看懂执行计划也就成了SQL优化的先决条件。 这里的SQL优化指的是SQL性能问题的定位，定位后就可以解决问题。
一． 查看执行计划的三种方法
1.1 设置autotrace
SQL> set autotrace on
SQL> select * from dave;
ID NAME
---------- ----------
8 安庆
1 dave
2 bl
1 bl
2 dave
3 dba
4 sf-express
5 dmm
已选择8行。
执行计划
----------------------------------------------------------
Plan hash value: 3458767806
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 8 | 64 | 2 (0)| 00:00:01 |
| 1 | TABLE ACCESS FULL| DAVE | 8 | 64 | 2 (0)| 00:00:01 |
--------------------------------------------------------------------------
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
4 consistent gets
0 physical reads
0 redo size
609 bytes sent via SQL*Net to client
416 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
8 rows processed
SQL>
1.2 使用SQL
SQL>EXPLAIN PLAN FOR sql语句;
SQL>SELECT plan_table_output FROM TABLE(DBMS_XPLAN.DISPLAY('PLAN_TABLE'));
示例：
SQL> EXPLAIN PLAN FOR SELECT * FROM DAVE;
已解释。
SQL> SELECT plan_table_output FROM TABLE(DBMS_XPLAN.DISPLAY('PLAN_TABLE'));
或者：
SQL> select * from table(dbms_xplan.display);
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
Plan hash value: 3458767806
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 8 | 64 | 2 (0)| 00:00:01 |
| 1 | TABLE ACCESS FULL| DAVE | 8 | 64 | 2 (0)| 00:00:01 |
--------------------------------------------------------------------------
已选择8行。
执行计划
----------------------------------------------------------
Plan hash value: 2137789089
--------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 8168 | 16336 | 29 (0)| 00:00:01 |
| 1 | COLLECTION ITERATOR PICKLER FETCH| DISPLAY | 8168 | 16336 | 29 (0)| 00:00:01 |
---------------------------------------------------------------------------------------------
统计信息
----------------------------------------------------------
25 recursive calls
12 db block gets
168 consistent gets
0 physical reads
0 redo size
974 bytes sent via SQL*Net to client
416 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
8 rows processed
SQL>
1.3 使用Toad,PL/SQL Developer工具
二． Cardinality（基数）/ rows
这是9i的一个执行计划，我们可以看到关键字Card：
执行计划
--------