首页 » ORACLE » Index join SQL性能故障 之 通过恢复统计信息还原SQL问题

Index join SQL性能故障 之 通过恢复统计信息还原SQL问题

上周正在开会讨论一套库的优化方案,突然有又收到了另一个性能告警,有套库的CPU突然暴涨负载非常高,随后其它业务因为CPU慢,业务逐渐积压,最终导致数据库主机命令都无响应, 最后定位了原因是与该SQL的一个对象的DDL drop partition, 因该SQL hard parse, 并且分区数量减少, 又因分区级统计信息不准确, CBO改变了执行计划使用了一个相对差的执行计划, 针对这个案例我做了大量的测试,这里对敏感信息做了处理简单的记录一下。

如果遇到这个案例应该搞清楚几个问题:
1,什么原因使执行计划改变
2,如何最快的修正执行计划
3,如何避免
4,如何在测试环境还原问题现象

期间做了以下尝试
1, 尝试kill 该sql的部分session,但session增度超过kill的速度
2,查看了SQL的执行计划,从SQL text简单看那个执行计划是也算合理
3,AWR中以前未采集到该SQL的执行计划,SQL CACHE和手动EXPLIAN plan 分析也是现在运行的执行计划(index join)
4, 尝试对表执行了一个grant使SQL重解析,问题依旧
5,尝试用SQL TEXT去其它同类库执行发现执行计划确实不一样(没用index join),当然后来我们从ASH中查到该sql确实是刚刚改变的执行计划
6, 尝试用SQL profile手动指定执行计划,我以前用该手段屡试不爽但是这次还是有点意外,因为执行计划中使用index join, oracle给我们提供了index_join hint但是没有提供no_index_join hint, sql中没有使用Table alias name,所以我使用的Table name让CBO不去使用其中一个索引,用了no_index(tablename indexname), 当然事后也在twitter及OTN上向Jonathan Lewis大师请教,得到JL一些宝贵意见,因为当时没有太多时间,不成功就尝试其它方法。后面我会记录多个成功的方法。
7, 收集表的统计信息,因为这是一个分区表,表级已超过了2亿记录,也不能删统计信息,因为此时动态采样对于大表更动弹不得, 所以先对表以分区为单位收集, 因为当前系统极慢,短时间不可能收集完,丢一个空口去收。
8, 尝试手动set几个分区索引的统计信息,因为没有参考但还执行计划还是未改变

就在尝试第8步过程中,没有再给机会实例重启了。往往考验都是这样,当你接手时系统已病入膏肓,一大群人会围在你身边不停的问怎么样了,顶着压力还要淡定条理的处理问题,再没太多的机会尝试,几分钟可能实例就宕掉了。当然如果您有好的建议可以去我的weibo或twitter或mail告诉我。

数据库起动后,立即收集了表及索引的统计信息,explian 确认执行计划已改变不再使用index join 和其它库相同,起应用故障恢复。

先来看一下问题时的SQL及执行计划

SQL> @xi 4x2qck7u5m4mk %
eXplain the execution plan for sqlid 4x2qck7u5m4mk child %...

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------
SQL_ID  4x2qck7u5m4mk, child number 4
-------------------------------------
SELECT       ATTRID,              ATTRVALUE   FROM       ANBOB_T1
WHERE       RECOID = :RECOID    AND       REGION = 399

Plan hash value: 3978804789
---------------------------------------------------------------------------------------------------------------
| Id  | Operation                  | Name                 | E-Rows | Pstart| Pstop |  OMem |  1Mem | Used-Mem |
---------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT           |                      |        |       |       |       |       |          |
|*  1 |  VIEW                      | index$_join$_001     |   1387K|       |       |       |       |          |
|*  2 |   HASH JOIN                |                      |        |       |       |  1000K|  1000K|  927K (0)|
|   3 |    PARTITION RANGE ITERATOR|                      |   1387K|    65 |    74 |       |       |          |
|*  4 |     INDEX RANGE SCAN       | IDX_ANBOB_T1         |   1387K|    65 |    74 |       |       |          |
|   5 |    PARTITION RANGE ITERATOR|                      |   1387K|    65 |    74 |       |       |          |
|*  6 |     INDEX FAST FULL SCAN   | IDX_ANBOB_T1_VALUE   |   1387K|    65 |    74 |       |       |          |
---------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - filter("RECOID"=TO_NUMBER(:RECOID))
   2 - access(ROWID=ROWID)
   4 - access("RECOID"=TO_NUMBER(:RECOID))
   6 - filter("REGION"=399)
Note
-----
   - dynamic sampling used for this statement (level=2)
SQL> @ind ANBOB.ANBOB_T1
Display indexes where table or index name matches %ANBOB.ANBOB_T1%...

