Postgres Pro
Downloads
Home   >   mailing lists

Thread: Few questions about my slow query

Few questions about my slow query

From
webmaster
Date:
Hi, I'm returning to postgresql performance problem, while I'm trying a lot
to understand way pg is performing queries I need your help... So I have two
tables:

refs (...some fields... , key is id_tit char(7) - indexed with default method);
temp_kat (...some fields..., key is id_ref char(7) - indexed with default
method);

Refs contains 80000 recs, temp_kat contains 5500 recs and all records from
temp_kat have a matching key value in refs... When I need "mark" records in
table refs (set the field exist for records existing also in temp_kat) I do
following query:

update refs set exist='true' where temp_kat.id_ref=refs.id_tit;

This query takes a lot of time - I don't know exactly how long because I had
to leave my office after 1.5 hour and the query was still running. I think
it's too long... Before executing I increased shared buffers to 1024 and both
tables are vacuumized. And I'm running the RedHat5.1 box on Celeron 266 with
64MB ram.

So the questions are:

1. Did I reach the maximum possible performance or should I rewrite query some
other way? 

2. Could I start postgres with some diff. options for ex. with the feature of
delayed flushing? My pg is 6.4.2 and I saw here there is a possibility to
run it at some diff. mode but I don't know how (no docs about it) and don't
know if it's only pg 6.5 feature

3. Should I change the indexing method from default one to some other? 

Many thanks for helping me; believe me that I'm not lazy to study but there is
not so much docs about the internal pg behaviour so I can only ask you...
--
Michal Samek, Tony distribuce s.r.o.
webmaster@tony.cz  (++420659/321350)
ICQ: 38607210

Re: Few questions about my slow query

From
webmaster
Date:
>Many thanks for helping me; believe me that I'm not lazy to study but there is
>not so much docs about the internal pg behaviour so I can only ask you...

OK, after posting this message I downloaded the 6.5 manual and I can see the
whole new chapter about pg internals, it's great :) I'm sorry I should read it
before I ask you. But still thanks if you want to help me.
--
Michal Samek, Tony distribuce s.r.o.
webmaster@tony.cz  (++420659/321350)
ICQ: 38607210

Re: [SQL] Few questions about my slow query

From
Tom Lane
Date:
webmaster <webmaster@tony.cz> writes:
> Refs contains 80000 recs, temp_kat contains 5500 recs and all records from
> temp_kat have a matching key value in refs... When I need "mark" records in
> table refs (set the field exist for records existing also in temp_kat) I do
> following query:

> update refs set exist='true' where temp_kat.id_ref=refs.id_tit;

> This query takes a lot of time

Hmm.  Are the id fields unique, or are there duplicates?  If there were,
say, 10 records in temp_kat with the same ID, then the corresponding
records in refs would each get updated 10 times.

It would help to see what EXPLAIN says about this query.  Since you have
indexes, a merge join would probably be the best strategy, but we can't
tell what's actually being used.

> 2. Could I start postgres with some diff. options for ex. with the feature of
> delayed flushing? My pg is 6.4.2 and I saw here there is a possibility to

I think you should update to 6.5.  You might also want to start the
postmaster with "-o -F" to turn off fsync, but this assumes that you
have confidence in your OS, hardware, and power supply --- in the event
of a system crash, you have a greater risk of losing data with -F.
        regards, tom lane