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';