SQL optimization tips -

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i beginner sql , have performance problems query. tips? query running on huge database...

select                 z_screenshots.guid,                 z_screenshots.player_name,                 z_screenshots.server,                 z_screenshots.map,                 z_screenshots.created,                 z_screenshots.uploaded,                 z_screenshots.uploader_id,                 z_screenshots.filesize             z_screenshots              inner join (                 select clients.guid                 clients clients.id not in (                     select clients.id                     clients                     inner join (                         select client_id                         penalties                         penalties.inactive = 0 and(penalties.type = 'ban' or (penalties.type = 'tempban' , from_unixtime(penalties.time_expire) > now()) )                         group penalties.client_id                     ) penalties                     on clients.id = penalties.client_id                 )             ) clients             on z_screenshots.guid = clients.guid              order z_screenshots.uploaded desc             limit ?, ?;

i advise rewrite query.

select     s.guid,     s.player_name,     s.server,     s.map,     s.created,     s.uploaded,     s.uploader_id,     s.filesize z_screenshots s  inner join clients c on s.guid = c.guid left join (     select distinct client_id     penalties     penalties.inactive = 0      , (penalties.type = 'ban' or (penalties.type = 'tempban' , from_unixtime(penalties.time_expire) > now()) ) ) p on c.id = p.client_id p.client_id null; order s.uploaded desc limit ?, ?;

when don't have client_id, query okay:

select     s.guid,     s.player_name,     s.server,     s.map,     s.created,     s.uploaded,     s.uploader_id,     s.filesize z_screenshots s  inner join clients c on s.guid = c.guid c.id not in  (     select distinct client_id     penalties     penalties.inactive = 0      , (penalties.type = 'ban' or (penalties.type = 'tempban' , from_unixtime(penalties.time_expire) > now()) ) ) order s.uploaded desc limit ?, ?;

regarding indexes, it's hard tell. clients, doesn't sound have distinct values. index better, closer result of formula gets 1.

count(distinct <column>) / count(<column>)

meaning, when have 5 distinct values , 5000000 rows, mysql (assuming because of limit clause) think it's cheaper read entire table. if case here, might consider having index on z_screenshots.uploaded instead, mysql @ least doesn't have sort. read explain know when index chosen , index is.


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