该sql如下：

Select  /*+ parallel(src, 8) */ distinct

src.systemname as systemname

,  src.databasename as databasename

,  src.tablename as tablename

,  src.username as username

from  meta_dbql_table_usage_exp_hst src

inner join DR_QRY_LOG_EXP_HST rl on

src.acctstringdate = rl.acctstringdate

and src.queryid = rl.queryid

And Src.Systemname = Rl.Systemname

and src.acctstringdate > sysdate - 30

And Rl.Acctstringdate > Sysdate - 30

inner join  meta_dr_qry_log_tgt_all_hst tgt on

upper(tgt.systemname) = upper('MOZART')

And Upper(tgt.Databasename) = Upper('GDW_TABLES')

And Upper(tgt.Tablename) = Upper('SSA_SLNG_LSTG_MTRC_SD')

AND src.acctstringdate = tgt.acctstringdate

and rl.statement_id = tgt.statement_id

and rl.systemname = tgt.systemname

And Tgt.Acctstringdate > Sysdate - 30

And Not(

Upper(Tgt.Systemname)=Upper(src.systemname)

And

Upper(Tgt.Databasename) = Upper(Src.Databasename)

And

Upper(Tgt.Tablename) = Upper(Src.Tablename)

)

And   tgt.Systemname is not null

And   tgt.Databasename Is Not Null

And   tgt.tablename is not null

SQL的简单分析总得来看，这个SQL就是三个表(meta_dbql_table_usage_exp_hst，DR_QRY_LOG_EXP_HST，meta_dr_qry_log_tgt_all_hst)的INNER JOIN，这三个表数据量都在百万级别，且都是分区表(以acctstringdate为分区键)，执行计划如下：

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

| Id  | Operation                              | Name                          | Rows  | Bytes | Cost  | Pstart| Pstop |

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

|   0 | SELECT STATEMENT                       |                               |     1 |   159 |  8654 |       |       |

|   1 |  PX COORDINATOR                        |                               |       |       |       |       |       |

|   2 |   PX SEND QC (RANDOM)                  | :TQ10002                      |     1 |   159 |  8654 |       |       |

|   3 |    SORT UNIQUE                         |                               |     1 |   159 |  8654 |       |       |

|   4 |     PX RECEIVE                         |                               |     1 |    36 |     3 |       |       |

|   5 |      PX SEND HASH                      | :TQ10001                      |     1 |    36 |     3 |       |       |

|*  6 |       TABLE ACCESS BY LOCAL INDEX ROWID| DR_QRY_LOG_EXP_HST            |     1 |    36 |     3 |       |       |

|   7 |        NESTED LOOPS                    |                               |     1 |   159 |  8633 |       |       |

|   8 |         NESTED LOOPS                   |                               |  8959 |  1076K|  4900 |       |       |

|   9 |          BUFFER SORT                   |                               |       |       |       |       |       |

|  10 |           PX RECEIVE                   |                               |       |       |       |       |       |

|  11 |            PX SEND BROADCAST           | :TQ10000                      |       |       |       |       |       |

|  12 |             PARTITION RANGE ITERATOR   |                               |     1 |    56 |  4746 |   KEY |    14 |

|* 13 |              TABLE ACCESS FULL         | META_DR_QRY_LOG_TGT_ALL_HST   |     1 |    56 |  4746 |   KEY |    14 |

|  14 |          PX BLOCK ITERATOR             |                               |  8959 |   586K|   154 |   KEY |   KEY |

|* 15 |           TABLE ACCESS FULL            | META_DBQL_TABLE_USAGE_EXP_HST |  8959 |   586K|   154 |   KEY |   KEY |

|  16 |         PARTITION RANGE ITERATOR       |                               |     1 |       |     2 |   KEY |   KEY |

|* 17 |          INDEX RANGE SCAN              | DR_QRY_LOG_EXP_HST_IDX        |     1 |       |     2 |   KEY |   KEY |

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

Predicate Information (identified by operation id):

