From: Reindl Harald
Date: April 20 2012 7:50am
Subject: Re: Why does the limit use the early row lookup.
List-Archive: http://lists.mysql.com/mysql/227213
Message-Id: <4F91152B.7000001@thelounge.net>
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Am 20.04.2012 04:29, schrieb =D5=C5=D6=BE=B8=D5:
> My point is that the limit can use late row lookup: lookup rows after
> checking indexes to optimize the select speed.
>=20
> But the mysql optimizer do it with the early row lookup: lookup all row=
s
> before checking indexes when the one fetch column is not in the indexes=
=2E
>=20
> Tell me why?

because the mysql optimizer until now is really
bad in many situations - order by rand() makes a
temporary table wil ALL data as example even with limit

select * from table order by rand() limit 10;
reads and writes the whole table to disk
have fun with large tables :-)


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