昨天同事参加了一个研讨会,有提到一个案例。一个通过dblink查询远端数据库，原来查询很快，但是远端数据库增加了一个索引之后，查询一下子变慢了。

经过分析，发现那个通过dblink的查询语句，查询远端数据库的时候，是走索引的，但是远端数据库添加索引之后，如果索引的个数超过20个，就会忽略第一个建立的索引，如果查询语句恰好用到了第一个建立的索引，被忽略之后，只能走Full Table Scan了。

听了这个案例，我查了一下，在oracle官方文档中，关于Managing a Distributed Database有一段话：

Several performance restrictions relate to access of remote objects:

Remote views do not have statistical data.
Queries on partitioned tables may not be optimized.
No more than 20 indexes are considered for a remote table.
No more than 20 columns are used for a composite index.

说到，如果远程数据库使用超过20个索引，这些索引将不被考虑。这段话，在oracle 9i起的文档中就已经存在，一直到12.2还有。

那么，超过20个索引，是新的索引被忽略了？还是老索引被忽略了？如何让被忽略的索引让oracle意识到?我们来测试一下。
（本文基于12.1.0.2的远程库和12.2.0.1的本地库进行测试，如果对测试过程没兴趣的，可以直接拉到文末看“综上”部分）

（一）初始化测试表：

--创建远程表：DROP TABLE t_remote; CREATE TABLE t_remote (col01 NUMBER,col02 NUMBER,col03 VARCHAR2(50),col04 NUMBER,col05 NUMBER,col06 VARCHAR2(50),col07 NUMBER,col08 NUMBER,col09 VARCHAR2(50),col10 NUMBER,col11 NUMBER,col12 VARCHAR2(50),col13 NUMBER,col14 NUMBER,col15 VARCHAR2(50),col16 NUMBER,col17 NUMBER,col18 VARCHAR2(50),col19 NUMBER,col20 NUMBER,col21 VARCHAR2(50),col22 NUMBER,col23 NUMBER,col24 VARCHAR2(50),col25 NUMBER,col26 NUMBER,col27 VARCHAR2(50));alter table t_remote modify (col01 not null);INSERT INTO t_remoteSELECTrownum, rownum, rpad('*',50,'*'),rownum, rownum, rpad('*',50,'*'),rownum, rownum, rpad('*',50,'*'),rownum, rownum, rpad('*',50,'*'),rownum, rownum, rpad('*',50,'*'),rownum, rownum, rpad('*',50,'*'),rownum, rownum, rpad('*',50,'*'),rownum, rownum, rpad('*',50,'*'),rownum, rownum, rpad('*',50,'*')FROM dualCONNECT BY level <= 10000;commit; create unique index t_remote_i01_pk on t_remote (col01);alter table t_remote add (constraint t_remote_i01_pk primary key (col01) using index t_remote_i01_pk);create index t_remote_i02 on t_remote (col02);create index t_remote_i03 on t_remote (col03);create index t_remote_i04 on t_remote (col04);create index t_remote_i05 on t_remote (col05);create index t_remote_i06 on t_remote (col06);create index t_remote_i07 on t_remote (col07);create index t_remote_i08 on t_remote (col08);create index t_remote_i09 on t_remote (col09);create index t_remote_i10 on t_remote (col10);create index t_remote_i11 on t_remote (col11);create index t_remote_i12 on t_remote (col12);create index t_remote_i13 on t_remote (col13);create index t_remote_i14 on t_remote (col14);create index t_remote_i15 on t_remote (col15);create index t_remote_i16 on t_remote (col16);create index t_remote_i17 on t_remote (col17);create index t_remote_i18 on t_remote (col18);create index t_remote_i19 on t_remote (col19);create index t_remote_i20 on t_remote (col20); exec dbms_stats.gather_table_stats(user,'T_REMOTE');--创建本地表：drop table t_local; CREATE TABLE t_local (col01 NUMBER,col02 NUMBER,col03 VARCHAR2(50),col04 NUMBER,col05 NUMBER,col06 VARCHAR2(50)); INSERT INTO t_localSELECTrownum, rownum, rpad('*',50,'*'),rownum, rownum, rpad('*',50,'*')FROM dualCONNECT BY level <= 50; COMMIT; create index t_local_i01 on t_local (col01);create index t_local_i02 on t_local (col02);create index t_local_i03 on t_local (col03);create index t_local_i04 on t_local (col04);create index t_local_i05 on t_local (col05);create index t_local_i06 on t_local (col06); exec dbms_stats.gather_table_stats(user,'t_local'); create database link dblink_remote CONNECT TO test IDENTIFIED BY test USING 'ora121'; SQL> select host_name from v$instance@dblink_remote; HOST_NAME----------------------------------------------------------------testdb2 SQL> select host_name from v$instance; HOST_NAME----------------------------------------------------------------testdb10 SQL>

