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Re: Significantly larger toast tables on 8.4?

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: "Stephen R(dot) van den Berg" <srb(at)cuci(dot)nl>
Cc: Alex Hunsaker <badalex(at)gmail(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Significantly larger toast tables on 8.4?
Date: 2009-01-02 16:01:56
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Lists: pgsql-hackers
"Stephen R. van den Berg" <srb(at)cuci(dot)nl> writes:
> What seems to be hurting the most is the 1MB upper limit.  What is the
> rationale behind that limit?

The argument was that compressing/decompressing such large chunks would
require a lot of CPU effort; also it would defeat attempts to fetch
subsections of a large string.  In the past we've required people to
explicitly "ALTER TABLE SET STORAGE external" if they wanted to make
use of the substring-fetch optimization, but it was argued that this
would make that more likely to work automatically.

I'm not entirely convinced by Alex' analysis anyway; the only way
those 39 large values explain the size difference is if they are
*tremendously* compressible, like almost all zeroes.  The toast
compressor isn't so bright that it's likely to get 10X compression
on typical data.

			regards, tom lane

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