| From: | Greg Stark <gsstark(at)mit(dot)edu> |
|---|---|
| To: | Simon Riggs <simon(at)2ndquadrant(dot)com> |
| Cc: | Greg Stark <gsstark(at)MIT(dot)EDU>, "Jim C(dot) Nasby" <jnasby(at)pervasive(dot)com>, josh(at)agliodbs(dot)com, pgsql-hackers(at)postgresql(dot)org |
| Subject: | Re: Improving N-Distinct estimation by ANALYZE |
| Date: | 2006-01-09 16:21:10 |
| Message-ID: | 87fynx1ifd.fsf@stark.xeocode.com |
| Views: | Raw Message | Whole Thread | Download mbox | Resend email |
| Thread: | |
| Lists: | pgsql-hackers |
> > These numbers don't make much sense to me. It seems like 5% is about as slow
> > as reading the whole file which is even worse than I expected. I thought I was
> > being a bit pessimistic to think reading 5% would be as slow as reading 20% of
> > the table.
I have a theory. My test program, like Postgres, is reading in 8k chunks.
Perhaps that's fooling Linux into thinking it's a sequential read and reading
in 32k chunks internally. That would effectively make a 25% scan a full table
scan. And a 5% scan would be a 20% scan which is about where I would have
expected the breakeven point to be.
--
greg
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