IN和INNER JOIN在我的情況下有什麼區別？

Question

在我的MySQL數據庫中，表事件具有一個組合索引，其中包含列closingeventid，timeStart和eventCode。表事件中有超過2100萬行。

這是兩個SQL，如果我在MySQL命令行中運行第一個SQL，進程mysqld-nt.exe的Mem Usage以每秒10 M的速度逐漸增加，它會上升到1.6 G左右，然後直線下降到30 M左右然後命令行返回Out of Memory異常。

但是，如果我使用第二個SQL，使用INNER JOIN而不是IN，那就沒關係。看來IN子句不會觸及任何索引。

爲什麼？任何想法來監視MySQL執行過程像MS SQL的執行計劃？

SELECT COUNT(*) AS 'cnt' 
FROM events 
WHERE 
    (timeStart < '2010-09-22 14:29:10’) 
AND (closingeventid IS NULL) 
AND (eventcode IN (SELECT DISTINCT evtcclosed FROM eventclose))` 

VS

SELECT COUNT(1) AS 'cnt' 
FROM events 
inner join 
      (SELECT DISTINCT evtcclosed FROM eventclose) ec 
    ON ec.evtcclosed = events.eventcode 
WHERE (timeStart < '2010-09-22 14:29:10’) AND (closingeventid IS NULL) 

Answer 1

MySQL優化可優化IN (subquery)條款非常糟糕。它爲外部查詢的每一行執行嵌套查詢。

要得到執行計劃 - 只是你的查詢之前加EXPLAIN關鍵字權

EXPLAIN SELECT ... 

Answer 2

感謝zerkms！

我試着EXPLAIN在兩個SQL上，但似乎他們都擊中了正確的索引idxEventClosed（closingeventid，timeStart，eventCode）。解釋看起來與下面的粗線相同。我仍然不清楚導致內存不足問題的原因。

mysql> EXPLAIN SELECT COUNT(*) AS 'cnt' FROM events WHERE (timeStart < '2010-10-29 14:29:10') AND (closingeventid IS NULL) AND (eventcode IN (SELECT D 
ISTINCT evtcclosed FROM eventclose)); 
+----+--------------------+------------+-------+-------------------------+----------------+---------+------+---------+------------------------------+ 
| id | select_type  | table  | type | possible_keys   | key   | key_len | ref | rows | Extra      | 
+----+--------------------+------------+-------+-------------------------+----------------+---------+------+---------+------------------------------+ 
| 1 | PRIMARY   | events  | range | idxStart,idxClosedEvent | idxClosedEvent | 17  | NULL | 4335955 | Using where; Using index  | 

| 2 | DEPENDENT SUBQUERY | eventclose | ALL | NULL     | NULL   | NULL | NULL |  10 | Using where; Using temporary | 

+----+--------------------+------------+-------+-------------------------+----------------+---------+------+---------+------------------------------+ 
2 rows in set (0.00 sec) 



mysql> EXPLAIN SELECT COUNT(1) AS 'cnt' FROM events inner join (SELECT DISTINCT evtcclosed FROM eventclose) ec on ec.evtcclosed = events.eventcode whe 
re (events.closingeventid is null) and (timeStart < '2010-10-28 14:29:10'); 

+----+-------------+------------+-------+---------------------------------+----------------+---------+------+---------+--------------------------+ 
| id | select_type | table  | type | possible_keys     | key   | key_len | ref | rows | Extra     | 
+----+-------------+------------+-------+---------------------------------+----------------+---------+------+---------+--------------------------+ 
| 1 | PRIMARY  | events  | range | evtNLoc,idxStart,idxClosedEvent | idxClosedEvent | 17  | NULL | 4330270 | Using where; Using index | 
**| 1 | PRIMARY  | <derived2> | ALL | NULL       | NULL   | NULL | NULL |  8 | Using where    |** 
| 2 | DERIVED  | eventclose | ALL | NULL       | NULL   | NULL | NULL |  10 | Using temporary   | 
+----+-------------+------------+-------+---------------------------------+----------------+---------+------+---------+--------------------------+ 
3 rows in set (0.00 sec)