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

6 - filter("RL"."STATEMENT_ID"="TGT"."STATEMENT_ID" AND "RL"."SYSTEMNAME"="TGT"."SYSTEMNAME" AND "SRC"."SYSTEMNAME"="RL"."SYSTEMNAME")

13 - filter(UPPER("TGT"."SYSTEMNAME")='MOZART' AND UPPER("TGT"."DATABASENAME")='GDW_TABLES' AND

UPPER("TGT"."TABLENAME")='SSA_SLNG_LSTG_MTRC_SD' AND "TGT"."ACCTSTRINGDATE">SYSDATE@!-30 AND "TGT"."SYSTEMNAME" IS NOT NULL

"TGT"."DATABASENAME" IS NOT NULL AND "TGT"."TABLENAME" IS NOT NULL)

15 - filter("SRC"."ACCTSTRINGDATE"="TGT"."ACCTSTRINGDATE" AND (UPPER("TGT"."SYSTEMNAME")<>UPPER("SRC"."SYSTEMNAME") OR

UPPER("TGT"."DATABASENAME")<>UPPER("SRC"."DATABASENAME") OR UPPER("TGT"."TABLENAME")<>UPPER("SRC"."TABLENAME")) AND

"SRC"."ACCTSTRINGDATE">SYSDATE@!-30)

17 - access("SRC"."QUERYID"="RL"."QUERYID" AND "SRC"."ACCTSTRINGDATE"="RL"."ACCTSTRINGDATE")

filter("RL"."ACCTSTRINGDATE">SYSDATE@!-30)

定位问题从上面执行计划中的表连接方式可以知道，这三个表之间进行了两次NESTED LOOP，问题出现在最里层的NESTED LOOP(对两个表都做了TABLE FULL SCAN)，因为表都是百万级别的(即时过滤后的数据量也不小)，性能问题就出现在内表(即被驱动表)META_DBQL_TABLE_USAGE_EXP_HST做了太多次的全表扫描。如果能把全表扫描转换成索引，则性能可以大幅度提高。

下面是NESTED LOOP的介绍：嵌套连接把要处理的数据集分为外部循环(驱动数据源)和内部循环(被驱动数据源)，外部循环只执行一次，内部循环执行的次数等于外部循环执行返回的数据个数。

这种连接的好处是内存使用非常少。

如果驱动数据源有限，且被驱动表在连接列上有相应的索引，则这种连接方式才是高效的。

下面是这三个表上索引的情况：

SQL> select index_name, table_name from user_indexes where table_name in ('DR_QRY_LOG_EXP_HST',upper('meta_dbql_table_usage_exp_hst'), upper('meta_dr_qry_log_tgt_all_hs

INDEX_NAME                                                   TABLE_NAME

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

META_DR_QRY_LOG_TGT_ALL_IDX                                  META_DR_QRY_LOG_TGT_ALL_HST

META_DBQL_TUSAGE_EHST_IDX                                    META_DBQL_TABLE_USAGE_EXP_HST

DR_QRY_LOG_EXP_HST_IDX                                       DR_QRY_LOG_EXP_HST

CREATE INDEX "GV"."META_DR_QRY_LOG_TGT_ALL_IDX" ON "GV"."META_DR_QRY_LOG_TGT_ALL_HST" ("STATEMENT_ID", "ACCTSTRINGDATE")

CREATE INDEX "GV"."META_DBQL_TUSAGE_EHST_IDX" ON "GV"."META_DBQL_TABLE_USAGE_EXP_HST" ("QUERYID", "ACCTSTRINGDATE")

CREATE INDEX "GV"."DR_QRY_LOG_EXP_HST_IDX" ON "GV"."DR_QRY_LOG_EXP_HST" ("QUERYID", "ACCTSTRINGDATE")

这三个索引都是本地分区索引(都包含分区键acctstringdate)，很显然，DR_QRY_LOG_EXP_HST表少了个索引，因为它与表meta_dr_qry_log_tgt_all_hst 在statement_id上做join，因此应该在它的statement_id上也创建本地分区索引如下：

create index DR_QRY_LOG_EXP_HST_IDX2 on gv.DR_QRY_LOG_EXP_HST (statement_id,ACCTSTRINGDATE) local;

