数据库性能问题总结--屡次发生的Oracle谓词越界

数据和云

发布于 2021-04-20 10:40:28

4510

发布于 2021-04-20 10:40:28

近期在客户现场屡次遇到由于统计信息过旧，导致执行计划选错引发的数据库性能问题，今天做个总结。

谓词越界常见发生在 where 谓词是时间字段的情况，总的来说统计信息记录的是一个过旧的时间，而 SQL 传入的时间是一个最新的时间范围（往往是 <time time1<c<time2）。由于统计信息不全，按照 CBO 计算出来的结果集就很小，在多表关联的情况下，CBO 就会选择认为的最优的关联方式，而实际执行时发现不是那么回事，有大量结果集需要扫描，就会爆发 SQL 性能问题。

谓词越界就是 select 的谓词的条件不在统计信息 low_value 和 high_value 之间，在实际选择结果集要大于 CBO 记录的结果集数量，即实际的 selectivity 偏大，这种情况下 CBO 评估出来的 selectivity 会出现严重的偏差，导致 CBO 选错执行计划。

测试验证

下面做一组测试，从执行计划 cost 看谓词越界的发生过程，先插入部分数据：

DECLARE
i INT;
BEGIN
i := 78179;
WHILE(i < 100000)
LOOP
i := i + 1;
INSERT INTO test_obj(object_id) VALUES(i);
COMMIT;
END LOOP;
END;
/

查看此时的 num_rows：

TEST@PROD1> select count(*) from test_obj;
  COUNT(*)
----------
     94283
TEST@PROD1> select max(object_ID),dump(max(object_id),16) from test_obj;
 
MAX(OBJECT_ID) DUMP(MAX(OBJECT_ID),16)
-------------- ----------------------------------------
        100000 Typ=2 Len=2: c3,b    
TEST@PROD1> select min(object_ID),dump(min(object_id),16) from test_obj;
 
MIN(OBJECT_ID   )               DUMP(MIN(OBJECT_ID),16)
------------------------------ ----------------------------------------
      2                          Typ=2 Len=2: c1,3        --C103

不收集统计信息，此时统计列统计信息过旧，HIGH_VALUE 依然是原来的值 78179。

TEST@PROD1> select  low_value ,high_value,num_distinct,num_nulls from  DBA_TAB_COL_STATISTICS where table_name='TEST_OBJ' and owner='TEST';
 
                                                                  Distinct     Number
LOW_VALUE                      HIGH_VALUE                           Values      Nulls
------------------------------ ------------------------------ ------------ ----------
C103                           C3085250                             72,462（原值）  0

查询结果返回 2081 行结果集。

TEST@PROD1> select count(*) from test_obj where object_id between 78200 and 81000;
  COUNT(*)
----------
      2801
计算公式为：
selectivity=((VAL2 - VAL1) / (HIGH_VALUE - LOW_VALUE)+2 / NUM_DISTINCT) * null_adjust
null_adjust=(NUM_ROES - NUM_NULLS) / NUM_ROES

计算结果为：
TEST@PROD1>  select round(((81000-78200)/(100000-2)+2/94283)*(94283-0)/94283*94283) from dual; 
 
ROUND(((81000-78200)/(100000-2)+2/94283)*(94283-0)/94283*94283)
---------------------------------------------------------------
                                                           2642

查看结果集发现 dictionary 值为 1，这明显是一个错误的执行计划，由于统计信息过旧，已经低于谓词条件区间（谓词过界）导致 CBO 低估了查询成本。

TEST@PROD1>  select count(*) from test_obj where object_id between 78200 and 81000;
 
Execution Plan
----------------------------------------------------------
Plan hash value: 2217143630
 
-------------------------------------------------------------------------------
| Id  | Operation          | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |          |     1 |     5 |   289   (1)| 00:00:04 |
|   1 |  SORT AGGREGATE    |          |     1 |     5 |            |          |
|*  2 |   TABLE ACCESS FULL| TEST_OBJ |     1 |     5 |   289   (1)| 00:00:04 |
-------------------------------------------------------------------------------
 
Predicate Information (identified by operation id):
---------------------------------------------------
 
   2 - filter("OBJECT_ID">=78200 AND "OBJECT_ID"<=81000)
 
 
Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
       1117  consistent gets
          0  physical reads
          0  redo size
        423  bytes sent via SQL*Net to client
        419  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

重新收集统计信息再次查看执行计划。

TEST@PROD1> exec dbms_stats.gather_table_stats('test','test_obj');
TEST@PROD1> select  low_value ,high_value,num_distinct,num_nulls from  DBA_TAB_COL_STATISTICS where table_name='TEST_OBJ' and owner='TEST';
 
                                              Distinct     Number
LOW_VALUE            HIGH_VALUE                 Values      Nulls
-------------------- -------------------- ------------ ----------
C103                 C30B                       94,283          0

此时统计信息 HIGH_VALUE 已经和最初计算的值相等，Typ=2 Len=2: c3,b。再次查看执行计划，此时 CBO 已经能够产生了正确的执行计划了。

执行计划为：

TEST@PROD1> select count(*) from test_obj where object_id between 78200 and 81000;
 
Execution Plan
----------------------------------------------------------
Plan hash value: 2217143630
 
-------------------------------------------------------------------------------
| Id  | Operation          | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |          |     1 |     5 |   314   (1)| 00:00:04 |
|   1 |  SORT AGGREGATE    |          |     1 |     5 |            |          |
|*  2 |   TABLE ACCESS FULL| TEST_OBJ |  2642 | 13210 |   314   (1)| 00:00:04 |
-------------------------------------------------------------------------------
 
Predicate Information (identified by operation id):
---------------------------------------------------
 
   2 - filter("OBJECT_ID">=78200 AND "OBJECT_ID"<=81000)
 
 
Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
       1117  consistent gets
          0  physical reads
          0  redo size
        423  bytes sent via SQL*Net to client
        419  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

谓词越界主要发生在大表，按照 Oracle 统计信息收集机制，表的数据变化量达到 10% 以上才会进行统计信息收集，大表不常收集统计信息就容易爆发谓词越界。