查询v$lock缓慢和direct path write temp等待

v$lock是常用的enqueue lock队列锁动态性能视图，不管是用户自己部署的监控脚本也好、还是enterprise manager都多少会使用到该V$LOCK视图， 但是在10g中遇到了v$lock查询缓慢的问题， 例如下面的查询会等待较多direct path write temp等待事件：    

select count(*) from v$lock;

  COUNT(*)
----------
       163

Elapsed: 00:00:60.90

Execution Plan
----------------------------------------------------------
Plan hash value: 2384831130

--------------------------------------------------------------------------------------
| Id  | Operation               | Name       | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT        |            |     1 |    50 |     1 (100)| 00:00:01 |
|   1 |  SORT AGGREGATE         |            |     1 |    50 |            |          |
|*  2 |   HASH JOIN             |            |     1 |    50 |     1 (100)| 00:00:01 |
|   3 |    MERGE JOIN CARTESIAN |            |   100 |  3800 |     0   (0)| 00:00:01 |
|*  4 |     FIXED TABLE FULL    | X$KSUSE    |     1 |    19 |     0   (0)| 00:00:01 |
|   5 |     BUFFER SORT         |            |   100 |  1900 |     0   (0)| 00:00:01 |
|   6 |      FIXED TABLE FULL   | X$KSQRS    |   100 |  1900 |     0   (0)| 00:00:01 |
|   7 |    VIEW                 | GV$_LOCK   |    10 |   120 |     0   (0)| 00:00:01 |
|   8 |     UNION-ALL           |            |       |       |            |          |
|*  9 |      FILTER             |            |       |       |            |          |
|  10 |       VIEW              | GV$_LOCK1  |     2 |    24 |     0   (0)| 00:00:01 |
|  11 |        UNION-ALL        |            |       |       |            |          |
|* 12 |         FIXED TABLE FULL| X$KDNSSF   |     1 |    64 |     0   (0)| 00:00:01 |
|* 13 |         FIXED TABLE FULL| X$KSQEQ    |     1 |    64 |     0   (0)| 00:00:01 |
|* 14 |      FIXED TABLE FULL   | X$KTADM    |     1 |    64 |     0   (0)| 00:00:01 |
|* 15 |      FIXED TABLE FULL   | X$KTATRFIL |     1 |    64 |     0   (0)| 00:00:01 |
|* 16 |      FIXED TABLE FULL   | X$KTATRFSL |     1 |    64 |     0   (0)| 00:00:01 |
|* 17 |      FIXED TABLE FULL   | X$KTATL    |     1 |    64 |     0   (0)| 00:00:01 |
|* 18 |      FIXED TABLE FULL   | X$KTSTUSC  |     1 |    64 |     0   (0)| 00:00:01 |
|* 19 |      FIXED TABLE FULL   | X$KTSTUSS  |     1 |    64 |     0   (0)| 00:00:01 |
|* 20 |      FIXED TABLE FULL   | X$KTSTUSG  |     1 |    64 |     0   (0)| 00:00:01 |
|* 21 |      FIXED TABLE FULL   | X$KTCXB    |     1 |    64 |     0   (0)| 00:00:01 |
--------------------------------------------------------------------------------------

direct path write temp
direct path write temp
direct path write temp
................

    显然仅返回100多条记录的v$LOCK视图的查询不该这么慢，也不该由SORT或HASH造成大量的临时空间使用， 究其根本还是FIXED TABLE即X$的内部表上的统计信息不准确导致的执行计划使用,通过使用RULE HINT可以马上获得较好的性能：    

select /*+ RULE */ count(*) from v$LOCK;

  COUNT(*)
----------
       190

Elapsed: 00:00:00.18

Execution Plan
----------------------------------------------------------
Plan hash value: 2026431807

-------------------------------------------------
| Id  | Operation                  | Name       |
-------------------------------------------------
|   0 | SELECT STATEMENT           |            |
|   1 |  SORT AGGREGATE            |            |
|   2 |   MERGE JOIN               |            |
|   3 |    SORT JOIN               |            |
|   4 |     MERGE JOIN             |            |
|   5 |      SORT JOIN             |            |
|   6 |       FIXED TABLE FULL     | X$KSQRS    |
|*  7 |      SORT JOIN             |            |
|   8 |       VIEW                 | GV$_LOCK   |
|   9 |        UNION-ALL           |            |
|* 10 |         FILTER             |            |
|  11 |          VIEW              | GV$_LOCK1  |
|  12 |           UNION-ALL        |            |
|* 13 |            FIXED TABLE FULL| X$KDNSSF   |
|* 14 |            FIXED TABLE FULL| X$KSQEQ    |
|* 15 |         FIXED TABLE FULL   | X$KTADM    |
|* 16 |         FIXED TABLE FULL   | X$KTATRFIL |
|* 17 |         FIXED TABLE FULL   | X$KTATRFSL |
|* 18 |         FIXED TABLE FULL   | X$KTATL    |
|* 19 |         FIXED TABLE FULL   | X$KTSTUSC  |
|* 20 |         FIXED TABLE FULL   | X$KTSTUSS  |
|* 21 |         FIXED TABLE FULL   | X$KTSTUSG  |
|* 22 |         FIXED TABLE FULL   | X$KTCXB    |
|* 23 |    SORT JOIN               |            |
|* 24 |     FIXED TABLE FULL       | X$KSUSE    |
-------------------------------------------------

    针对上述问题考虑为FIXED TABLE收集统计信息，可以使用DBMS_STATS.GATHER_FIXED_OBJECTS_STATS标准存储过程，特别是对于版本升级上来的数据库，特别需要考虑执行该存储过程更新FIXED TABLE STATISTICS:    

SQL> set timing on;
SQL> exec DBMS_STATS.GATHER_FIXED_OBJECTS_STATS;

PL/SQL procedure successfully completed.

Elapsed: 00:01:24.87

      Create fixed table statistics Directly after catupgrd.sql has been completed This will speed up processing for recompilation with utlrp.sql Create fixed table statistics again after a week with regular production workload This task should be done only a few times per year