From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | "Diehl, Jeffrey" <jdiehl(at)sandia(dot)gov> |
Cc: | "'Haller Christoph'" <ch(at)rodos(dot)fzk(dot)de>, pgsql-sql(at)postgresql(dot)org |
Subject: | Re: Out of free buffers... HELP! |
Date: | 2001-09-20 06:55:54 |
Message-ID: | 1701.1000968954@sss.pgh.pa.us |
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Lists: | pgsql-sql |
"Diehl, Jeffrey" <jdiehl(at)sandia(dot)gov> writes:
> Ok, can someone explain to me why this first query might run faster than the
> second?
> select src,dst,count(dst) from data;
> select src,dst,count(*) from data;
Hmm, I'd expect the second to be marginally faster. count(*) counts the
number of rows matching the WHERE condition; count(foo) counts the
number of rows matching the WHERE condition for which foo is not NULL.
So count(foo) ought to involve a couple extra cycles to test for
non-NULL-ness of the specified field. But it's hard to believe you
could measure the difference --- what results are you getting?
regards, tom lane
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