标签：autotrac explain plan 10046
http://blog.csdn.net/robo23/article/details/42616067
文章里简单地比较了EXPLAIN PLAN FOR和SET AUTOTRACE的方法来打印执行计划。
但偶然的机会我发现SET AUTOTRCE尽管有真实去执行SQL，但打印出来的执行计划并不一定准确。看如下实验：
T表有10万条记录，A字段有普通索引，A字段的选择性很好：
<span style="font-size:14px;">SQL> set autotrace on;
SQL> alter session set events '10046 trace name context forever,level 12'; <<<<
Session altered.
SQL> variable a number;
SQL> execute :a := 99999; <<<<<<<
PL/SQL procedure successfully completed.
SQL> select /*bind_peek*/count(b) from t where a < :a; <<<<<<<<<<
COUNT(B)
----------
99998
Execution Plan
----------------------------------------------------------
Plan hash value: 3565334791
----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 8 | 5 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 8 | | |
| 2 | TABLE ACCESS BY INDEX ROWID| T | 5000 | 40000 | 5 (0)| 00:00:01 |
|* 3 | <strong>INDEX RANGE SCAN</strong> | IND_T_A | 900 | | 3 (0)| 00:00:01 |
----------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - access("A"<TO_NUMBER(:A))
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
185 consistent gets
0 physical reads
0 redo size
528 bytes sent via SQL*Net to client
524 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
SQL> oradebug tracefile_name;
/u01/app/oracle/diag/rdbms/orcl/orcl/trace/orcl_ora_24324.trc <<<<<<<<</span>
接下来,我们去检查一下trace文件内容:
trace文件里清楚地标识着执行计划是走全表扫描,与SET AUTOTRACE打印出来的路径有出路。
再来看以下两处说明：
图一:
图二:
图一说明,SET AUTOTRACE确实有去执行SQL；图二说明，SET AUTOTRACE的执行计划仍然来至于EXPLAIN PLAN.
这才恍然大悟,SET AUTOTRACE打印出来的执行计划也有可能是不准确的，还是10046最准确。
标签：autotrac explain plan 10046
原文地址：http://blog.csdn.net/robo23/article/details/43883187
