表收集错误导致执行计划错误

Posted 2020-07-09 侯志清-江西南昌

有时候，表收集信息错误，导致执行计划错误问题。
SQL> insert into test values(1,‘user1‘);
已创建 1 行。
SQL> commit;
提交完成。
SQL> select * from test;
        ID NAME
---------- ----------------------------------------
         1 user1
SQL> insert into test values(2,‘user2‘);
已创建 1 行。
SQL> commit;
提交完成。
SQL> DECLARE
  2  BEGIN
  3  dbms_stats.gather_table_stats(
  4     ownname=>‘hzq‘,
  5     tabname=>‘test‘,
  6     cascade=>true
  7     );
  8  END;
  9  /
PL/SQL 过程已成功完成。
SQL> set autotrace trace explain;
SQL> select * from test where id>1;
执行计划
----------------------------------------------------------
Plan hash value: 2500689602
---------------------------------------------------------------------------------------------------
| Id  | Operation                           | Name        | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                    |             |     1 |     9 |     2   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID BATCHED| TEST        |     1 |     9 |     2   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN                  | IDX_TEST_ID |     1 |       |     1   (0)| 00:00:01 |
---------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   2 - access("ID">1)
SQL> begin
  2  for i in  1..100000 loop
  3  insert into test values(i,‘user‘||i);
  4  commit;
  5  end loop;
  6  end;
  7  /
PL/SQL 过程已成功完成。
--插入10万行数据之后，执行计划本应该走全表扫描，但是还是走了索引范围扫描
SQL> select * from test where id>1;
执行计划
----------------------------------------------------------
Plan hash value: 2500689602
---------------------------------------------------------------------------------------------------
| Id  | Operation                           | Name        | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                    |             |     1 |     9 |     2   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID BATCHED| TEST        |     1 |     9 |     2   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN                  | IDX_TEST_ID |     1 |       |     1   (0)| 00:00:01 |
---------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   2 - access("ID">1)
--重新收集之后，索引计划会纠正。
SQL> DECLARE
  2  BEGIN
  3  dbms_stats.gather_table_stats(
  4     ownname=>‘hzq‘,
  5     tabname=>‘test‘,
  6     cascade=>true
  7     );
  8  END;
  9  /
PL/SQL 过程已成功完成。
SQL> select * from test where id>1;
执行计划
----------------------------------------------------------
Plan hash value: 1357081020
--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |   100K|  1464K|   103   (1)| 00:00:01 |
|*  1 |  TABLE ACCESS FULL| TEST |   100K|  1464K|   103   (1)| 00:00:01 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - filter("ID">1)