Comments on: A Second Rule of Thumb for Choosing Column Order in Indexes http://www.tokutek.com/2009/06/a_second_rule_of_thumb_for_choosing_column_order_in_indexes/ Thu, 02 Feb 2012 04:10:04 +0000 hourly 1 http://wordpress.org/?v=3.2.1 By: Shlomi Noach http://www.tokutek.com/2009/06/a_second_rule_of_thumb_for_choosing_column_order_in_indexes/#comment-123 Shlomi Noach Wed, 24 Jun 2009 18:42:48 +0000 http://a_second_rule_of_thumb_for_choosing_column_order_in_indexes#comment-123 Hi, "...an index of (b,a) will always be as good, and likely better, than an index of (a,b)..." A bit of understatement :) unless this is a covering index (in your example it is), with (a,b), the 'b' part cannot be utilized, and the index is as good as (a) for this particular query. the (b,a) index will answer this query, and will also support queries like: WHERE b=5 AND a BETWEEN 10 AND 20 ORDER BY a DESC etc. Hi,

“…an index of (b,a) will always be as good, and likely better, than an index of (a,b)…”
A bit of understatement :)
unless this is a covering index (in your example it is), with (a,b), the ‘b’ part cannot be utilized, and the index is as good as (a) for this particular query.
the (b,a) index will answer this query, and will also support queries like: WHERE b=5 AND a BETWEEN 10 AND 20 ORDER BY a DESC
etc.

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