analyze) twice a day to every 30
> minutes (how long it takes a vacuum analyze to run - another
> seeming problem because it shouldn't take so long?).
That really depends on the table size and the kind of traffic it sees.
> I've done a VACUUM FULL and that sometimes helps, but obviously is
> not a long-term solution for a true OLTP system. Why I said
> "sometimes helps": earlier this week I did a VACUUM FULL, VACUUM
> ANALYZE on the entire schema. A query that was supposed to use the
> 25MB index above (that matched the parameters of the partial index
> exactly) was still not doing so. I had to DROP and re-CREATE the
> index (and do another ANALYZE) to get the database to (correctly)
> use the index.
VACUUM FULL rewrites large portions of your table in order to compact
space. Since you're physically moving the tuples around it requires
updating the indexes on the table and thus can be a fast track to
bloated indexes. The VACUUM FULL docs even point this out and
suggests running REINDEX after a VACUUM FULL. You do realize that we
have a REINDEX command, right? No need to drop and recreate
separately when they are consecutive actions, just use REINDEX.
>
>
> Another problem: sometimes I get these unexplainable delays in
> using a 'stored procedure' even though all its SQL calls run just
> fine individually. For example, calling a particular function will
> take 1 minute to execute even though entering the SQL commands
> individually through psql will total up to about 1 second. When I
> log 'long-duration queries', I only see the top function call in
> the log. When I put 'RAISE NOTICE' statements in the various PL/
> pgSQL functions, I only see them displayed for the function I call
> directly; any underlying functions called from that function does
> not show any of their NOTICE statements. Because of this I can't
> track down where the delays are occurring for me to try and correct
> the problem (not that there should be one in the first place)! I
> should also note that there is not any lock contention (double-
> checked with pgadmin3) showing that would "artificially" delay the
> queries...
>
>
>
> I've played with quite a few of the parameters like
> vacuum_cost_delay, work_mem, shared_buffers, turned fsync off, etc.
> in the postgresql.conf file. The server I've been given to use is
> quite old (read: not the fastest anymore), but normally shows a
> load average of 0.25.to 0.50 or so.
>
>
>
> I guess my questions boil down to what I can do other than use the
> EXPLAIN, EXPLAIN ANALYZE, VACUUM ANALYZE, etc. to get my schema
> performing acceptably across the board in a consistent manner. If
> there is more information that I can give out that would improve
> responses, please let me know.
As Merlin said, it would help to see some of those query plans along
with those tables' definitions. In addition, your memory and disk/I/
O related config setting along with the system specs would help
alot. Without concrete examples and numbers, the answers to all of
your questions are "It depends -- on those concrete examples and
numbers."

Erik Jones

Software Developer | Emma®
erik(at)myemma(dot)com
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