sql - MySQL query optimization using coalesce -

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can explain me why second query faster first? (first 1 takes around 40 seconds, second 1 less 0.1)

select releases.feature_code, releases.platform_id, version, release_date, general_available, uri, filesize, display_name, feature_name.full_name, if ((select exists(select 1 release_exceptions  releases.feature_code = release_exceptions.feature_code  , releases.platform_id = release_exceptions.platform_id   , releases.version = release_exceptions.version) = 1),  1, 0) release_exceptions  releases  left outer join platform using(platform_id) left outer join feature_name using(feature_code)  order version desc;

~~~

query 2 ------------------

~~~

select feature_code, version, release_date, general_available, platform_id, uri, filesize, display_name, feature_name.full_name, coalesce(release_exceptions, 0) release_exceptions  mus.releases  left outer join ( select distinct feature_code, version, platform_id, 1 release_exceptions mus.release_exceptions ) release_exceptions using (feature_code, version, platform_id)  left outer join platform using(platform_id) left outer join feature_name using(feature_code)  ;

it seems way generates release_exceptions column big difference in speeds. coworker wrote second one, unable explain why it's faster.

http://imgur.com/dxluo6s,noryv7v#0

first image first query (note - feature_code, platform_id, , version added indexes after posting, results in first 1 being faster now)

its not coalse that's causing speed up. it's removal of sub query. first runs query per row

the second single sub query


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