Oracle執行計劃與統計信息的一些總結

 

[日期：2011-08-05]
來源：Linux社區  做者：wangshengfeng1986211
[字體：大 中 小]

2010-07-01 15:03

一、SET AUTOTRACE ON EXPLAIN
（set autot on exp)
SQLPLUS的命令，在執行SQL語句的同時顯示執行計劃，設置EXP(LAIN)的目的是隻顯示執行計劃而不顯示統計信息.。
二、SQL>explain plan for select ````````;
SQL>select * from table(dbms_xplan.display);

執行了set autotrace on explain語句以後，接下來的查詢、插入、更新、刪除語句就會顯示執行計劃，直到執行「set autotrace off;」語句。若是是設置了set autotrace on，除了會顯示執行計劃以外，還會顯示一些有用的統計信息。

執行EXPLAIN PLAN FOR 能夠只顯示執行計劃,而後執行以下查詢

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.

三、SQL>exec dbms_stats.delete_table_stats(USER,'表');(刪除表的統計信息）

SQL>exec dbms_stats.gather_table_stats(USER,'表',METHOD_OPT=>'FOR ALL COLUMNS SIZE 100')（收集表的統計信息）

四、AUTOTRACE的幾個經常使用選項

       set autotrace off ---------------- 不生成autotrace 報告，這是缺省模式
set autotrace on explain ------ autotrace只顯示優化器執行路徑報告
set autotrace on statistics -- 只顯示執行統計信息
set autotrace on ----------------- 包含執行計劃和統計信息
set autotrace traceonly ------ 同set autotrace on，可是不顯示查詢輸

(1). set autotrace on explain; --只顯示執行計劃
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;--只顯示統計信息
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 只是不顯示查詢輸出
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;--比較實用的選項，只顯示執行計劃，可是與set autotrace on explain;相比不會執行語句，對於僅僅查看大表的Explain Plan很是管用。
SQL> set autotrace traceonly explain;
SQL> select * from dba_objects;
已用時間: 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)

五、analyze

analyze table hr.employees compute(estimate) statistics;(compute收集每一行數據的統計信息，比較耗時；estimate收集一部分數據行的統計信息）

select t.owner,t.table_name,t.tablespace_name,t.blocks,t.empty_blocks,t.avg_spacefrom dba_tables twhere t.owner='HR';