You can't use an index to select records in a range, and order them. The order by will
cause a filesort in that case.
Additionally indexes are always read left to right. So an index on ('user_id',
'product_id') will help when doing WHERE user_id=N AND product_id IN (1,2,3), but wouldn't
help for just the condtion on product_id.
See the manual for full details on how mysql uses indexes:
From: Jonas Galvez [mailto:jonasgalvez@stripped]
Sent: Friday, October 01, 2010 11:48 AM
Subject: Indexing question
Suppose I wanted to be able to perform queries against three columns of my
table: 'user_id', 'product_id' and 'created'. Most of the time I'll just be
range-selecting records from the table ordering by 'created'. But I may also
want to select where 'user_id' = something and 'product_id' in (list, of,
ids), ordered by 'created'. Do I need two separate indexes, one on 'created'
and another on ('user_id', 'product_id', 'created'), or does having only the
latter suffice the former case?
-- Jonas, http://jonasgalvez.com.br
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