1、"表姐"的报表SQL问题
今天"表姐"有个比较紧急的工作，有一张历史费用报表下班前必须查询出来，已经倒腾2个小时了，查询并发数已经加到40个，报表还是查询不出来，而且这个报表每月都会折磨表姐，眼看就要下班了，时间紧近，紧急求助。经常被单位各位"表姐"骚扰，而且还特别紧急。。。。DBA的苦，好吧，放下手头工作，先帮"表姐"处理问题。。。。2、问题分析
DBA：你的SQL发我看看？表姐：select /*+ parallel(T 40)*/* FROM ht.ht_tab_fy TWHERE EXISTS (SELECT 1 FROM ht2.ht_DEPT M WHERE T.DEPT_ID = M.DEPT_ID AND M.DEPT_TREE LIKE '%611002%')AND T.DATA_MONTH = '201709' ;DBA：这个查询结果发我看看？explain plan forselect * FROM ht.ht_tab_fy TWHERE EXISTS (SELECT 1 FROM ht2.ht_DEPT M WHERE T.DEPT_ID = M.DEPT_ID AND M.DEPT_TREE LIKE '%611002%')AND T.DATA_MONTH = '201709' ;select * from table(dbms_xplan.display());表姐：-------------------------------------------------------------------------------| Id | Operation | Name | Rows | Bytes | Cost (%CPU)|-------------------------------------------------------------------------------| 0 | SELECT STATEMENT | | 25466 | 3531K| 80186 (2)|| 1 | HASH JOIN RIGHT SEMI| | 25466 | 3531K| 80186 (2)|| 2 | TABLE ACCESS FULL | ht_DEPT | 58 | 2494 | 8 (0)|| 3 | TABLE ACCESS FULL | ht_tab_fy | 118K| 11M| 80176 (2)|-------------------------------------------------------------------------------DBA：这两个查询结果给我select version from v$instance;select name,value from v$parameter where name like '%multiblock%';SQL> select name,value from v$parameter where name like '%multiblock%';SQL> select version from v$instance;VERSION-----------------10.2.0.5.0NAME VALUE-------------------------------------------------- --------------------------------------------------db_file_multiblock_read_count 163、优化方案
DBA：从新收集下统计信息：exec dbms_stats.gather_table_stats(ownname=>'ht2',tabname=>'ht_DEPT',estimate_percent=>100,method_opt=>'for all columns size repeat',no_invalidate=>false,degree=>8,cascade=>true);exec dbms_stats.gather_table_stats(ownname=>'ht',tabname=>'ht_tab_fy',estimate_percent=>100,method_opt=>'for all columns size repeat',no_invalidate=>false,degree=>8,cascade=>true);然后再执行你的语句：alter session set db_file_multiblock_read_count=128; --前面加一条select * FROM ht.ht_tab_fy TWHERE EXISTS (SELECT 1 FROM ht2.ht_DEPT M WHERE T.DEPT_ID = M.DEPT_ID AND M.DEPT_TREE LIKE '%611002%')AND T.DATA_MONTH = '201709' ;4、优化结果
表姐：结果秒出，太感谢了，眼泪哗哗的！！！！5、现在问题来了？
表姐的语句为什么得到了优化？原理是什么？