TABLE_OWNER     TABLE_NAME    INDEX_NAME         POS# COLUMN_NAME                    DSC
--------------- ------------- ------------------ ---- ------------------------------ ----
ANBOB            ANBOB_T1      IDX_ANBOB_T1          1 RECOID
                               IDX_ANBOB_T1_VALUE    1 ATTRID
                                                     2 ATTRVALUE
                                                     3 REGION


INDEX_OWNER          TABLE_NAME                     INDEX_NAME                     IDXTYPE    UNIQ STATUS   PART TEMP  H     LFBLKS           NDK   NUM_ROWS       CLUF LAST_ANALYZED     DEGREE VISIBILIT
-------------------- ------------------------------ ------------------------------ ---------- ---- -------- ---- ---- -- ---------- ------------- ---------- ---------- ----------------- ------ ---------
ANBOB                 ANBOB_T1                    IDX_ANBOB_T1                NORMAL     NO   N/A      YES  N     1          0             0          0          0 20141214 22:30:50 1      VISIBLE
                     ANBOB_T1                    IDX_ANBOB_T1_VALUE           NORMAL     NO   N/A      YES  N     1          0             0          0          0 20150527 00:25:59 1      VISIBLE

Note:
就是一个简单的select语句,SQL涉及表的所有字段都有索引,不过分布在两个索引上,ORACLE 优化器会选择了使用index join的执行计划,避免了对table的访问路径, 因当时cpu过高,查询慢没成功记录两个索引的分区信息,不过可以确认表有个索引最后一次收集是20141214。Index join 也叫index hash join ,只指从多个索引中通过hash join取到值减少对表的访问。INDEX JOIN 的执行计划存在bug,从上面可以看到两个索引及hash后的e-rows是相同的,关于Index JOIN原理不再多描述。

#收集统计信息后的执行计划
SQL>  explain plan for    SELECT ATTRID, ATTRVALUE
           FROM ANBOB.ANBOB_T1
          WHERE RECOID = '1111' AND REGION = 399;
Explained.

SQL> @x2

PLAN_TABLE_OUTPUT
-----------------------------------------------------------------------------------------------------------
Plan hash value: 491038779

----------------------------------------------------------------------------------------------------------------------
| Id  | Operation                          | Name            | Rows  | Bytes | Cost (%CPU)| Time     | Pstart| Pstop |
----------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                   |                 |     1 |    28 |    22   (0)| 00:00:01 |       |       |
|   1 |  PARTITION RANGE ITERATOR          |                 |     1 |    28 |    22   (0)| 00:00:01 |    65 |    74 |
|*  2 |   TABLE ACCESS BY LOCAL INDEX ROWID| ANBOB_T1        |     1 |    28 |    22   (0)| 00:00:01 |    65 |    74 |
|*  3 |    INDEX RANGE SCAN                | IDX_ANBOB_T1    |     1 |       |    21   (0)| 00:00:01 |    65 |    74 |
----------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   2 - filter("REGION"=399)
   3 - access("RECOID"=1111)

Note:
可以看到CBO使用了另一执行计划,不再使用INDEX_JOIN,COST非常小且响应时间非常快, 细心的可能发现index join的执行计划中谓词中有TO_NUMBER, 那应该是程序绑定变量类型可能是非number类型, CBO窥探到了变量,所以不是因为变量的问题使用了hash join的执行计划。就是因为统计信息的问题。刚好我们这个数据库有BCV同步,现在就可以在测试库上还原问题,再分析,其实只要还原统计信息就可以。下面我们在测试库还原这个问题。

SQL> alter session set NLS_TIMESTAMP_TZ_FORMAT='yyyy-mm-dd:hh24:mi:ss';
Session altered.

SQL> @tab ANBOB.ANBOB_T1
Show tables matching condition "%ANBOB.ANBOB_T1%" (if schema is not specified then current user's tables only are shown)...

OWNER                TABLE_NAME                     TYPE     NUM_ROWS        BLOCKS     EMPTY AVGSPC ROWLEN TAB_LAST_ANALYZED DEGREE     COMPRESS
-------------------- ------------------------------ ---- ------------ ------------- --------- ------ ------ ----------------- ---------- --------
ANBOB                 ANBOB_T1                    PTAB    283829133       1796808         0      0     35 20160217 16:50:55          1

SQL> select table_name,PARTITION_NAME,stats_update_time from dba_tab_stats_history where owner='ANBOB' AND table_name='ANBOB_T1';

