現(xiàn)象：一條SQL突然運(yùn)行的特別慢。

select uidTable.column_value, first_name||' '

||last_name, company, job_title, upper(member_level),

upper(service_value)

from (select * from table(select cast(multiset

(select b from bbb)as Taaa) from dual)) uidTable,member

where uidTable.column_value = member.login_id(+)

and member.site='alibaba' and member.site='test';

出錯(cuò)原因：用戶增加了一個(gè)條件member.site=test,造成連接的順序變化了，原來(lái)的驅(qū)動(dòng)表是uidTable(最多1024條記錄），現(xiàn)在變成了member表做驅(qū)動(dòng)(600W條）。所以這條語(yǔ)句變的巨慢。

但是既然是外連接，為什么連接的順序會(huì)改變呢？因?yàn)橥膺B接的連接順序不是由COST決定的，而是由連接的條件決定的。發(fā)現(xiàn)執(zhí)行計(jì)劃如下：

-------------------------------------------------------

| Id | Operation | Name | Rows | Bytes | Cost |

--------------------------------------------------------

| 0 | SELECT STATEMENT | | 1018 | 72278 | 8155 |

| 1 | NESTED LOOPS | | 1018 | 72278 | 8155 |

| 2 | VIEW | | 4072 | 69224 | 11 |

| 3 | COLLECTION ITERATOR SUBQUERY FETCH| | | | |

| 4 | TABLE ACCESS FULL | DUAL | 4072 | | 11 |

| 5 | TABLE ACCESS FULL | BBB | 41 | 287 | 2 |

| 6 | TABLE ACCESS BY INDEX ROWID | MEMBER | 1 | 54 | 2 |

|* 7 | INDEX UNIQUE SCAN | MEMBER_SITE_LID_PK | 4 | | 1 |

-------------------------------------------------

為什么根本就沒(méi)有執(zhí)行外連接呢？問(wèn)題出在member.site='test'這個(gè)條件上，因?yàn)閷?duì)外連接的表加了條件，造成外連接失效。改為member.site(+)='test'后，問(wèn)題徹底解決。

---------------------------------------------------

| Id | Operation | Name | Rows | Bytes | Cost |

-----------------------------------------------------

| 0 | SELECT STATEMENT | | 1018 | 72278 | 8155 |

| 1 | NESTED LOOPS | | 1018 | 72278 | 8155 |

| 2 | VIEW | | 4072 | 69224 | 11 |

| 3 | COLLECTION ITERATOR SUBQUERY FETCH| | | | |

| 4 | TABLE ACCESS FULL | DUAL | 4072 | | 11 |

| 5 | TABLE ACCESS FULL | BBB | 41 | 287 | 2 |

| 6 | TABLE ACCESS BY INDEX ROWID | MEMBER | 1 | 54 | 2 |

|* 7 | INDEX UNIQUE SCAN | MEMBER_SITE_LID_PK | 4 | | 1 |

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