[20201224]sql优化困惑.txt

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[20201224]sql优化困惑.txt --//昨天优化生产系统一条语句，不小心折腾一个下午，回家的路上才想起如何优化，自己一下子没有转过来，浪费大量的时间。 1.环境： > @ ver1 PORT_STRING VERSION BANNER ------------------------------ -------------- -------------------------------------------------------------------------------- x86_64/Linux 2.4.xx 11.2.0.4.0 Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production > @ bind_cap 1n7yhk3p9cd66 '' C200 -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- select * from ( select huanzhexin0_.id as col_0_0_, huanzhexin0_.binglihao as col_1_0_, huanzhexin0_.xingming as col_2_0_, huanzhexin0_.xingbie as col_3_0_, huanzhexin0_.jtdz as col_4_0_, huanzhexin0_ .shengri as col_5_0_, jcd2_.id as col_6_0_, jiuzhen1_.id as col_7_0_, jcd2_.jcdh as col_8_0_, jcd2_.kd_time as col_9_0_, jcd2_.kdys as col_10_0_, (select yuangong3_.xingming from yuangong yuangong3_ w here yuangong3_.gonghao=jcd2_.kdys) as col_11_0_, (select bumen4_.bmmc from bumen bumen4_ where bumen4_.id=jcd2_.kdks_id) as col_12_0_, jcd2_.kdks_id as col_13_0_, jcd2_.biaoti as col_14_0_, (select y uangong5_.xingming from yuangong yuangong5_ where yuangong5_.gonghao=jcd2_.jcys) as col_15_0_, jcd2_.jcjs_time as col_16_0_ from huanzhexinxi huanzhexin0_, jiuzhen jiuzhen1_, jcd jcd2_ where huanzhexi n0_.id=jiuzhen1_.huanzhe_id and jiuzhen1_.id=jcd2_.jiuzhen_id and jcd2_.biaoshi=:"SYS_B_0" and (jcd2_.state is null) order by jcd2_.jcjs_time ) where rownum <= :1 SQL_ID CHILD_NUMBER WAS NAME POSITION MAX_LENGTH LAST_CAPTURED DATATYPE_STRING VALUE_STRING ------------- ------------ --- ---------------------------------------- ------------ ------------ ------------------- --------------- ------------ 1n7yhk3p9cd66 0 YES :SYS_B_0 1 22 2020-07-21 11:26:31 NUMBER 56 YES :1 2 22 2020-07-21 11:26:31 NUMBER 100 --//理论讲这样语句不会输出许多行，逻辑读也不会很高。猜测操作完成后修改state非NULL。 > @ d_buffer 1n7yhk3p9cd66 60 EXECUTIONS1 BUFFER_GETS1 ELAPSED_TIME1 ROWS_PROCESSED1 每次buffer_gets 每次执行时间平均处理记录数 --------------- --------------- --------------- --------------- --------------- --------------- --------------- 2098552 576708129 360817107935 32134 274.81240827008 171936.2245658 .01531246306977 ... sleep 60 , waiting .... EXECUTIONS2 BUFFER_GETS2 ELAPSED_TIME2 ROWS_PROCESSED2 每次buffer_gets 每次执行时间平均处理记录数 --------------- --------------- --------------- --------------- --------------- --------------- --------------- 2098562 576797449 360819016026 32134 274.85366122135 171936.31449821 .01531239010332 总buffer_gets 每次buffer_gets 执行次数总执行时间每次执行时间总处理记录数平均处理记录数 --------------- --------------- --------------- --------------- --------------- --------------- --------------- 89320 8932 10 1908091 190809.1 0 0 --//每分钟执行10次。又是一个经典的刷新语句。每次接近0.19秒。