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POMySQL slow query even though primary index is being used and good EXPLAIN plan
 

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  1. POMySQL slow query even though primary index is being used and good EXPLAIN plan
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    <p>I have a highly trafficked table with 1.3M rows which is seeing a spate of slow queries of the following type:</p> <pre><code>UPDATE app_info SET data1=269223, data2=0, data3=164, last_update='2012-08-30' WHERE slice_id=7636 AND app_id=375 AND user_id=21012286 AND mode_id=1; </code></pre> <p>Yet, the explain plan for this query indicates an optimal plan (we're using the primary key):</p> <pre><code>explain select * from app_info where slice_id=7636 and app_id=375 and user_id=21012286 and mode_id=1\G *************************** 1. row *************************** id: 1 select_type: SIMPLE table: app_info type: const possible_keys: PRIMARY key: PRIMARY key_len: 18 ref: const,const,const,const rows: 1 Extra: </code></pre> <p>Here's the slow query log:</p> <pre><code> Time: 120830 3:23:37 # User@Host: rest_service[rest_service] @ app01.peak.mindjolt.com [10.0.0.174] # Thread_id: 10091395 Schema: platform Last_errno: 0 Killed: 0 # Query_time: 68.559347 Lock_time: 0.000045 Rows_sent: 0 Rows_examined: 1 Rows_affected: 1 Rows_read: 2 # Bytes_sent: 52 Tmp_tables: 0 Tmp_disk_tables: 0 Tmp_table_sizes: 0 # InnoDB_trx_id: 575CBF3B9 UPDATE app_info SET data1=269223, data2=0, data3=164, last_update='2012-08-30' WHERE slice_id=7636 AND app_id=375 AND user_id=21012286 AND mode_id=1; </code></pre> <p>About 30% of all queries are taking >1s, and about 10% of all queries are taking >10s (!)</p> <p>What could be causing this query to run slow? As far as I can tell, the plan is perfect, only one row was scanned, and no time was spent acquiring the lock. So, what goes?</p> <p>Update: forgot to include server specs, this is on a 64G Quad Xeon X5650 2.66GHz (24 cores), Mysql 5.1.52-rel11.6-log Percona server 11.6, 12 disk PERC H700 RAID Array. This server has worked perfectly fine for a long time (uptime indicate 565 days up).</p> <p>Update 2: This table has only one index, which is a primary index consisting of the tuple (app_id, user_id, slice_id, mode_id). Additionally, this is a write-only master, three other slave servers handle all reads.</p>
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    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PTQuestion
    1. USBlanka
    1. USBlanka
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    1. This table or related slice is empty.
    1. This table or related slice is empty.
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      1. POMySQL slow query even though primary index is being used and good EXPLAIN plan
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
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      1. POMySQL slow query even though primary index is being used and good EXPLAIN plan
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      1. VTUpMod
    1. CO`EXPLAIN` does not work on UPDATE queries as far a I know so far
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      1. POMySQL slow query even though primary index is being used and good EXPLAIN plan
      1. USAdrian Cornish
    2. COpossibly, but doesn't the slow query log show that the update query only scanned one row, just as my explain plan on a modified select query shows?
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      1. POMySQL slow query even though primary index is being used and good EXPLAIN plan
      1. USBlanka
 

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