可以看到，远程表有27个字段，目前还只是在前20个字段建立了索引，且第一个字段是主键。本地表，有6个字段，6个字段都建索引。

（二）第一轮测试，远程表上有20个索引。

测试场景1：

在远程表20索引的情况下，本地表和远程表关联，用本地表的第一个字段关联远程表的第一个字段：

select l.col06,l.col05,l.col04,r.col27, r.col26,r.col25from t_local l, t_remote@dblink_remote rwhere l.col01=r.col01;select * from table( dbms_xplan.display_cursor(null, null, 'typical LAST') );PLAN_TABLE_OUTPUT---------------------------------------------------------------------------------------------------------SQL_ID 04schqc3d9rgm, child number 0-------------------------------------select l.col06,l.col05,l.col04,r.col27, r.col26,r.col25 from t_local l,t_remote@dblink_remote r where l.col01=r.col01Plan hash value: 631452043-----------------------------------------------------------------------------------------------| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Inst |IN-OUT|-----------------------------------------------------------------------------------------------| 0 | SELECT STATEMENT | | | | 53 (100)| | | || 1 | NESTED LOOPS | | 50 | 6300 | 53 (0)| 00:00:01 | | || 2 | TABLE ACCESS FULL| T_LOCAL | 50 | 3000 | 3 (0)| 00:00:01 | | || 3 | REMOTE | T_REMOTE | 1 | 66 | 1 (0)| 00:00:01 | DBLIN~ | R->S |-----------------------------------------------------------------------------------------------Remote SQL Information (identified by operation id):---------------------------------------------------- 3 - SELECT "COL01","COL25","COL26","COL27" FROM "T_REMOTE" "R" WHERE :1="COL01" (accessing 'DBLINK_REMOTE' ) 23 rows selected.SQL> -- 我们这里注意一下，WHERE :1="COL01"的存在，正是因为这个条件，所以在远程是走了主键而不是全表扫。我们把这个语句带入到远程执行。远程：SQL> explain plan for 2 SELECT "COL01","COL25","COL26","COL27" FROM "T_REMOTE" "R" WHERE :1="COL01";PLAN_TABLE_OUTPUT------------------------------------------------------------------------------------------------------------Plan hash value: 829680338-----------------------------------------------------------------------------------------------| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |-----------------------------------------------------------------------------------------------| 0 | SELECT STATEMENT | | 1 | 63 | 2 (0)| 00:00:01 || 1 | TABLE ACCESS BY INDEX ROWID| T_REMOTE | 1 | 63 | 2 (0)| 00:00:01 ||* 2 | INDEX UNIQUE SCAN | T_REMOTE_I01_PK | 1 | | 1 (0)| 00:00:01 |-----------------------------------------------------------------------------------------------Predicate Information (identified by operation id):PLAN_TABLE_OUTPUT--------------------------------------------------------------------------------------------------------------------------------------------------------------- 2 - access("COL01"=TO_NUMBER(:1))14 rows selected.

