mysql not exists优化_MySQL优化--NOT EXISTS和LEFT JOIN方式差异
在MySQL中,我们可以将NOT EXISTS语句转换为LEFT JOIN语句来进行优化,哪为什么会有性能提升呢?
使用NOT EXISTS方式SQL为:
SELECT count(1)FROMt_monitor mWHERE NOT exists(SELECT 1
FROM t_alarm_realtime ASaWHERE a.resource_id=m.resource_idAND a.resource_type=m.resource_typeAND a.monitor_name=m.monitor_name)
而使用LEFT JOIN方式SQL为:
SELECT count(1)FROMt_monitor mLEFT JOIN t_alarm_realtime ASaON a.resource_id=m.resource_idAND a.resource_type=m.resource_typeAND a.monitor_name=m.monitor_nameWHERE a.resource_id is NULL
从查询效果来看,NOT EXISTS 方式耗时29.38秒,而LEFT JOIN方式耗时1.20秒,性能提升25倍左右。
查看NOT EXISTS方式的执行计划:
*************************** 1. row ***************************id:1select_type:PRIMARY
table: m
partitions:NULLtype:indexpossible_keys:NULL
key: idx_id_name_type
key_len:119ref:NULLrows:578436filtered:100.00Extra: Usingwhere; Using index
*************************** 2. row ***************************id:2select_type: DEPENDENT SUBQUERYtable: a
partitions:NULLtype: eq_ref
possible_keys: idx_id_name_typekey: idx_id_name_type
key_len:119ref: cmdb.m.resource_id,cmdb.m.monitor_name,cmdb.m.resource_type
rows:1filtered:100.00Extra: Usingindex
查看LEFT JOIN方式的执行计划:
*************************** 1. row ***************************id:1select_type: SIMPLEtable: m
partitions:NULLtype:indexpossible_keys:NULL
key: idx_id_name_type
key_len:119ref:NULLrows:578436filtered:100.00Extra: Usingindex
*************************** 2. row ***************************id:1select_type: SIMPLEtable: a
partitions:NULLtype: eq_ref
possible_keys: idx_id_name_typekey: idx_id_name_type
key_len:119ref: cmdb.m.resource_id,cmdb.m.monitor_name,cmdb.m.resource_type
rows:1filtered:100.00Extra: Usingwhere; Not exists; Using index
使用SQL PROFILE查看NOT EXISTS 执行过程:
+--------------------+----------+----------+------------+-------------------+---------------------+--------------+---------------+
| Status | Duration | CPU_user | CPU_system | Context_voluntary | Context_involuntary | Block_ops_in | Block_ops_out |
+--------------------+----------+----------+------------+-------------------+---------------------+--------------+---------------+
| executing | 0.000023 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| Sending data | 0.000026 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| executing | 0.000023 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| Sending data | 0.000026 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| executing | 0.000022 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| Sending data | 0.000026 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| executing | 0.000023 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| Sending data | 0.000026 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| executing | 0.000023 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| Sending data | 0.000026 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| executing | 0.000023 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| Sending data | 0.000028 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| executing | 0.000022 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| Sending data | 0.000026 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| executing | 0.000023 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| Sending data | 0.000026 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| executing | 0.000023 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| Sending data | 0.000026 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| executing | 0.000023 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| Sending data | 0.000026 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| executing | 0.000023 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| Sending data | 0.000026 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| executing | 0.000023 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| Sending data | 0.000029 | 0.001000 | 0.000000 | 0 | 0 | 0 | 0 |
| executing | 0.000023 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| Sending data | 0.000026 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| executing | 0.000022 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| Sending data | 0.000026 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| executing | 0.000023 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| Sending data | 0.000027 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| executing | 0.000023 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| Sending data | 0.000026 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| executing | 0.000023 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| Sending data | 0.000026 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| executing | 0.000023 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| Sending data | 0.000026 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| executing | 0.000023 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| Sending data | 0.000026 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| executing | 0.000023 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| Sending data | 0.000026 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| executing | 0.000023 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| Sending data | 0.000026 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| executing | 0.000023 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| Sending data | 0.000026 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| executing | 0.000023 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| Sending data | 0.000026 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| executing | 0.000023 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| Sending data | 0.000028 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| executing | 0.000023 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| Sending data | 0.000026 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| executing | 0.000023 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| Sending data | 0.000026 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| executing | 0.000023 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| Sending data | 0.000026 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| executing | 0.000023 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| Sending data | 0.000026 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| executing | 0.000023 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| Sending data | 0.000026 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| executing | 0.000023 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| Sending data | 0.000026 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| executing | 0.000023 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| Sending data | 0.000026 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| executing | 0.000023 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| Sending data | 0.000026 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| executing | 0.000025 | 0.000000 | 0.000999 | 0 | 0 | 0 | 0 |
