今天碰到一個非常奇怪的問題問題，一條SQL語句在PL/SQL developer中很慢，需要9s，問題SQL:
SELECT * FROM GG_function_location f WHERE f.parent_id ='03000000000001'; 表GG_function_location有5千萬的數據，parent_id上是有索引的。
診斷第一步：就在PL/SQL developer中按F5，看到的執行計划是走索引的，應該不會慢啊。
第二步：在sqlplus中用autotrace看，非常快，0.06s。
第三部：我想要重現這種慢，於是在PL/SQL developer中開一個窗口，天啊！單獨執行SQL非常慢，但使用下面的語句就非常快，真是太神奇了。
alter session set tracefile_identifier = 'gg_test';
alter session set events '10046 trace name context forever ,level 12' ;
SELECT * FROM GG_function_location f WHERE f.parent_id ='03000000000001' ;
alter session set events '10046 trace name context off' ;
第四部：我想到v$sql中找到這條SQL的執行計划，終於有了發現。
SQL> select banner from v$version;
BANNER
--------------------------------------------------------------------------------
Oracle Database 12c Enterprise Edition Release 12.1.0.1.0 - 64bit Production
PL/SQL Release 12.1.0.1.0 - Production
CORE 12.1.0.1.0 Production
TNS for Linux: Version 12.1.0.1.0 - Production
NLSRTL Version 12.1.0.1.0 - Production
SQL> select s.SQL_TEXT,s.SQL_ID
from v$sql swhere s.SQL_TEXT like
'SELECT * FROM GG_function_location f WHERE f.parent_id =%'
and s.SQL_TEXT not like '%AND%';
SQL_TEXT SQL_ID
------------------------------------------------------------------------- ------------
SELECT * FROM GG_function_location f WHERE f.parent_id ='03000000000001' dk02nb8mkchna
SELECT * FROM GG_function_location f WHERE f.parent_id ='03000000000001' 2zav8x5kwxb32
SELECT * FROM GG_function_location f WHERE f.parent_id ='03000000000001' bc0k800k6u0x3
先找到SQL_ID，再找到對應的執行計划
select hash_value, child_number, sql_text from v$sql swhere s.SQL_ID = 'bc0k800k6u0x3';
select * from table(dbms_xplan.display_cursor(611124131, 0, 'advanced'));
執行計划一：
HASH_VALUE 656818826, child number 0
-------------------------------------
SELECT * FROM GG_function_location f WHERE f.parent_id ='03000000000001'
Plan hash value: 1550360901
-----------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Pstart| Pstop |
-----------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 7 (100)| | | |
| 1 | TABLE ACCESS BY GLOBAL INDEX ROWID BATCHED| GG_FUNCTION_LOCATION | 3 | 999 | 7 (0)| 00:00:01 | ROWID | ROWID |
|* 2 | INDEX RANGE SCAN | IDX_GG_FL_PARENT_ID | 3 | | 4 (0)| 00:00:01 | | |
-----------------------------------------------------------------------------------------------------------------------------------
執行計划二：
HASH_VALUE 611124131, child number 0
-------------------------------------
SELECT * FROM GG_function_location f WHERE f.parent_id ='03000000000001'
Plan hash value: 3374024865
------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Pstart| Pstop |
------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 68 (100)| | | |
| 1 | PARTITION LIST ALL | | 1 | 247 | 68 (0)| 00:00:01 | 1 | 2 |
| 2 | PARTITION LIST ALL| | 1 | 247 | 68 (0)| 00:00:01 | 1 | 20 |
|* 3 | TABLE ACCESS FULL| GG_FUNCTION_LOCATION | 1 | 247 | 68 (0)| 00:00:01 | 1 | 40 |
------------------------------------------------------------------------------------------------------------
分析：我判斷是解析這條SQL語句走錯了執行計划，SELECT * FROM GG_function_location f WHERE f.parent_id ='03000000000001'，於是我把改為
SELECT /*+gg*/* FROM GG_function_location f WHERE f.parent_id ='03000000000001',非常快。接近就簡單了，把索引刪除后，重建，會讓此SQL重新解析。
解決方案：drop index IDX_GG_FL_PARENT_ID;
create index IDX_GG_FL_PARENT_ID on GG_FUNCTION_LOCATION (PARENT_ID) nologging;