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Re: hashjoin chosen over 1000x faster plan

From: "Kevin Grittner" <Kevin(dot)Grittner(at)wicourts(dot)gov>
To: "Simon Riggs" <simon(at)2ndquadrant(dot)com>
Cc: <pgsql-performance(at)postgresql(dot)org>,"Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Subject: Re: hashjoin chosen over 1000x faster plan
Date: 2007-10-10 19:35:58
Message-ID: 470CE34D.EE98.0025.0@wicourts.gov (view raw or flat)
Thread:
Lists: pgsql-performance
>>> On Wed, Oct 10, 2007 at  1:54 PM, in message
<1192042492(dot)4233(dot)334(dot)camel(at)ebony(dot)site>, Simon Riggs <simon(at)2ndquadrant(dot)com>
wrote: 
> 
> But the planner doesn't work on probability. It works on a best-guess
> selectivity, as known at planning time.
 
The point I'm trying to make is that at planning time the
pg_statistic row for this "Charge"."reopHistSeqNo" column showed
stanullfrac as 0.989; it doesn't seem to have taken this into account
when making its guess about how many rows would be joined when it was
compared to the primary key column of the "CaseHist" table.  I'm
suggesting that it might be a good thing if it did.
 
-Kevin
 



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