Message-ID: <552468A3.9090004@BlueTreble.com>
Date: Tue, 7 Apr 2015 18:30:43 -0500
From: Jim Nasby <Jim.Nasby@BlueTreble.com>
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To: Tom Lane <tgl@sss.pgh.pa.us>, Kevin Grittner <kgrittn@ymail.com>
CC: "Carson, Leonard" <lcarson@sdsc.edu>, "pgsql-performance@postgresql.org"
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Subject: Re: views much slower in 9.3 than 8.4
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On 3/30/15 10:52 AM, Tom Lane wrote:
> Kevin Grittner<kgrittn@ymail.com>  writes:
>> >Tom Lane<tgl@sss.pgh.pa.us>  wrote:
>>> >>But the other problem is that the planner considers less-than-1%
>>> >>differences in cost estimates to be "in the noise", which means
>>> >>that it's not going to consider cost differences of less than
>>> >>1480 units in the remaining join steps to be significant.  This
>>> >>is how come we end up with the apparently brain-dead decisions to
>>> >>use seqscans on some of the other tables such as "pi" and "ac":
>>> >>comparing the seqscan to a potential inner indexscan, the total
>>> >>cost of the join is "the same" according to the 1% rule,
>> >The 1% rule itself might be something to add to the R&D list.

> Perhaps.  But it does make for a significant difference in planner speed,
> and I would argue that any case where it really hurts is by definition
> a cost estimation failure somewhere else.

What I wish we had was some way to represent "confidence" in the 
accuracy of a specific plan node, with the goal of avoiding plans that 
cost out slightly cheaper but if we guessed wrong on something will blow 
up spectacularly. Nested loops are an example; if you miscalculate 
either of the sides by very much you can end up with a real mess unless 
the rowcounts were already pretty trivial to begin with.
-- 
Jim Nasby, Data Architect, Blue Treble Consulting
Data in Trouble? Get it in Treble! http://BlueTreble.com


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