Comments on: COUNT(*) vs COUNT(col)

By: dalin

dalin — Tue, 03 Nov 2009 01:55:23 +0000

And to correct my own typo I meant
SELECT 1 FROM foo WHERE bar=”baz” LIMIT 1

the limit being very important

By: dalin

dalin — Tue, 03 Nov 2009 01:54:30 +0000

It should be noted that often when people use COUNT they really just want to know if there’s anything there and aren’t particularly interested in a accurate count. In this case a far more performant approach is
SELECT 1 FROM foo WHERE bar=”baz”
The query completes as soon as the first row is found where bar=”baz”.

By: Simple Pagination - CodeCall Programming Forum

Simple Pagination - CodeCall Programming Forum — Thu, 01 Oct 2009 12:47:50 +0000

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By: 关于 count(*) 和 count(col) 的种种 - Pro Evolution Swine

关于 count(*) 和 count(col) 的种种 - Pro Evolution Swine — Thu, 30 Apr 2009 08:24:42 +0000

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By: لدى حلين لمعرفة عدد السجلات اى الحلول تنصحوتى - سوالف سوفت

لدى حلين لمعرفة عدد السجلات اى الحلول تنصحوتى - سوالف سوفت — Fri, 12 Sep 2008 11:31:00 +0000

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By: howard

howard — Sat, 23 Jun 2007 10:33:15 +0000

hello, can i say:

the rule of thumb is…

1. use count(primary_key)
2. use count(indexed_field_and_not_null)
3. use count(indexed_field)
4. use count(*)

how abt the length of the index, does it matter?

thanks.

By: » Easy MySQL Performance Tips

» Easy MySQL Performance Tips — Wed, 06 Jun 2007 12:12:05 +0000

[...] Never do a COUNT(*) (or anything *, says Zach). Instead, replace the * with the name of the column you’re searching against (and is hopefully indexed). That way some queries can execute entirely in the keycache (while * forces MySQL to read every matching row from the table). [...]

By: peter

peter — Mon, 30 Apr 2007 02:05:59 +0000

Pavel,

For primary key it should be the same as primary key can’t have null values.
It is same as count(val2) in my example.

By: Pavel

Pavel — Sat, 28 Apr 2007 23:02:57 +0000

What about count(id) (id is primary key)?
Is count(id) slower than count(*)?

Using innodb + where and myisam + where;

By: peter

peter — Thu, 26 Apr 2007 16:34:32 +0000

Raj,

Good point. It looks like a bug to me as Innodb automatically picks up “shorter” indexes while in practice even first index you tried is shorter.

The other interesting point I should note – smaller index may not be faster for huge tables, because if you insert data in order primary key tree is often sequential while secondary indexes may be scattered having few sequential pages.