欢迎您访问程序员文章站本站旨在为大家提供分享程序员计算机编程知识!
您现在的位置是: 首页  >  数据库

Oracle执行计划与统计信息的一些总结

程序员文章站 2022-06-08 07:58:19
...

SQLPLUS的命令,在执行SQL语句的同时显示执行计划,设置EXP(LAIN)的目的是只显示执行计划而不显示统计信息.。2、SQLgt;explain

1、SET AUTOTRACE ON EXPLAIN
(set autot on exp)
SQLPLUS的命令,在执行SQL语句的同时显示执行计划,设置EXP(LAIN)的目的是只显示执行计划而不显示统计信息.。
2、SQL>explain plan for select ````````;
SQL>select * from table(dbms_xplan.display);

执行了set autotrace on explain语句之后,接下来的查询、插入、更新、删除语句就会显示执行计划,直到执行“set autotrace off;”语句。如果是设置了set autotrace on,,除了会显示执行计划之外,还会显示一些有用的统计信息。

执行EXPLAIN PLAN FOR 可以只显示执行计划,然后执行如下查询

SQL> select * from table(dbms_xplan.display);

如:

SQL> explain plan for select * from emp where deptno='20';

Explained.

SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
Plan hash value: 3956160932

--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 5 | 150 | 3 (0)| 00:00:01 |
|* 1 | TABLE ACCESS FULL| EMP | 5 | 150 | 3 (0)| 00:00:01 |
--------------------------------------------------------------------------

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

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------

1 - filter("DEPTNO"=20)

13 rows selected.

3、SQL>exec dbms_stats.delete_table_stats(USER,'表');(删除表的统计信息)

SQL>exec dbms_stats.gather_table_stats(USER,'表',METHOD_OPT=>'FOR ALL COLUMNS SIZE 100')(收集表的统计信息)

4、AUTOTRACE的几个常用选项  

set autotrace off ---------------- 不生成autotrace 报告,这是缺省模式
set autotrace on explain ------ autotrace只显示优化器执行路径报告
set autotrace on statistics -- 只显示执行统计信息
set autotrace on ----------------- 包含执行计划和统计信息
set autotrace traceonly ------ 同set autotrace on,但是不显示查询输


(1). set autotrace on explain; --只显示执行计划
SQL> set autotrace on explain;
SQL>

select count(*) from dba_objects;

COUNT(*)
----------
31820

Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE
1 0 SORT (AGGREGATE)
2 1 VIEW OF 'DBA_OBJECTS'
3 2 UNION-ALL
4 3 FILTER
5 4 TABLE ACCESS (BY INDEX ROWID) OF 'OBJ$'
6 5 NESTED LOOPS
7 6 TABLE ACCESS (FULL) OF 'USER$'
8 6 INDEX (RANGE SCAN) OF 'I_OBJ2' (UNIQUE)
9 4 TABLE ACCESS (BY INDEX ROWID) OF 'IND$'
10 9 INDEX (UNIQUE SCAN) OF 'I_IND1' (UNIQUE)
11 3 NESTED LOOPS
12 11 TABLE ACCESS (FULL) OF 'USER$'
13 11 INDEX (RANGE SCAN) OF 'I_LINK1' (NON-UNIQUE)

(2). set autotrace on statistics;--只显示统计信息
SQL> set autotrace on statistics;
SQL> select count(*) from dba_objects;

COUNT(*)
----------
31820

Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
25754 consistent gets
0 physical reads
0 redo size
383 bytes sent via SQL*Net to client
503 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed

(3). set autotrace traceonly;--同set autotrace on 只是不显示查询输出
SQL> set autotrace traceonly;
SQL> select count(*) from dba_objects;

Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE
1 0 SORT (AGGREGATE)
2 1 VIEW OF 'DBA_OBJECTS'
3 2 UNION-ALL
4 3 FILTER
5 4 TABLE ACCESS (BY INDEX ROWID) OF 'OBJ$'
6 5 NESTED LOOPS
7 6 TABLE ACCESS (FULL) OF 'USER$'
8 6 INDEX (RANGE SCAN) OF 'I_OBJ2' (UNIQUE)
9 4 TABLE ACCESS (BY INDEX ROWID) OF 'IND$'
10 9 INDEX (UNIQUE SCAN) OF 'I_IND1' (UNIQUE)
11 3 NESTED LOOPS
12 11 TABLE ACCESS (FULL) OF 'USER$'
13 11 INDEX (RANGE SCAN) OF 'I_LINK1' (NON-UNIQUE)

Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
25754 consistent gets
0 physical reads
0 redo size
383 bytes sent via SQL*Net to client
503 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed

(4).set autotrace traceonly explain;--比较实用的选项,只显示执行计划,但是与set autotrace on explain;相比不会执行语句,对于仅仅查看大表的Explain Plan非常管用。
SQL> set autotrace traceonly explain;
SQL> select * from dba_objects;
已用时间: 00: 00: 00.00

Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE
1 0 VIEW OF 'DBA_OBJECTS'
2 1 UNION-ALL
3 2 FILTER
4 3 TABLE ACCESS (BY INDEX ROWID) OF 'OBJ$'
5 4 NESTED LOOPS
6 5 TABLE ACCESS (FULL) OF 'USER$'
7 5 INDEX (RANGE SCAN) OF 'I_OBJ2' (UNIQUE)
8 3 TABLE ACCESS (BY INDEX ROWID) OF 'IND$'
9 8 INDEX (UNIQUE SCAN) OF 'I_IND1' (UNIQUE)
10 2 TABLE ACCESS (BY INDEX ROWID) OF 'LINK$'
11 10 NESTED LOOPS
12 11 TABLE ACCESS (FULL) OF 'USER$'
13 11 INDEX (RANGE SCAN) OF 'I_LINK1' (NON-UNIQUE)

5、analyze

analyze table hr.employees compute(estimate) statistics;(compute收集每一行数据的统计信息,比较耗时;estimate收集一部分数据行的统计信息)

select t.owner,t.table_name,t.tablespace_name,t.blocks,t.empty_blocks,t.avg_space
from dba_tables t
where t.owner='HR';