Re: Index on two columns not used - Mailing list pgsql-performance

From Arnaud Lesauvage
Subject Re: Index on two columns not used
Date
Msg-id 4536698B.7010001@freesurf.fr
Whole thread Raw
In response to Re: Index on two columns not used  (Tom Lane <tgl@sss.pgh.pa.us>)
List pgsql-performance
Tom Lane a écrit :
> Arnaud Lesauvage <thewild@freesurf.fr> writes:
>> When I join these two tables, the 2-column index of the first table is
>> not used.
>> Why does the query planner think that this plan  is better ?
>
> Hm, is gid by itself nearly unique in these tables?  If so, the merge
> join would get only marginally more efficient by using both columns as
> merge conditions.  Heikki's probably right to guess that the planner
> thinks it's better to use the smaller index.
>
> However, if there are lots of duplicate gids, then it ought to have
> preferred the two-column merge ...

gid is the primary key of the first table, so absolutely unique.
Thanks for the information Tom !

--
Arnaud

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