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Re: TOAST usage setting

From: Gregory Stark <stark(at)enterprisedb(dot)com>
To: "Bruce Momjian" <bruce(at)momjian(dot)us>
Cc: <pgsql-hackers(at)postgresql(dot)org>, "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Subject: Re: TOAST usage setting
Date: 2007-05-29 17:06:47
Message-ID: 87abvno8yg.fsf@oxford.xeocode.com (view raw or flat)
Thread:
Lists: pgsql-hackers
"Bruce Momjian" <bruce(at)momjian(dot)us> writes:

>> No, we did substring() too :)
>
> Uh, I looked at text_substring(), and while there is an optimization to
> do character counting for encoding length == 1, it is still accessing
> the data.

Sure but it'll only access the first chunk. There are two chunks in your test.
It might be interesting to run tests accessing 0 (length()), 1 (substr()), and
2 chunks (hashtext()).

Or if you're concerned with the cpu cost of hashtext you could calculate the
precise two bytes you need to access with substr to force it to load both
chunks. But I think the real cost of unnecessary toasting is the random disk
i/o so the cpu cost is of secondary interest.

-- 
  Gregory Stark
  EnterpriseDB          http://www.enterprisedb.com


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