我们可以看到，对于远程表的执行计划，这是走主键的。

测试场景2：

在远程表20索引的情况下，本地表和远程表关联，用本地表的第一个字段关联远程表的第20个字段：

select l.col06,l.col05,l.col04,r.col27, r.col26,r.col25from t_local l, t_remote@dblink_remote rwhere l.col01=r.col20;select * from table( dbms_xplan.display_cursor(null, null, 'typical LAST') );PLAN_TABLE_OUTPUT---------------------------------------------------------------------------------------------------------SQL_ID 5rwtbwcnv0tsm, child number 0-------------------------------------select l.col06,l.col05,l.col04,r.col27, r.col26,r.col25 from t_local l,t_remote@dblink_remote r where l.col01=r.col20Plan hash value: 631452043-----------------------------------------------------------------------------------------------| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Inst |IN-OUT|-----------------------------------------------------------------------------------------------| 0 | SELECT STATEMENT | | | | 103 (100)| | | || 1 | NESTED LOOPS | | 50 | 6300 | 103 (0)| 00:00:01 | | || 2 | TABLE ACCESS FULL| T_LOCAL | 50 | 3000 | 3 (0)| 00:00:01 | | || 3 | REMOTE | T_REMOTE | 1 | 66 | 2 (0)| 00:00:01 | DBLIN~ | R->S |-----------------------------------------------------------------------------------------------Remote SQL Information (identified by operation id):---------------------------------------------------- 3 - SELECT "COL20","COL25","COL26","COL27" FROM "T_REMOTE" "R" WHERE :1="COL20" (accessing 'DBLINK_REMOTE' ) 23 rows selected.SQL> 远程：PLAN_TABLE_OUTPUT------------------------------------------------------------------------------------------------------------Plan hash value: 3993494813----------------------------------------------------------------------------------------------------| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |----------------------------------------------------------------------------------------------------| 0 | SELECT STATEMENT | | 1 | 63 | 2 (0)| 00:00:01 || 1 | TABLE ACCESS BY INDEX ROWID BATCHED| T_REMOTE | 1 | 63 | 2 (0)| 00:00:01 ||* 2 | INDEX RANGE SCAN | T_REMOTE_I20 | 1 | | 1 (0)| 00:00:01 |----------------------------------------------------------------------------------------------------Predicate Information (identified by operation id):PLAN_TABLE_OUTPUT--------------------------------------------------------------------------------------------------------------------------------------------------------------- 2 - access("COL20"=TO_NUMBER(:1))14 rows selected.SQL>