| Sending data | 0.000031 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| executing | 0.000023 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| Sending data | 0.000028 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| executing | 0.000023 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| Sending data | 0.000027 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| executing | 0.000023 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| Sending data | 0.000027 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| executing | 0.000023 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| Sending data | 0.000027 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| executing | 0.000023 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| Sending data | 0.000027 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| executing | 0.000023 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| Sending data | 0.000027 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| executing | 0.000023 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| Sending data | 0.000027 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| executing | 0.000023 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| Sending data | 0.000027 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| executing | 0.000023 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| Sending data | 0.000028 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| executing | 0.000023 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| Sending data | 0.000027 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| executing | 0.000023 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| Sending data | 0.000027 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| executing | 0.000022 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| Sending data | 0.000027 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| executing | 0.000023 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| Sending data | 0.000027 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| executing | 0.000023 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| Sending data | 0.000033 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| end | 0.000024 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| query end | 0.000028 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| closing tables | 0.000027 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| freeing items | 0.000039 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| logging slow query | 0.000059 | 0.000000 | 0.000000 | 0 | 0 | 0 | 16 |
| cleaning up | 0.000033 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
+--------------------+----------+----------+------------+-------------------+---------------------+--------------+---------------+
使用SQL PROFILE查看LEFT JOIN 执行过程:
+----------------------+----------+----------+------------+-------------------+---------------------+--------------+---------------+
| Status | Duration | CPU_user | CPU_system | Context_voluntary | Context_involuntary | Block_ops_in | Block_ops_out |
+----------------------+----------+----------+------------+-------------------+---------------------+--------------+---------------+
| starting | 0.000162 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| checking permissions | 0.000025 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| checking permissions | 0.000025 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| Opening tables | 0.000033 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| init | 0.000049 | 0.001000 | 0.000000 | 0 | 0 | 0 | 0 |
| System lock | 0.000030 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| optimizing | 0.000033 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| statistics | 0.000050 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| preparing | 0.000037 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| executing | 0.000025 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| Sending data | 1.200899 | 1.547764 | 0.124981 | 7460 | 116 | 0 | 8608 |
| end | 0.000103 | 0.000000 | 0.000000 | 2 | 0 | 0 | 0 |
| query end | 0.000028 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| closing tables | 0.000028 | 0.000000 | 0.000000 | 0 | 0 | 0 | 0 |
| freeing items | 0.000039 | 0.000000 | 0.000000 | 2 | 0 | 0 | 8 |
| logging slow query | 0.000052 | 0.000000 | 0.000000 | 0 | 0 | 0 | 24 |
| cleaning up | 0.000030 | 0.000000 | 0.000000 | 1 | 0 | 0 | 0 |
+----------------------+----------+----------+------------+-------------------+---------------------+--------------+---------------+
两种执行方式对比:
1、从执行计划来看,两个表都使用了索引,区别在于NOT EXISTS使用“DEPENDENT SUBQUERY”方式,而LEFT JOIN使用普通表关联的方式
2、从执行过程来看,LEFT JOIN方式主要消耗Sending data的上,在NOT EXISTS方式主要消耗在"executing"和“Sending data”两项上,受限于PROFILE只能记录100行结果,因此超过57万个"executing"和“Sending data”的组合项没有显示,虽然每次"executing"和“Sending data”的组合项消耗时间较少(约50毫秒),但由于执行次数较高,导致最终执行时间较长(50μs*578436=28921800us=28.92s)
如何在NOT EXISTS和LEFT JOIN中选择:
1、当外层数据较少时,子查询循环次数较少,使用NOT EXISTS并不会导致严重的性能问题,推荐使用NOT EXISTS方式。
2、当外层数据较大时,子查询消耗随外层数据量递增,查询性能较差,推荐使用LEFT JOIN方式
总结:
按照存在即合理是客观唯心主义的理论,NOT EXISTS以更直观地方式实现业务需求,在SQL复杂度上要远低于LEFT JOIN,且在生产执行计划时,NOT EXISTS方式相对更稳定些,LEFT JOIN可能会随统计信息变化而生产不同的执行计划。
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