而且经常是输出记录为0. --//执行计划如下： Plan hash value: 1469246793 ---------------------------------------------------------------------------------------------------------------------------------------------------- | Id | Operation | Name | E-Rows |E-Bytes|E-Temp | Cost (%CPU)| E-Time | OMem | 1Mem | Used-Mem | ---------------------------------------------------------------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | | | | 4736 (100)| | | | | | 1 | TABLE ACCESS BY INDEX ROWID | YUANGONG | 1 | 13 | | 2 (0)| 00:00:01 | | | | |* 2 | INDEX RANGE SCAN | I_YUANGONG_GONGHAO | 1 | | | 1 (0)| 00:00:01 | 1025K| 1025K| | | 3 | TABLE ACCESS BY INDEX ROWID | BUMEN | 1 | 14 | | 1 (0)| 00:00:01 | | | | |* 4 | INDEX UNIQUE SCAN | PK_BUMEN | 1 | | | 0 (0)| | 1025K| 1025K| | | 5 | TABLE ACCESS BY INDEX ROWID | YUANGONG | 1 | 13 | | 2 (0)| 00:00:01 | | | | |* 6 | INDEX RANGE SCAN | I_YUANGONG_GONGHAO | 1 | | | 1 (0)| 00:00:01 | 1025K| 1025K| | |* 7 | COUNT STOPKEY | | | | | | | | | | | 8 | VIEW | | 46745 | 107M| | 4736 (1)| 00:00:57 | | | | |* 9 | SORT ORDER BY STOPKEY | | 46745 | 6893K| 7800K| 4736 (1)| 00:00:57 | 1024 | 1024 | | |* 10 | HASH JOIN | | 46745 | 6893K| 4848K| 3276 (1)| 00:00:40 | 8577K| 2802K| 9729K (0)| | 11 | TABLE ACCESS STORAGE FULL FIRST ROWS | HUANZHEXINXI | 75182 | 3964K| | 370 (1)| 00:00:05 | 1025K| 1025K| | |* 12 | HASH JOIN | | 46745 | 4427K| 3120K| 2430 (1)| 00:00:30 | 9672K| 4837K| 7815K (0)| | 13 | TABLE ACCESS STORAGE FULL FIRST ROWS| JIUZHEN | 145K| 1416K| | 372 (1)| 00:00:05 | 1025K| 1025K| | |* 14 | TABLE ACCESS STORAGE FULL FIRST ROWS| JCD | 46745 | 3971K| | 1687 (1)| 00:00:21 | 1025K| 1025K| | ---------------------------------------------------------------------------------------------------------------------------------------------------- 2.优化过程略： --//我加入的提示如下： /*+ gather_plan_statistics leading(jcd2_ jiuzhen1_ huanzhexin0_) index(jcd2_ I_JCD_BIAOSHI_STATE_JCJS_TIME) index(jiuzhen1_ PK_JIUZHEN) index(huanzhexin0_ PK_HUANZHEXINXI) use_nl( jiuzhen1_) use_nl( huanzhexin0_ ) cardinality(jcd2_ 200) */ --// I_JCD_BIAOSHI_STATE_JCJS_TIME 索引包含3个字段 BIAOSHI，STATE，JCJS_TIME。 --//如果我带入jcd2_.biaoshi=:"SYS_B_0"的值53，biaoshi=53,state is null很多，执行统计信息如下： ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | Id | Operation | Name | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | | | 4214 (100)| | 100 |00:00:00.44 | 164K| | | | | 1 | TABLE ACCESS BY INDEX ROWID | YUANGONG | 6370 | 1 | 13 | 2 (0)| 00:00:01 | 6354 |00:00:00.02 | 1927 | | | | |* 2 | INDEX RANGE SCAN | I_YUANGONG_GONGHAO | 6370 | 1 | | 1 (0)| 00:00:01 | 6354 |00:00:00.01 | 3 | 1025K| 1025K| | | 3 | TABLE ACCESS BY INDEX ROWID | BUMEN | 6 | 1 | 14 | 1 (0)| 00:00:01 | 6 |00:00:00.01 | 9 | | | | |* 4 | INDEX UNIQUE SCAN | PK_BUMEN | 6 | 1 | | 0 (0)| | 6 |00:00:00.01 | 3 | 1025K| 1025K| | | 5 | TABLE ACCESS BY INDEX ROWID | YUANGONG | 15 | 1 | 13 | 2 (0)| 00:00:01 | 14 |00:00:00.01 | 7 | | | | |* 6 | INDEX RANGE SCAN | I_YUANGONG_GONGHAO | 15 | 1 | | 1 (0)| 00:00:01 | 14 |00:00:00.01 | 3 | 1025K| 1025K| | |* 7 | COUNT STOPKEY | | 1 | | | | | 100 |00:00:00.44 | 164K| | | | | 8 | VIEW | | 