From: Tom Lane <tgl@sss.pgh.pa.us>
To: Yves Dorfsman <yves@zioup.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: extract(year from date) doesn't use index but maybe could?
Date: Sun, 19 Apr 2015 18:29:54 -0400
Message-ID: <6398.1429482594@sss.pgh.pa.us> (raw)
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Yves Dorfsman <yves@zioup.com> writes:
> What about functions that are simpler such as upper()/lower()?
If you think those are simpler, you're much mistaken :-(. For instance,
"lower(first_name) = 'yves'" would have to be translated to something
like "first_name IN ('yves', 'yveS', 'yvEs', 'yvES', ..., 'YVES')"
-- 16 possibilities altogether, or 2^N for an N-character string.
(And that's just assuming ASCII up/down-casing, never mind the interesting
rules in some non-English languages.) In a case-sensitive index, those
various strings aren't going to sort consecutively, so we'd end up needing
a separate index probe for each possibility.
extract(year from date) agrees with timestamp comparison up to boundary
cases, that is a few hours either way at a year boundary depending on the
timezone situation. So you could translate it to a lossy-but-indexable
timestamp comparison condition and not expect to scan too many index items
that don't satisfy the original extract() condition. But I don't see how
to make something like that work for mapping case-insensitive searches
onto case-sensitive indexes.
regards, tom lane
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To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, yves@zioup.com
Subject: Re: extract(year from date) doesn't use index but maybe could?
In-Reply-To: <6398.1429482594@sss.pgh.pa.us>
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