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Re: select distinct slower in 5.0 than 4.1
Posted by: Saar Barhoom
Date: December 20, 2005 10:29AM

The data sets are EXACTLY identical. This is a regular benchmark test I use for my application.

Anyway - I found out the reason:
The table has two indexes. The first index is a BTREE index on lastname. The second index is a unique constraint on PID, lastname.

While 4.1 optimizer is using wisely the first index, 5.0 prefers the second!

My guess is that this happens probably because of some problem in the logic that makes it prefer second since it contains both columns. But - since PID is the first column in the index - it turns out this choice is worst.

What do you think?

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Written By
December 18, 2005 12:24PM
December 19, 2005 10:49AM
Re: select distinct slower in 5.0 than 4.1
December 20, 2005 10:29AM
December 20, 2005 11:36AM

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