1 | 200 | 472K| 4214 (1)| 00:00:51 | 100 |00:00:00.44 | 164K| | | | |* 9 | SORT ORDER BY STOPKEY | | 1 | 200 | 30400 | 4214 (1)| 00:00:51 | 100 |00:00:00.44 | 164K| 22528 | 22528 |20480 (0)| | 10 | NESTED LOOPS | | 1 | 200 | 30400 | 4213 (1)| 00:00:51 | 48454 |00:00:00.39 | 162K| | | | | 11 | NESTED LOOPS | | 1 | 200 | 30400 | 4213 (1)| 00:00:51 | 48454 |00:00:00.31 | 114K| | | | | 12 | NESTED LOOPS | | 1 | 200 | 19600 | 4013 (1)| 00:00:49 | 48454 |00:00:00.22 | 70013 | | | | | 13 | TABLE ACCESS BY INDEX ROWID| JCD | 1 | 200 | 17600 | 3813 (1)| 00:00:46 | 48454 |00:00:00.07 | 5407 | | | | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |* 14 | INDEX RANGE SCAN | I_JCD_BIAOSHI_STATE_JCJS_TIME | 1 | 9315 | | 45 (0)| 00:00:01 | 48454 |00:00:00.02 | 97 | 1025K| 1025K| | | 15 | TABLE ACCESS BY INDEX ROWID| JIUZHEN | 48454 | 1 | 10 | 1 (0)| 00:00:01 | 48454 |00:00:00.12 | 64606 | | | | |* 16 | INDEX UNIQUE SCAN | PK_JIUZHEN | 48454 | 1 | | 0 (0)| | 48454 |00:00:00.06 | 16152 | 1025K| 1025K| | |* 17 | INDEX UNIQUE SCAN | PK_HUANZHEXINXI | 48454 | 1 | | 0 (0)| | 48454 |00:00:00.07 | 44430 | 1025K| 1025K| | | 18 | TABLE ACCESS BY INDEX ROWID | HUANZHEXINXI | 48454 | 1 | 54 | 1 (0)| 00:00:01 | 48454 |00:00:00.06 | 48454 | | | | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- --//实际上扫描jcd获得100条记录就可以停止，而实际读取48454行，如果控制呢？感觉oracle的优化器那里出了问题。 3.分析： --//我在这里折腾很长时间，一直没有调整出来。后来我单独把表jcd拿出来执行，才知道问题在哪里。 alter session set statistics_level = all; select * from (select id from jcd where biaoshi=53 and state is null order by jcjs_time) where rownum<=10; Plan hash value: 1740280897 ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | Id | Operation | Name | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | | | 1188 (100)| | 10 |00:00:00.24 | 1070 | | | | |* 1 | COUNT STOPKEY | | 1 | | | | | 10 |00:00:00.24 | 1070 | | | | | 2 | VIEW | | 1 | 8191 | 103K| 1188 (1)| 00:00:15 | 10 |00:00:00.24 | 1070 | | | | |* 3 | SORT ORDER BY STOPKEY | | 1 | 8191 | 175K| 1188 (1)| 00:00:15 | 10 |00:00:00.24 | 1070 | 2048 | 2048 | 2048 (0)| |* 4 | VIEW | index$_join$_002 | 1 | 8191 | 175K| 1186 (1)| 00:00:15 | 48496 |00:00:00.22 | 1070 | | | | |* 5 | HASH JOIN | | 1 | | | | | 48644 |00:00:00.21 | 1070 | 4967K| 3974K| 4037K (0)| |* 6 | INDEX RANGE SCAN | I_JCD_BIAOSHI_STATE_JCJS_TIME | 1 | 8191 | 175K| 225 (0)| 00:00:03 | 48644 |00:00:00.01 | 98 | 1025K| 1025K| | | 7 | INDEX STORAGE FAST FULL SCAN| PK_JCD | 1 | 8191 | 175K| 1200 (1)| 00:00:15 | 331K|00:00:00.06 | 972 | 1025K| 1025K| | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- --//实际读取48644行。id=6.我希望就是id＝6，仅仅读取100行就ok了。怎么控制不住呢。浪费N多时间。 --//回家的路上才想起来，实际上问题出在order by的选择上。加入biaoshi,state就可以控制仅仅扫描很少的行。 select * from (select id from jcd where biaoshi=53 and state is null order by biaoshi,state,jcjs_time) where rownum<=10; --//补充一点，只要order by 中jcjs_time不在第一的位置，执行计划都是一样的。 