RE : RE : [BUGS] BUG #3519: Postgres takes the wrong query plan resulting in performance issues

Sorry,
This output is coming from PG 8.1.19
I'm attaching the one that is coming from 8.2.4
Thanks and sorry for the confusion


-----Message d'origine-----
De : Heikki Linnakangas [email protected: hli...@gmail.com] De la part de Heikki Linnakangas
Envoyé : 6 août 2007 15:32
À : Mouhamadou Dia
Cc : [email protected: pgsql...@postgresql.org]
Objet : Re: RE : [BUGS] BUG #3519: Postgres takes the wrong query plan resulting in performance issues

Mouhamadou Dia wrote:
> I'm sending in attachment the output of the explain analyze command and the create table statements of tables involved in the query.

Wait, you said that the query takes 20 seconds on 8.2, but the explain
analyze output says that it actually took 50 seconds. Is this the output
from 8.2.4?

Hmm. I don't see anything terribly wrong in the planner's estimates. The
only estimate that's off is the # of rows in pror_org matching the qual
orgt_cd = 'CHAIN', 27 estimated vs 1 actual. You could try increasing
the statistics target for that column to get that estimate right. That
might tip the planner to choose a plan with nested loop joins instead of
hash joins.

Have you played with enable_seqscan=off or enable_hashjoin=off? That's
not a good long term solution, but it would be interesting to see what
happens.

"Heikki Linnakangas" <heikki@enterprisedb.com> writes:

> Have you played with enable_seqscan=off or enable_hashjoin=off? That's
> not a good long term solution, but it would be interesting to see what
> happens.

I think this is a case where Postgres just doesn't know it can re-order near
an outer join. Outer joins often can't be re-ordered and Postgres isn't a
general theorem prover, it can't always figure out whether it's safe to
re-order them.

The structure of your query is a whole series of left outer joins, the result
of which is then (inner) joined with one more table. The outer joins return a
whole lot of records but the inner join is only going to match a few of them.

The only hope you have of a reasonable plan here is if Postgres can figure out
that it can do the inner join first so that it only has to perform the outer
join on the resulting records.

I think it could actually re-order the inner query to happen first in this
case. But I'm not certain, it's tricky to tell. But the fact that Oracle finds
a way to execute it quickly gives me some confidence that it ought to be
possible since I think Oracle does get join orderings reasonably right. I'm
not so sure about Informix.

Gregory Stark <stark@enterprisedb.com> writes:
> The structure of your query is a whole series of left outer joins, the result
> of which is then (inner) joined with one more table. The outer joins return a
> whole lot of records but the inner join is only going to match a few of them.

Hmmm ... actually I see 6 tables inside the join-tree and four more
loose in the FROM-clause, ten relations altogether.  Which means the OP
is falling foul of from_collapse_limit, and it's not investigating every
possible join order.  Try setting from_collapse_limit to more than 10.

   regards, tom lane

The three other loose ones are attached to a table inside outer joins though.
Doesn't that prevent any possibility of them being done earlier? But the first
one looks like it ought to be driving the join.

I've played with both parameters but it doesn't help in this case
Thanks


-----Message d'origine-----
De : Heikki Linnakangas [email protected: hli...@gmail.com] De la part de Heikki Linnakangas
Envoyé : 6 août 2007 16:58
À : Mouhamadou Dia
Cc : [email protected: pgsql...@postgresql.org]
Objet : Re: RE : RE : [BUGS] BUG #3519: Postgres takes the wrong query plan resulting in performance issues

Hmm. I don't see anything terribly wrong in the planner's estimates. The
only estimate that's off is the # of rows in pror_org matching the qual
orgt_cd = 'CHAIN', 27 estimated vs 1 actual. You could try increasing
the statistics target for that column to get that estimate right. That
might tip the planner to choose a plan with nested loop joins instead of
hash joins.

Have you played with enable_seqscan=off or enable_hashjoin=off? That's
not a good long term solution, but it would be interesting to see what
happens.

Thanks Tom,
By setting from_collapse_limit to more than 10, the query takes 133ms instead of 20s.
My question is: why even if from_collapse_limit is set to 8 (it's default value), the same query takes 30ms just by changing the order of PRPT_PRT and PROR_ORG tables in the query?


-----Message d'origine-----
De : Tom Lane [email protected: t...@sss.pgh.pa.us]
Envoyé : 6 août 2007 21:31
À : Gregory Stark
Cc : Heikki Linnakangas; Mouhamadou Dia; [email protected: pgsql...@postgresql.org]
Objet : Re: RE : RE : [BUGS] BUG #3519: Postgres takes the wrong query plan resulting in performance issues

Gregory Stark <stark@enterprisedb.com> writes:
> The structure of your query is a whole series of left outer joins, the result
> of which is then (inner) joined with one more table. The outer joins return a
> whole lot of records but the inner join is only going to match a few of them.

Hmmm ... actually I see 6 tables inside the join-tree and four more
loose in the FROM-clause, ten relations altogether.  Which means the OP
is falling foul of from_collapse_limit, and it's not investigating every
possible join order.  Try setting from_collapse_limit to more than 10.

   regards, tom lane