TABLE_NAME            PARTITION_NAME                 STATS_UPDATE_TIME
--------------------- ------------------------------ --------------------------
...                
ANBOB_T1              PART_398_MAX                   2016-02-17:16:50:55
ANBOB_T1              PART_399_201507                2016-02-17:16:50:26
ANBOB_T1              PART_399_201507                2016-02-17:16:50:55
ANBOB_T1              PART_399_201508                2016-02-17:16:50:26
ANBOB_T1              PART_399_201508                2016-02-17:16:50:55
ANBOB_T1              PART_399_201509                2016-02-17:16:50:26
ANBOB_T1              PART_399_201509                2016-02-17:16:50:55
ANBOB_T1              PART_399_201510                2016-02-17:16:50:26
ANBOB_T1              PART_399_201510                2016-02-17:16:50:55
ANBOB_T1              PART_399_201511                2016-02-17:16:50:26
ANBOB_T1              PART_399_201511                2016-02-17:16:50:55
ANBOB_T1              PART_399_201512                2016-02-17:16:50:26
ANBOB_T1              PART_399_201512                2016-02-17:16:50:55
ANBOB_T1              PART_399_201601                2016-02-17:16:50:26
ANBOB_T1              PART_399_201601                2016-02-17:16:50:55
ANBOB_T1              PART_399_201602                2016-02-17:16:50:26
...

SQL> select DBMS_STATS.GET_STATS_HISTORY_AVAILABILITY  from dual;

GET_STATS_HISTORY_AVAILABILITY
---------------------------------------------------------------------------
2016-01-23:09:28:05

SQL>  select DBMS_STATS.GET_STATS_HISTORY_RETENTION from dual;

GET_STATS_HISTORY_RETENTION
---------------------------
                         31

Tip:
统计信息历史保留期当前是31天也是默认值,我们可以恢复到问题时刻的统计信息,对表统计信息的保留周期也可以修改使用DBMS_STATS.ALTER_STATS_HISTORY_RETENTION,注意恢复后还可以恢复回来,恢复操作不会修改dba_tab_stats_history的记录。

— 恢复到问题时间点时的统计信息

 SQL> begin
     dbms_stats.restore_table_stats('ANBOB','ANBOB_T1',
     to_timestamp('2016-02-17:15:00:55','yyyy-mm-dd:hh24:mi:ss'));
     end;
     /

PL/SQL procedure successfully completed.
						 
 SQL> @tab ANBOB.ANBOB_T1
Show tables matching condition "%ANBOB.ANBOB_T1%" (if schema is not specified then current user's tables only are shown)...

OWNER     TABLE_NAME      TYPE     NUM_ROWS        BLOCKS     EMPTY AVGSPC ROWLEN TAB_LAST_ANALYZED DEGREE     COMPRESS
--------- --------------- ---- ------------ ------------- --------- ------ ------ ----------------- ---------- --------
ANBOB      ANBOB_T1       PTAB                                                                               1

SQL>  @ind ANBOB.ANBOB_T1
Display indexes where table or index name matches %ANBOB.ANBOB_T1%...

TABLE_OWNER     TABLE_NAME    INDEX_NAME                 POS# COLUMN_NAME                    DSC
--------------- ------------- -------------------------- ---- ------------------------------ ----
ANBOB            ANBOB_T1     IDX_ANBOB_T1                1 RECOID
                              IDX_ANBOB_T1_VALUE          1 ATTRID
                                                          2 ATTRVALUE
                                                          3 REGION


INDEX_OWNER          TABLE_NAME                     INDEX_NAME                     IDXTYPE    UNIQ STATUS   PART TEMP  H     LFBLKS           NDK   NUM_ROWS       CLUF LAST_ANALYZED     DEGREE VISIBILIT
-------------------- ------------------------------ ------------------------------ ---------- ---- -------- ---- ---- -- ---------- ------------- ---------- ---------- ----------------- ------ ---------
ANBOB                 ANBOB_T1                    IDX_ANBOB_T1                NORMAL     NO   N/A      YES  N     1          0             0          0          0 20141214 22:30:50 1      VISIBLE
                     ANBOB_T1                    IDX_ANBOB_T1_VALUE           NORMAL     NO   N/A      YES  N     1          0             0          0          0 20150527 00:25:59 1      VISIBLE
                     
SQL> @tabpart2 ANBOB.ANBOB_T1

 TABLE_OWNER        TABLE_NAME              POS PARTITION_NAME          NUM_ROWS     BLOCKS LAST_ANALYZED     GLO INT
------------------ ---------------- ---------- --------------------- ---------- ---------- ----------------- --- ---
...
ANBOB                                        65 PART_399_201507                                               NO  NO
ANBOB                                        66 PART_399_201508                                               NO  NO
ANBOB                                        67 PART_399_201509                                               NO  NO
ANBOB                                        68 PART_399_201510                                               NO  NO
ANBOB                                        69 PART_399_201511                                               NO  NO
ANBOB                                        70 PART_399_201512                                               NO  NO
ANBOB                                        71 PART_399_201601                                               NO  NO
ANBOB                                        72 PART_399_201602                                               NO  NO
ANBOB                                        73 PART_399_MAX                                                  NO  NO
...

