查看执行计划的方法

1. Explain Plan For SQL
   不实际执行SQL语句，生成的计划未必是真实执行的计划
   必须要有plan_table

2. SQLPLUS AUTOTRACE
   除set autotrace traceonly explain外均实际执行SQL，但仍未必是真实计划
   必须要有plan_table

3. SQL TRACE
   需要启用10046戒者SQL_TRACE
   一般用tkprof看的更清楚些，当然10046里本身也有执行计划信息

4. V$SQL和V$SQL_PLAN
   可以查询到多个子游标的计划信息了，但是看起来比较费劲

5. Enterprise Manager
   可以图形化显示执行计划，但并非所有环境有EM可用

6. 其他第三方工具
   注意 PL/SQL developer之类工具F5看到的执行计划未必是真实的

推荐的方法 DBMS_XPLAN

select * from table(dbms_xplan….);

dbms_xplan.display()

数据来源是Plan Table

dbms_xplan.display_cursor

数据来源是Shared pool中的游标缓存

FUNCTION DISPLAY_CURSOR RETURNS DBMS_XPLAN_TYPE_TABLE
 Argument Name                  Type                    In/Out Default?
 ------------------------------ ----------------------- ------ --------
 SQL_ID                         VARCHAR2                IN     DEFAULT
 CURSOR_CHILD_NO                NUMBER(38)              IN     DEFAULT
 FORMAT                         VARCHAR2                IN     DEFAULT

推荐的使用参数为：

select * from table(dbms_xplan.display_cursor('sqlId',null,'ADVANCED  ALLSTATS LAST PEEKED_BINDS'));

如果sqlId为NULL，则显示当前session的执行计划。

select * from table(dbms_xplan.display_cursor(null,null,'ADVANCED  ALLSTATS LAST PEEKED_BINDS'));

其中format的解释如下：

IOSTATS: Assuming that basic plan statistics are
  ---                 collected when SQL statements are executed (either by
  ---                 using the gather_plan_statistics hint or by setting the
  ---                 parameter statistics_level to ALL), this format will show
  ---                 IO statistics for all (or only for the last as shown below)
  ---                 executions of the cursor.
  ---
  ---        MEMSTATS: Assuming that PGA memory management is enabled (i.e
  ---                  pga_aggregate_target parameter is set to a non 0 value),
  ---                  this format allows to display memory management
  ---                  statistics (e.g. execution mode of the operator, how
  ---                  much memory was used, number of bytes spilled to
  ---                  disk, ...). These statistics only apply to memory
  ---                  intensive operations like hash-joins, sort or some bitmap
  ---                  operators.
  ---
  ---        ROWSTATS: Assuming that basic plan statistics are
  ---                  collected when SQL statements are executed (either by
  ---                  using the gather_plan_statistics hint or by setting the
  ---                  parameter statistics_level to ALL), this format will show
  ---                  row count statistics for all (or only for the last as
  ---                  shown below) executions of the cursor.
  ---
  ---        ALLSTATS: A shortcut for 'IOSTATS MEMSTATS ROWSTATS'
  ---
  ---        LAST: By default, plan statistics are shown for all executions of
  ---              the cursor. The keyword LAST can be specified to see only
  ---              the statistics for the last execution.
  ---
  ---        PEEKED_BINDS：显示解析时使用的绑定变量。

dbms_xplan.display_awr

数据来源是AWR仓库基表WRH$_SQL_PLAN

FUNCTION DISPLAY_AWR RETURNS DBMS_XPLAN_TYPE_TABLE
 Argument Name                  Type                    In/Out Default?
 ------------------------------ ----------------------- ------ --------
 SQL_ID                         VARCHAR2                IN
 PLAN_HASH_VALUE                NUMBER(38)              IN     DEFAULT
 DB_ID                          NUMBER(38)              IN     DEFAULT
 FORMAT                         VARCHAR2                IN     DEFAULT
 CON_ID                         NUMBER(38)              IN     DEFAULT

dbms_xplan.display_sqlset

数据来源是SQL Set视图

以上内容主要整理自maclean的oracle执行计划教学视频和ppt：
www.askmaclean.com/archives/read-sql-execution-plan.html