性能对比新的执行计划如下：

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

| Id  | Operation                              | Name                          | Rows  | Bytes | Cost  | Pstart| Pstop |

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

|   0 | SELECT STATEMENT                       |                               |     1 |   159 |  4838 |       |       |

|   1 |  SORT UNIQUE                           |                               |     1 |   159 |  4838 |       |       |

|*  2 |   TABLE ACCESS BY LOCAL INDEX ROWID    | META_DBQL_TABLE_USAGE_EXP_HST |     1 |    67 |     3 |       |       |

|   3 |    NESTED LOOPS                        |                               |     1 |   159 |  4816 |       |       |

|   4 |     NESTED LOOPS                       |                               |    18 |  1656 |  4762 |       |       |

|   5 |      PARTITION RANGE ITERATOR          |                               |     1 |    56 |  4746 |   KEY |    14 |

|*  6 |       TABLE ACCESS FULL                | META_DR_QRY_LOG_TGT_ALL_HST   |     1 |    56 |  4746 |   KEY |    14 |

|   7 |      PARTITION RANGE ITERATOR          |                               |    18 |   648 |    16 |   KEY |    14 |

|*  8 |       TABLE ACCESS BY LOCAL INDEX ROWID| DR_QRY_LOG_EXP_HST            |    18 |   648 |    16 |   KEY |    14 |

|*  9 |        INDEX RANGE SCAN                | DR_QRY_LOG_EXP_HST_IDX2       |    31 |       |    15 |   KEY |    14 |

|  10 |     PARTITION RANGE ITERATOR           |                               |     1 |       |     2 |   KEY |   KEY |

|* 11 |      INDEX RANGE SCAN                  | META_DBQL_TUSAGE_EHST_IDX     |     1 |       |     2 |   KEY |   KEY |

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

Predicate Information (identified by operation id):

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

2 - filter((UPPER("TGT"."SYSTEMNAME")<>UPPER("SRC"."SYSTEMNAME") OR

UPPER("TGT"."DATABASENAME")<>UPPER("SRC"."DATABASENAME") OR UPPER("TGT"."TABLENAME")<>UPPER("SRC"."TABLENAME"))

AND "SRC"."SYSTEMNAME"="RL"."SYSTEMNAME")

6 - filter(UPPER("TGT"."SYSTEMNAME")='MOZART' AND UPPER("TGT"."DATABASENAME")='GDW_TABLES' AND

UPPER("TGT"."TABLENAME")='SSA_SLNG_LSTG_MTRC_SD' AND "TGT"."ACCTSTRINGDATE">SYSDATE@!-30 AND "TGT"."SYSTEMNAME"

IS NOT NULL AND "TGT"."DATABASENAME" IS NOT NULL AND "TGT"."TABLENAME" IS NOT NULL)

8 - filter("RL"."SYSTEMNAME"="TGT"."SYSTEMNAME")

9 - access("RL"."STATEMENT_ID"="TGT"."STATEMENT_ID" AND "RL"."ACCTSTRINGDATE">SYSDATE@!-30 AND

"RL"."ACCTSTRINGDATE" IS NOT NULL)

11 - access("SRC"."QUERYID"="RL"."QUERYID" AND "SRC"."ACCTSTRINGDATE"="RL"."ACCTSTRINGDATE")

filter("SRC"."ACCTSTRINGDATE"="TGT"."ACCTSTRINGDATE" AND "SRC"."ACCTSTRINGDATE">SYSDATE@!-30)

从新的的执行计划可以看出，它的第一个NESTED LOOP果然用了最新创建的索引。

下面是执行时间：

已用时间:  00: 00: 02.16

两秒种搞定，远远超出他期望的5s ：)

方法总结NESTED LOOP高效的条件：驱动数据源有限，且被驱动表在连接列上有相应的索引。