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From: Vitalii Tymchyshyn <tivv00@gmail.com>
To: pgsql-performance@postgresql.org
Subject: Re: anti-join chosen even when slower than old plan
Date: Fri, 12 Nov 2010 12:10:01 +0200
Message-ID: <4CDD1279.5090809@gmail.com> (raw)
In-Reply-To: <AANLkTi=c6ZvbgkzkRvtf2BTdzYoKGNhWU1CTRShsBoeN@mail.gmail.com>
References: <4CD9663502000025000374B1@gw.wicourts.gov>
	<20961.1289347680@sss.pgh.pa.us>
	<4CDA62C60200002500037512@gw.wicourts.gov>
	<AANLkTi=eNXKEBo5dXJDd4+aHhfmkibH++fqjj12+DA3o@mail.gmail.com>
	<4CDACBBF020000250003755B@gw.wicourts.gov>
	<4CDB7BAA.7020506@vmsinfo.com>
	<4CDBB0A40200002500037571@gw.wicourts.gov>
	<AANLkTi=dzhRTTNm1LkyNCAC5+PWbp1jWtKFh4T1Bu4nj@mail.gmail.com>
	<18450.1289499783@sss.pgh.pa.us>
	<19088.1289501929@sss.pgh.pa.us>
	<AANLkTi=dnDwLEyxienHs7ZGP1k18YQzaQMPY7KP-hjMT@mail.gmail.com>
	<19796.1289504156@sss.pgh.pa.us>
	<AANLkTi=c6ZvbgkzkRvtf2BTdzYoKGNhWU1CTRShsBoeN@mail.gmail.com>

I'd say there are two Qs here:

1) Modify costs based on information on how much of the table is in 
cache. It would be great  if this can be done, but I'd prefer to have it 
as admin knobs (because of plan stability). May be both admin and 
automatic ways can be followed with some parallel (disableable) process 
modify knobs on admin behalf. In this case different strategies to 
automatically modify knobs can be applied.

2) Modify costs for part of table retrieval. Then you need to define 
"part". Current ways are partitioning and partial indexes. Some similar 
to partial index thing may be created, that has only "where" clause and 
no data. But has statistics and knobs (and may be personal bufferspace 
if they are introduced). I don't like to gather data about "last X 
percents" or like, because it works only in clustering and it's hard for 
optimizer to decide if it will be enough to scan only this percents for 
given query.

Best regards, Vitalii Tymchyshyn



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