Question

运行执行计划而不收集表统计信息（使用dbms_stat.gather_table_stat）和收集表统计信息然后运行执行计划将对执行计划进行任何更改。有人请说明收集表统计信息以及执行计划如何依赖于收集统计信息

Answer 1

执行计划基于统计数据。如果您知道它并且以下示例对您没有用，请说出来并且我将删除此帖子。谢谢。例如，我创建了两个表：

create table ag_test1 (id1 number, val varchar2(50));
alter table ag_test1 add constraint pk_ag_test1 primary key (id1);
create table ag_test2 (id2 number, val varchar2(50));
alter table ag_test2 add constraint pk_ag_test2 primary key (id2);

之后我收集了这些表格的统计数据：

begin
  dbms_stats.gather_table_stats(ownname => user,tabname => 'AG_TEST1', cascade =>true);
  dbms_stats.gather_table_stats(ownname => user,tabname => 'AG_TEST2', cascade =>true);
end;

运行查询：

select /* test_query_1 */
       t1.val as tbl1, t2.val as tbl2
from ag_test1 t1
join ag_test2 t2 on (t1.id1 = t2.id2);

查看执行计划（因为你记得表包含0行）：

--------------------------------------------------------------------------------------------
| Id  | Operation                    | Name        | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |             |       |       |     2 (100)|          |
|   1 |  NESTED LOOPS                |             |     1 |    80 |     2   (0)| 00:00:01 |
|   2 |   NESTED LOOPS               |             |     1 |    80 |     2   (0)| 00:00:01 |
|   3 |    TABLE ACCESS FULL         | AG_TEST1    |     1 |    40 |     2   (0)| 00:00:01 |
|*  4 |    INDEX UNIQUE SCAN         | PK_AG_TEST2 |     1 |       |     0   (0)|          |
|   5 |   TABLE ACCESS BY INDEX ROWID| AG_TEST2    |     1 |    40 |     0   (0)|          |
--------------------------------------------------------------------------------------------

让我们填写此表（每行包含1M行）：

insert all
when 1=1 then 
  into ag_test1 (id1, val)
  values (lvl, val1)
when 1=1 then 
  into ag_test2 (id2, val)
  values (lvl, val2)
select level as lvl, 'TBL1_VAL_'||level as val1, 'TBL2_VAL_'||level as val2
from dual
connect by level <=1000000;
commit;

再次运行查询并查看：

--------------------------------------------------------------------------------------------
| Id  | Operation                    | Name        | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |             |       |       |     2 (100)|          |
|   1 |  NESTED LOOPS                |             |     1 |    80 |     2   (0)| 00:00:01 |
|   2 |   NESTED LOOPS               |             |     1 |    80 |     2   (0)| 00:00:01 |
|   3 |    TABLE ACCESS FULL         | AG_TEST1    |     1 |    40 |     2   (0)| 00:00:01 |
|*  4 |    INDEX UNIQUE SCAN         | PK_AG_TEST2 |     1 |       |     0   (0)|          |
|   5 |   TABLE ACCESS BY INDEX ROWID| AG_TEST2    |     1 |    40 |     0   (0)|          |
--------------------------------------------------------------------------------------------

什么都没改变。 1M行的执行计划等于0行。因为Oracle使用软解析（查找并使用以前的计划）。让我们使用no_invalidate子句收集统计信息（ no_invalidate =＆gt; false ）：

begin
  dbms_stats.gather_table_stats(ownname => user,tabname => 'AG_TEST1', cascade =>true, no_invalidate => false);
  dbms_stats.gather_table_stats(ownname => user,tabname => 'AG_TEST2', cascade =>true, no_invalidate => false);
end;

查看执行计划：

---------------------------------------------------------------------------------------
| Id  | Operation          | Name     | Rows  | Bytes |TempSpc| Cost (%CPU)| Time     |
---------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |          |       |       |       |  5120 (100)|          |
|*  1 |  HASH JOIN         |          |  1000K|    40M|    31M|  5120   (1)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| AG_TEST1 |  1000K|    20M|       |   995   (1)| 00:00:01 |
|   3 |   TABLE ACCESS FULL| AG_TEST2 |  1000K|    20M|       |   995   (1)| 00:00:01 |
---------------------------------------------------------------------------------------

oracle中的GATHER_TABLE_STATS和执行计划

1 个答案: