| From: | Josh Berkus <josh(at)agliodbs(dot)com> |
|---|---|
| To: | Martijn van Oosterhout <kleptog(at)svana(dot)org>, Gavin Flower <GavinFlower(at)archidevsys(dot)co(dot)nz> |
| Cc: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Greg Stark <stark(at)mit(dot)edu>, Simon Riggs <simon(at)2ndquadrant(dot)com>, Peter Geoghegan <pg(at)heroku(dot)com>, Jim Nasby <jim(at)nasby(dot)net>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: ANALYZE sampling is too good |
| Date: | 2013-12-11 22:49:18 |
| Message-ID: | 52A8EBEE.9070506@agliodbs.com |
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| Lists: | pgsql-hackers |
On 12/11/2013 02:39 PM, Martijn van Oosterhout wrote:
> In this discussion we've mostly used block = 1 postgresql block of 8k.
> But when reading from a disk once you've read one block you can
> basically read the following ones practically for free.
>
> So I wonder if you could make your sampling read always 16 consecutive
> blocks, but then use 25-50% of the tuples. That way you get many more
> tuples for the same amount of disk I/O seeks..
Yeah, that's what I meant by "tune this for the FS". We'll probably
have to test a lot of different "block sizes" on different FSes before
we arrive at a reasonable size, and even then I'll bet we have to offer
a GUC.
--
Josh Berkus
PostgreSQL Experts Inc.
http://pgexperts.com
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