ORACLE零碎笔记

1. 行级锁

    Session1: select * from table for update;这样的话是把表锁了，如果Session2仍然用“select * from table for update ”查询就会被堵塞。

    现在的需求是把锁定的行屏蔽掉，让另外的SESSION查的时候不会被堵塞，只是屏蔽了锁定的行。例子如下：

session1: 

 >   select * from table;

  >   field1  field2  

  >      1    asdfa     

  >      2    hgrew

  >      3    poptoe

  3 rows selected

  > select * from table where field1=1 for update;

session2:

  >  select * from table for update skip locked;

  >  field1  field2

  >   2      hgrew

  >   3      poptoe

  2 rows selected

这样就查询到只有2条了。SQLSERVER好像也有类似的：select   *   from table with(UPDLOCK,READPAST)   where   id   =  1

注:不过这样的方法还是比较危险的，通常是用字段做标记，UPDATE字段表示占用或者空闲

2. SQL优化

原SQL，用时69.375S:

select count(*)  cnt from sfism4.r_wip_tracking_t
where  pallet_no='WC7182021T673856OQ' 
or pallet_no in (select Pallet_no from sfism4.r_wip_tracking_t
  where serial_number='WC7182021T673856OQ' and Pallet_No is not null and Pallet_no<>'N/A')

优化后SQL，用时0.047S：

select count(w.serial_number)  from sfism4.r_wip_tracking_t w
where  exists(select s.serial_number from sfism4.r_wip_tracking_t s
  where (s.serial_number='WC7182021T673856OQ' or  s.pallet_no='WC7182021T673856OQ') and s.Pallet_No is not null and s.Pallet_no<>'N/A' and w.pallet_no = s.pallet_no)

说明一下，表sfism4.r_wip_tracking_t中，serial_number有唯一索引，pallet_no也有索引，全表2495691条记录

分析：

原SQL的执行计划：

SELECT STATEMENT, GOAL = CHOOSE   Cost=7536 Cardinality=1 Bytes=4
 SORT AGGREGATE    Cardinality=1 Bytes=4
  FILTER     
   TABLE ACCESS FULL Object owner=SFISM4 Object name=R_WIP_TRACKING_T Cost=7536 Cardinality=106288 Bytes=425152
   FILTER     
    TABLE ACCESS BY INDEX ROWID Object owner=SFISM4 Object name=R_WIP_TRACKING_T Cost=3 Cardinality=1 Bytes=19
     INDEX UNIQUE SCAN Object owner=SFISM4 Object name=WIP_SN Cost=2 Cardinality=2124150 

优化后的执行计划：

SELECT STATEMENT, GOAL = CHOOSE   Cost=74 Cardinality=1 Bytes=23
 SORT AGGREGATE    Cardinality=1 Bytes=23
  NESTED LOOPS   Cost=74 Cardinality=478 Bytes=10994
   SORT UNIQUE     
    TABLE ACCESS BY INDEX ROWID Object owner=SFISM4 Object name=R_WIP_TRACKING_T Cost=28 Cardinality=22 Bytes=418
     BITMAP CONVERSION TO ROWIDS     
      BITMAP OR     
       BITMAP CONVERSION FROM ROWIDS     
        INDEX RANGE SCAN Object owner=SFISM4 Object name=WIP_SN Cost=2  
       BITMAP CONVERSION FROM ROWIDS     
        INDEX RANGE SCAN Object owner=SFISM4 Object name=R107_PALLET_NO Cost=3  
   INDEX RANGE SCAN Object owner=SFISM4 Object name=R107_PALLET_NO Cost=2 Cardinality=22 Bytes=88
  猜测是OR的原因导致没有走索引

 3.左联接丢失数据

select * from table1 left join table2   on table1.name=table2.name   and table2.id=1001
----------------------------------
select * from table1 left join table2   on table1.name=table2.name where   table2.id=1001

这里,第二个SQL会丢失部分数据

性能上 老婆大人推荐:可以写成select * from table1 left join (select * from table2 where table2.xxx=1001) on table1.yyy=table2.zzz

这样就可以走table2.xxx的索引了,而且table1的所有记录都能显示出来

4.查看锁对象

  select * from v$locked_object;根据OBJECT_ID查看哪个OBJECT被锁定，select * from dba_objects where object_id = 49549;

  根据SESSION_ID 查看哪个SESSION锁的，select * from V$session;

最后灭了SESSION,ALTER SYSTEM KILL SESSION '11,822';  (SID,SERIAL#)