From: Øystein Grøvlen
Date: February 12 2013 2:11pm
Subject: Re: debugging how query plans are made
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On 02/12/13 02:54 PM, Zardosht Kasheff wrote:
> Hello all,
>
> In MySQL and/or MariaDB, are there any hooks or methods one can use to
> learn why a query plan is chosen over alternatives? We sometimes
> notice that a different query plan is chosen for us than InnoDB (which
> may be fine, given we are a different storage engine), and we would
> like to better understand why. Is there any debug parameter that gets
> MySQL to output decisions made in the optimizer?

MySQL 5.6 has optimizer tracing which outputs the decisions made by the 
optimizer.  See http://dev.mysql.com/doc/internals/en/optimizer-tracing.html

Hope this helps,

-- 
Øystein