select??count(id)?from?cin_offlinemessage?where?`datetime`?>=?'2012-03-15'?and?state=0;? 結果是全表掃描。 mysql>?explain??select??count(id)?from?cin_offlinemessage?where?`datetime`?>=?'2012-03-15'?and?state=0;??? +----+-------------+--------------------+------+------------------------------+------+---------+------+----------+-------------+ |?id?|?select_type?|?table??????????????|?type?|?possible_keys????????????????|?key??|?key_len?|?ref??|?rows?????|?Extra???????| +----+-------------+--------------------+------+------------------------------+------+---------+------+----------+-------------+ |??1?|?SIMPLE??????|?cin_offlinemessage?|?ALL??|?cin_offflinemessage_datetime?|?NULL?|?NULL????|?NULL?|?45046637?|?Using?where?| +----+-------------+--------------------+------+------------------------------+------+---------+------+----------+-------------+ 1?row?in?set?(0.00?sec) 雖然有索引，但沒有用上，掃描的行數太多了（8位數），優化器認為全表掃描比索引來的快。 后我修改了下 mysql>?explain??select??count(id)?from?cin_offlinemessage?where?`datetime`?between?'2012-03-15?00:00:00'?and?'2012-03-16?23:59:59'??and?state=0; +----+-------------+--------------------+-------+------------------------------+------------------------------+---------+------+---------+-------------+ |?id?|?select_type?|?table??????????????|?type??|?possible_keys????????????????|?key??????????????????????????|?key_len?|?ref??|?rows????|?Extra???????| +----+-------------+--------------------+-------+------------------------------+------------------------------+---------+------+---------+-------------+ |??1?|?SIMPLE??????|?cin_offlinemessage?|?range?|?cin_offflinemessage_datetime?|?cin_offflinemessage_datetime?|?9???????|?NULL?|?3534912?|?Using?where?| +----+-------------+--------------------+-------+------------------------------+------------------------------+---------+------+---------+-------------+ 1?row?in?set?(0.00?sec) 這樣就用上了索引。（掃描了7位數）
本文轉自 liang3391 51CTO博客，原文鏈接:http://blog.51cto.com/liang3391/816803
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