Re: Postgres optimizer

On Fri, 2007-08-03 at 13:58 -0400, Mouhamadou Dia wrote:
> Hello,
>
> I have a Postgres instance (version 8.1) running on a Solaris 10
> machine. When I run the following query
>
>
>
> SELECT *  FROM PROR_ORG,  ( ( ( ( (PRPT_PRT LEFT OUTER JOIN
> PRPT_PRTADR ON
>
> PRPT_PRT.PRT_NRI = PRPT_PRTADR.PRT_NRI AND
> PRPT_PRTADR.ADR_F_DEF=true)
>
> LEFT OUTER JOIN PLGE_CTY ON PRPT_PRTADR.CTY_NRI = PLGE_CTY.CTY_NRI)
> LEFT
>
> OUTER JOIN PLGE_CTY1 PLGE_CTY_PLGE_CTY1 ON PLGE_CTY.CTY_NRI =
>
> PLGE_CTY_PLGE_CTY1.CTY_NRI AND PLGE_CTY_PLGE_CTY1.LNG_CD = 'fr')
> LEFT
>
> OUTER JOIN PLGE_CTRSD ON PRPT_PRTADR.CTRSD_CD = PLGE_CTRSD.CTRSD_CD
>
> AND PRPT_PRTADR.CTR_ISO_CD = PLGE_CTRSD.CTR_ISO_CD)  LEFT OUTER JOIN
>
> PLGE_CTR ON PRPT_PRTADR.CTR_ISO_CD = PLGE_CTR.CTR_ISO_CD) , PROR_ORG1
>
> PROR_ORG_PROR_ORG1, PROR_ORGT, PROR_ORGT1 PROR_ORGT_PROR_ORGT1
>
> WHERE (  (PROR_ORG.ORGT_CD = PROR_ORGT.ORGT_CD) AND
>
> (PROR_ORGT.ORGT_CD = PROR_ORGT_PROR_ORGT1.ORGT_CD AND
>
> PROR_ORGT_PROR_ORGT1.LNG_CD = 'fr') AND (PROR_ORG.PRT_NRI =
>
> PROR_ORG_PROR_ORG1.PRT_NRI AND PROR_ORG_PROR_ORG1.LNG_CD = 'fr') AND
>
> (PROR_ORG.PRT_NRI = PRPT_PRT.PRT_NRI) )  AND ( ((PROR_ORG.ORGT_CD
> ='CHAIN')) )
>
>
>
> it takes 45 seconds to run. In this case the optimizer does a
> sequential scan of the PRPT_PRT table (which is the largest one)
> despite the existence of an index on PRT_NRI column of PRPT_PRT table.
>
>  I’ve activated the GEQO but it still takes nearly the same time to
> run (between 40 and 45s).
>
> When I change the order of PRPT_PRT and PROR_ORG tables, it takes only
> 30 milliseconds to run. In this case the optimizer uses the index on
> PRT_NRI column of PRPT_PRT table, what is normal and what I was
> expecting.
>
> Is there a known problem with the Postgres optimizer?
>
> For your information, the same query takes 20 milliseconds to run on
> Informix and 60 milliseconds to run on Oracle independently of the
> order of the tables in the query.
>
>
>
> PRPT_PRT has 1.3 millions rows
>
> PRPT_PRTADR has 300.000 rows
>
> PROR_ORG has 1500 rows
>
> These are the largest tables, all the others are small tables. All
> statistics are up to date.

If I recall correctly, PG 8.2 was the first version where the planner
supported reordering outer joins.  Prior releases would get poor
performance unless the joins were listed in the right order.

So it is quite possible that upgrading to 8.2 would solve your problem.
Do you have the ability to try that?

-- Mark Lewis