我们可以看到，对于远程表的执行计划，这是走索引范围扫描的。

测试场景3：

在远程表20索引的情况下，本地表和远程表关联，用本地表的第2个字段关联远程表的第2个字段：

select l.col06,l.col05,l.col04,r.col27, r.col26,r.col25from t_local l, t_remote@dblink_remote rwhere l.col02=r.col02;select * from table( dbms_xplan.display_cursor(null, null, 'typical LAST') );PLAN_TABLE_OUTPUT---------------------------------------------------------------------------------------------------------SQL_ID 81ctrx5huhfvq, child number 0-------------------------------------select l.col06,l.col05,l.col04,r.col27, r.col26,r.col25 from t_local l,t_remote@dblink_remote r where l.col02=r.col02Plan hash value: 631452043-----------------------------------------------------------------------------------------------| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Inst |IN-OUT|-----------------------------------------------------------------------------------------------| 0 | SELECT STATEMENT | | | | 103 (100)| | | || 1 | NESTED LOOPS | | 50 | 6300 | 103 (0)| 00:00:01 | | || 2 | TABLE ACCESS FULL| T_LOCAL | 50 | 3000 | 3 (0)| 00:00:01 | | || 3 | REMOTE | T_REMOTE | 1 | 66 | 2 (0)| 00:00:01 | DBLIN~ | R->S |-----------------------------------------------------------------------------------------------Remote SQL Information (identified by operation id):---------------------------------------------------- 3 - SELECT "COL02","COL25","COL26","COL27" FROM "T_REMOTE" "R" WHERE :1="COL02" (accessing 'DBLINK_REMOTE' ) 23 rows selected.SQL> 远程：SQL> explain plan for 2 SELECT "COL02","COL25","COL26","COL27" FROM "T_REMOTE" "R" WHERE :1="COL02";Explained.SQL> select * from table(dbms_xplan.display());PLAN_TABLE_OUTPUT------------------------------------------------------------------------------------------------------------Plan hash value: 2505594687----------------------------------------------------------------------------------------------------| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |----------------------------------------------------------------------------------------------------| 0 | SELECT STATEMENT | | 1 | 63 | 2 (0)| 00:00:01 || 1 | TABLE ACCESS BY INDEX ROWID BATCHED| T_REMOTE | 1 | 63 | 2 (0)| 00:00:01 ||* 2 | INDEX RANGE SCAN | T_REMOTE_I02 | 1 | | 1 (0)| 00:00:01 |----------------------------------------------------------------------------------------------------Predicate Information (identified by operation id):PLAN_TABLE_OUTPUT--------------------------------------------------------------------------------------------------------------------------------------------------------------- 2 - access("COL02"=TO_NUMBER(:1))14 rows selected.SQL>

我们可以看到，对于远程表的执行计划，这是走索引范围扫描的。

测试场景4：

在远程表20索引的情况下，本地表和远程表关联，用本地表的第2个字段关联远程表的第20个字段：

select l.col06,l.col05,l.col04,r.col27, r.col26,r.col25from t_local l, t_remote@dblink_remote rwhere l.col02=r.col20;select * from table( dbms_xplan.display_cursor(null, null, 'typical LAST') );PLAN_TABLE_OUTPUT---------------------------------------------------------------------------------------------------------SQL_ID 407pxjh9mgbry, child number 0-------------------------------------select l.col06,l.col05,l.col04,r.col27, r.col26,r.col25 from t_local l,t_remote@dblink_remote r where l.col02=r.col20Plan hash value: 631452043-----------------------------------------------------------------------------------------------| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Inst |IN-OUT|-----------------------------------------------------------------------------------------------| 0 | SELECT STATEMENT | | | | 103 (100)| | | || 1 | NESTED LOOPS | | 50 | 6300 | 103 (0)| 00:00:01 | | || 2 | TABLE ACCESS FULL| T_LOCAL | 50 | 3000 | 3 (0)| 00:00:01 | | || 3 | REMOTE | T_REMOTE | 1 | 66 | 2 (0)| 00:00:01 | DBLIN~ | R->S |-----------------------------------------------------------------------------------------------Remote SQL Information (identified by operation id):---------------------------------------------------- 3 - SELECT "COL20","COL25","COL26","COL27" FROM "T_REMOTE" "R" WHERE :1="COL20" (accessing 'DBLINK_REMOTE' )23 rows selected.SQL> 远程：SQL> explain plan for 2 SELECT "COL20","COL25","COL26","COL27" FROM "T_REMOTE" "R" WHERE :1="COL20";Explained.SQL> select * from table(dbms_xplan.display());PLAN_TABLE_OUTPUT------------------------------------------------------------------------------------------------------------Plan hash value: 3993494813----------------------------------------------------------------------------------------------------| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |----------------------------------------------------------------------------------------------------| 0 | SELECT STATEMENT | | 1 | 63 | 2 (0)| 00:00:01 || 1 | TABLE ACCESS BY INDEX ROWID BATCHED| T_REMOTE | 1 | 63 | 2 (0)| 00:00:01 ||* 2 | INDEX RANGE SCAN | T_REMOTE_I20 | 1 | | 1 (0)| 00:00:01 |----------------------------------------------------------------------------------------------------Predicate Information (identified by operation id):PLAN_TABLE_OUTPUT--------------------------------------------------------------------------------------------------------------------------------------------------------------- 2 - access("COL20"=TO_NUMBER(:1))14 rows selected.SQL>

