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POmore records takes less time
 

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  1. POmore records takes less time
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    <p>This is almost driving me insane</p> <p>I do the following query: </p> <pre><code>SELECT * FROM `photo_person` WHERE photo_person.photo_id IN (SELECT photo_id FROM photo_person WHERE `photo_person`.`person_id` ='1') </code></pre> <p>When I change the id, I get different processing time. Although it's all the same queries and tables. By changing the person_id I get the following:</p> <p>-- person_id=1 ( 3 total, Query took 0.4523 sec)</p> <p>-- person_id=2 ( 99 total, Query took 0.1340 sec)</p> <p>-- person_id=3 ( 470 total, Query took 0.0194 sec)</p> <p>-- person_id=4 ( 1,869 total, Query took 0.0024 sec)</p> <p>I do not understand how with the increase of the number of records/results the query time is lower. The table structures are very straight forward</p> <p>UPDATE: I have already disabled mysql query cache, so every time I run the query, I would get the same exact value (of course it varies on the milisecond level but this is can be neglected)</p> <p>UPDATE: table is MyISAM</p> <pre><code>CREATE TABLE IF NOT EXISTS `photo_person` ( `entry_id` int(11) NOT NULL AUTO_INCREMENT, `photo_id` int(11) NOT NULL DEFAULT '0', `person_id` int(11) NOT NULL DEFAULT '0', PRIMARY KEY (`entry_id`), UNIQUE KEY `PhotoID` (`photo_id`,`person_id`), KEY `photo_id` (`photo_id`), KEY `person_id` (`person_id`) ) ENGINE=MyISAM DEFAULT CHARSET=utf8 AUTO_INCREMENT=182072 ; </code></pre> <p>Here is the results of the profiling</p> <pre><code>+----------+------------+-----------------------------+ | Query_ID | Duration |Query | +----------+------------+-----------------------------+ | 1 | 0.45541200 | SELECT ...`person_id` ='1') | | 2 | 0.44833700 | SELECT ...`person_id` ='2') | | 3 | 0.45587800 | SELECT ...`person_id` ='3') | | 4 | 0.45074900 | SELECT ...`person_id` ='4') | +----------+------------+-----------------------------+ </code></pre> <p>now since the number are the same, it must be the caching :( So the aparently the caching kicks in a certain number of records or bytes</p> <pre><code>mysql&gt; SHOW VARIABLES LIKE "%cac%"; +------------------------------+------------+ | Variable_name | Value | +------------------------------+------------+ | binlog_cache_size | 32768 | | have_query_cache | YES | | key_cache_age_threshold | 300 | | key_cache_block_size | 1024 | | key_cache_division_limit | 100 | | max_binlog_cache_size | 4294963200 | | query_cache_limit | 1024 | | query_cache_min_res_unit | 4096 | | query_cache_size | 1024 | | query_cache_type | ON | | query_cache_wlock_invalidate | OFF | | table_definition_cache | 256 | | table_open_cache | 64 | | thread_cache_size | 8 | +------------------------------+------------+ </code></pre> <p>14 rows in set (0.00 sec)</p>
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      1. POmore records takes less time
      1. USRaptor
      1. VTFavorite
    1. COinteresting. Is it related to query overheads ? What database schema engine are you using (e.g. InnoDB / MyISAM) ?
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      1. USRaptor
    2. COhow many times did you run the queries to get those results? some queries might hit the query cache, try them all again and see if you get different results
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      1. USstivlo
    3. CODid you re-run the queries an number of times. The DB could be caching the relevant data speeding-up latter queries.
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      1. USBrett Walker
 

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