SQL> @indpart ANBOB.IDX_ANBOB_T1

PARTITION_POSITION INDEX_NAME           PARTITION_NAME       H     LFBLKS    NDK   NUM_ROWS LAST_ANALYZED     STATUS
------------------ -------------------- ------------------- -- ---------- ------ ---------- ----------------- --------
 ...
                65 IDX_ANBOB_T1      PART_399_201507      1          0      0          0 20150625 01:23:33 USABLE
                66 IDX_ANBOB_T1      PART_399_201508                                                       USABLE
                67 IDX_ANBOB_T1      PART_399_201509                                                       USABLE
                68 IDX_ANBOB_T1      PART_399_201510                                                       USABLE
                69 IDX_ANBOB_T1      PART_399_201511                                                       USABLE
                70 IDX_ANBOB_T1      PART_399_201512                                                       USABLE
                71 IDX_ANBOB_T1      PART_399_201601      1          0      0          0 20151222 16:31:44 USABLE
                72 IDX_ANBOB_T1      PART_399_201602                                                       USABLE
                73 IDX_ANBOB_T1      PART_399_MAX                                                          USABLE
...

SQL> @indpart ANBOB.IDX_ANBOB_T1_VALUE

PARTITION_POSITION INDEX_NAME             PARTITION_NAME     H     LFBLKS    NDK   NUM_ROWS LAST_ANALYZED     STATUS
------------------ ---------------------- ----------------- -- ---------- ------ ---------- ----------------- --------
                ...
                65 IDX_ANBOB_T1_VALUE   PART_399_201507    1          0      0          0 20150625 01:23:33 USABLE
                66 IDX_ANBOB_T1_VALUE   PART_399_201508                                                     USABLE
                67 IDX_ANBOB_T1_VALUE   PART_399_201509                                                     USABLE
                68 IDX_ANBOB_T1_VALUE   PART_399_201510                                                     USABLE
                69 IDX_ANBOB_T1_VALUE   PART_399_201511                                                     USABLE
                70 IDX_ANBOB_T1_VALUE   PART_399_201512                                                     USABLE
                71 IDX_ANBOB_T1_VALUE   PART_399_201601    1          0      0          0 20151222 16:31:44 USABLE
                72 IDX_ANBOB_T1_VALUE   PART_399_201602                                                     USABLE
                73 IDX_ANBOB_T1_VALUE   PART_399_MAX                                                        USABLE
				...
 
 SQL> explain plan for SELECT    /*anbob*/   ATTRID,              ATTRVALUE   FROM       ANBOB.ANBOB_T1
    WHERE       RECOID = :RECOID    AND       REGION = 399;

Explained.

SQL> @x2

PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------------
Plan hash value: 3978804789

-------------------------------------------------------------------------------------------------------------------
| Id  | Operation                  | Name                 | Rows  | Bytes | Cost (%CPU)| Time     | Pstart| Pstop |
-------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT           |                      |   985K|   178M|   197   (1)| 00:00:03 |       |       |
|*  1 |  VIEW                      | index$_join$_001     |   985K|   178M|   197   (1)| 00:00:03 |       |       |
|*  2 |   HASH JOIN                |                      |       |       |            |          |       |       |
|   3 |    PARTITION RANGE ITERATOR|                      |   985K|   178M|   136  (92)| 00:00:02 |    65 |    74 |
|*  4 |     INDEX RANGE SCAN       | IDX_ANBOB_T1         |   985K|   178M|   136  (92)| 00:00:02 |    65 |    74 |
|   5 |    PARTITION RANGE ITERATOR|                      |   985K|   178M|   232   (1)| 00:00:03 |    65 |    74 |
|*  6 |     INDEX FAST FULL SCAN   | IDX_ANBOB_T1_VALUE   |   985K|   178M|   232   (1)| 00:00:03 |    65 |    74 |
-------------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter("RECOID"=TO_NUMBER(:RECOID))
   2 - access(ROWID=ROWID)
   4 - access("RECOID"=TO_NUMBER(:RECOID))
   6 - filter("REGION"=399)

Note
-----
   - dynamic sampling used for this statement (level=2)

Note:
到这里故障时的SQL的执行计划恢复了,我们又恢复了index join执行计划, 因篇幅原因先到这里,下篇会继续使用sql profile修正该执行计划…

打赏

对不起,这篇文章暂时关闭评论。