Plan hash value: 2856453538 ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | Id | Operation | Name | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | 0 | SELECT STATEMENT | | 1 | | | 8 (100)| | 10 |00:00:00.01 | 12 | | | | |* 1 | COUNT STOPKEY | | 1 | | | | | 10 |00:00:00.01 | 12 | | | | | 2 | VIEW | | 1 | 10 | 130 | 8 (0)| 00:00:01 | 10 |00:00:00.01 | 12 | | | | | 3 | TABLE ACCESS BY INDEX ROWID| JCD | 1 | 10 | 220 | 8 (0)| 00:00:01 | 10 |00:00:00.01 | 12 | | | | |* 4 | INDEX RANGE SCAN | I_JCD_BIAOSHI_STATE_JCJS_TIME | 1 | | | 3 (0)| 00:00:01 | 41 |00:00:00.01 | 4 | 1025K| 1025K| | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ --//这样就可以很好的控制逻辑读，扫描行数。 --//这样上面的语句修改如下： SELECT * FROM ( SELECT /*+ gather_plan_statistics */ huanzhexin0_.id AS col_0_0_ ,huanzhexin0_.binglihao AS col_1_0_ ,huanzhexin0_.xingming AS col_2_0_ ,huanzhexin0_.xingbie AS col_3_0_ ,huanzhexin0_.jtdz AS col_4_0_ ,huanzhexin0_.shengri AS col_5_0_ ,jcd2_.id AS col_6_0_ ,jiuzhen1_.id AS col_7_0_ ,jcd2_.jcdh AS col_8_0_ ,jcd2_.kd_time AS col_9_0_ ,jcd2_.kdys AS col_10_0_ , (SELECT yuangong3_.xingming FROM yuangong yuangong3_ WHERE yuangong3_.gonghao = jcd2_.kdys) AS col_11_0_ , (SELECT bumen4_.bmmc FROM bumen bumen4_ WHERE bumen4_.id = jcd2_.kdks_id) AS col_12_0_ ,jcd2_.kdks_id AS col_13_0_ ,jcd2_.biaoti AS col_14_0_ , (SELECT yuangong5_.xingming FROM yuangong yuangong5_ WHERE yuangong5_.gonghao = jcd2_.jcys) AS col_15_0_ ,jcd2_.jcjs_time AS col_16_0_ FROM huanzhexinxi huanzhexin0_, jiuzhen jiuzhen1_, jcd jcd2_ WHERE huanzhexin0_.id = jiuzhen1_.huanzhe_id AND jiuzhen1_.id = jcd2_.jiuzhen_id AND jcd2_.biaoshi = :"SYS_B_0" AND (jcd2_.state IS NULL) ORDER BY jcd2_.biaoshi,jcd2_.state,jcd2_.jcjs_time) WHERE ROWNUM <= :1; --//执行计划如下: :"SYS_B_0" = 53. --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | Id | Operation | Name | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | | | 32 (100)| | 100 |00:00:00.01 | 353 | | | | | 1 | TABLE ACCESS BY INDEX ROWID | YUANGONG | 29 | 1 | 13 | 2 (0)| 00:00:01 | 17 |00:00:00.01 | 4 | | | | |* 2 | INDEX RANGE SCAN | I_YUANGONG_GONGHAO | 29 | 1 | | 1 (0)| 00:00:01 | 17 |00:00:00.01 | 3 | 1025K| 1025K| | | 3 | TABLE ACCESS BY INDEX ROWID | BUMEN | 3 | 1 | 14 | 1 (0)| 00:00:01 | 3 |00:00:00.01 | 5 | | | | |* 4 | INDEX UNIQUE SCAN | PK_BUMEN | 3 | 1 | | 0 (0)| | 3 |00:00:00.01 | 2 | 1025K| 1025K| | | 5 | TABLE ACCESS BY INDEX ROWID | YUANGONG | 15 | 1 | 13 | 2 (0)| 00:00:01 | 14 |00:00:00.01 | 7 | | | | |* 6 | INDEX RANGE SCAN | I_YUANGONG_GONGHAO | 15 | 1 | | 1 (0)| 00:00:01 | 14 |00:00:00.01 | 3 | 1025K| 1025K| | |* 7 | COUNT STOPKEY | | 1 | | | | | 100 |00:00:00.01 | 353 | | | | | 8 | VIEW | | 1 | 11 | 26587 | 32 (0)| 00:00:01 | 100 |00:00:00.01 | 353 | | | | | 9 | NESTED LOOPS | | 1 | 11 | 1672 | 32 (0)| 00:00:01 | 100 |00:00:00.01 | 337 | | | | | 10 | NESTED LOOPS | | 1 | 12 | 1672 | 32 (0)| 00:00:01 | 100 |00:00:00.01 | 237 | | | | | 11 | NESTED LOOPS | | 1 | 12 | 1176 | 20 (0)| 00:00:01 | 100 |00:00:00.01 | 196 | | | | | 12 | TABLE ACCESS BY INDEX ROWID| JCD | 1 | 9315 | 800K| 8 (0)| 00:00:01 | 100 |00:00:00.01 | 46 | | | | |* 13 | INDEX RANGE SCAN | I_JCD_BIAOSHI_STATE_JCJS_TIME | 1 | 12 | | 3 (0)| 00:00:01 | 100 |00:00:00.01 | 3 | 1025K| 1025K| | | 14 | TABLE ACCESS BY INDEX ROWID| JIUZHEN | 100 | 1 | 10 | 1 (0)| 00:00:01 | 100 |00:00:00.01 | 150 | | | | |* 15 | INDEX UNIQUE SCAN | PK_JIUZHEN | 100 | 1 | | 0 (0)| | 100 |00:00:00.01 | 50 | 1025K| 1025K| | |* 16 | INDEX UNIQUE SCAN | PK_HUANZHEXINXI | 100 | 1 | | 0 (0)| | 100 |00:00:00.01 | 41 | 1025K| 1025K| | | 17 | TABLE ACCESS BY INDEX ROWID | HUANZHEXINXI | 100 | 1 | 54 | 1 (0)| 00:00:01 | 100 |00:00:00.01 | 100 | | | | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- --//执行计划如下: :"SYS_B_0" = 56.这个逻辑读更少。 --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | Id | Operation | Name | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | | | 32 (100)| | 0 |00:00:00.01 | 3 | | | | | 1 | TABLE ACCESS BY INDEX ROWID | YUANGONG | 0 | 1 | 13 | 2 (0)| 00:00:01 | 0 |00:00:00.01 | 0 | | | | |* 2 | INDEX RANGE SCAN | I_YUANGONG_GONGHAO | 0 | 1 | | 1 (0)| 00:00:01 | 0 |00:00:00.01 | 0 | 1025K| 1025K| | | 3 | TABLE ACCESS BY INDEX ROWID | BUMEN | 0 | 1 | 14 | 1 (0)| 00:00:01 | 0 |00:00:00.01 | 0 | | | | |* 4 | INDEX UNIQUE SCAN | PK_BUMEN | 0 | 1 | | 0 (0)| | 0 |00:00:00.01 | 0 | 1025K| 1025K| | | 5 | TABLE ACCESS BY INDEX ROWID | YUANGONG | 0 | 1 | 13 | 2 (0)| 00:00:01 | 0 |00:00:00.01 | 0 | | | | |* 6 | INDEX RANGE SCAN | I_YUANGONG_GONGHAO | 0 | 1 | | 1 (0)| 00:00:01 | 0 |00:00:00.01 | 0 | 1025K| 1025K| | |* 7 | COUNT STOPKEY | | 1 | | | | | 0 |00:00:00.01 | 3 | | | | | 8 | VIEW | | 1 | 11 | 26587 | 32 (0)| 00:00:01 | 0 |00:00:00.01 | 3 | | | | | 9 | NESTED LOOPS | | 1 | 11 | 1672 | 32 (0)| 00:00:01 | 0 |00:00:00.01 | 3 | | | | | 10 | NESTED LOOPS | | 1 | 12 | 1672 | 32 (0)| 00:00:01 | 0 |00:00:00.01 | 3 | | | | | 11 | NESTED LOOPS | | 1 | 12 | 1176 | 20 (0)| 00:00:01 | 0 |00:00:00.01 | 3 | | | | | 12 | TABLE ACCESS BY INDEX ROWID| JCD | 1 | 9315 | 800K| 8 (0)| 00:00:01 | 0 |00:00:00.01 | 3 | | | | |* 13 | INDEX RANGE SCAN | I_JCD_BIAOSHI_STATE_JCJS_TIME | 1 | 12 | | 3 (0)| 00:00:01 | 0 |00:00:00.01 | 3 | 1025K| 1025K| | | 14 | TABLE ACCESS BY INDEX ROWID| JIUZHEN | 0 | 1 | 10 | 1 (0)| 00:00:01 | 0 |00:00:00.01 | 0 | | | | |* 15 | INDEX UNIQUE SCAN | PK_JIUZHEN | 0 | 1 | | 0 (0)| | 0 |00:00:00.01 | 0 | 1025K| 1025K| | |* 16 | INDEX UNIQUE SCAN | PK_HUANZHEXINXI | 0 | 1 | | 0 (0)| | 0 |00:00:00.01 | 0 | 1025K| 1025K| | | 17 | TABLE ACCESS BY INDEX ROWID | HUANZHEXINXI | 0 | 1 | 54 | 1 (0)| 00:00:01 | 0 |00:00:00.01 | 0 | | | | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 3.总结： --//实际上还是细节很重要,如果当时我加入提示实际上优化已经完成，这个语句带入就是56，根本不会变，记录很少，逻辑读不会很高 --//，但是仔细再思考一步，就可以发现开发没有写好sql语句。 --//另外再次说明交流很重要，根本没必要在上面浪费这么多时间，估计问一下周围的人许多很快得到结果。可惜在我周围连一个问问题 --//的人都没有..........

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