我们可以看到，对于远程表的执行计划，这是走索引范围扫描的。

（三）建立第21个索引：

create index t_remote_i21 on t_remote (col21);exec dbms_stats.gather_table_stats(user,'T_REMOTE');

（四）远程表上现在有21个索引，重复上面4个测试：

测试场景1：

PLAN_TABLE_OUTPUT---------------------------------------------------------------------------------------------------------SQL_ID 04schqc3d9rgm, child number 1-------------------------------------select l.col06,l.col05,l.col04,r.col27, r.col26,r.col25 from t_local l,t_remote@dblink_remote r where l.col01=r.col01Plan hash value: 830255788-----------------------------------------------------------------------------------------------| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Inst |IN-OUT|-----------------------------------------------------------------------------------------------| 0 | SELECT STATEMENT | | | | 156 (100)| | | ||* 1 | HASH JOIN | | 50 | 6300 | 156 (0)| 00:00:01 | | || 2 | TABLE ACCESS FULL| T_LOCAL | 50 | 3000 | 3 (0)| 00:00:01 | | || 3 | REMOTE | T_REMOTE | 10000 | 644K| 153 (0)| 00:00:01 | DBLIN~ | R->S |-----------------------------------------------------------------------------------------------Predicate Information (identified by operation id):--------------------------------------------------- 1 - access("L"."COL01"="R"."COL01")Remote SQL Information (identified by operation id):---------------------------------------------------- 3 - SELECT "COL01","COL25","COL26","COL27" FROM "T_REMOTE" "R" (accessing 'DBLINK_REMOTE' ) 28 rows selected.SQL>--我们看到，这里已经没有了之前的 WHERE :1="COL01",即使不带入到远程看执行计划，我们也可以猜到它是全表扫。远程：SQL> explain plan for 2 SELECT "COL01","COL25","COL26","COL27" FROM "T_REMOTE" "R";Explained.SQL> select * from table(dbms_xplan.display());PLAN_TABLE_OUTPUT------------------------------------------------------------------------------------------------------------Plan hash value: 4187688566------------------------------------------------------------------------------| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |------------------------------------------------------------------------------| 0 | SELECT STATEMENT | | 10000 | 615K| 238 (0)| 00:00:01 || 1 | TABLE ACCESS FULL| T_REMOTE | 10000 | 615K| 238 (0)| 00:00:01 |------------------------------------------------------------------------------8 rows selected.SQL>

我们可以看到，对于远程表的执行计划，如果关联条件是远程表的第一个字段，第一个字段上的索引是被忽略的，执行计划是选择全表扫描的。

测试场景2：

PLAN_TABLE_OUTPUT---------------------------------------------------------------------------------------------------------SQL_ID 5rwtbwcnv0tsm, child number 1-------------------------------------select l.col06,l.col05,l.col04,r.col27, r.col26,r.col25 from t_local l,t_remote@dblink_remote r where l.col01=r.col20Plan hash value: 631452043-----------------------------------------------------------------------------------------------| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Inst |IN-OUT|-----------------------------------------------------------------------------------------------| 0 | SELECT STATEMENT | | | | 103 (100)| | | || 1 | NESTED LOOPS | | 50 | 6300 | 103 (0)| 00:00:01 | | || 2 | TABLE ACCESS FULL| T_LOCAL | 50 | 3000 | 3 (0)| 00:00:01 | | || 3 | REMOTE | T_REMOTE | 1 | 66 | 2 (0)| 00:00:01 | DBLIN~ | R->S |-----------------------------------------------------------------------------------------------Remote SQL Information (identified by operation id):---------------------------------------------------- 3 - SELECT "COL20","COL25","COL26","COL27" FROM "T_REMOTE" "R" WHERE :1="COL20" (accessing 'DBLINK_REMOTE' ) 23 rows selected.SQL> 远程：SQL> explain plan for 2 SELECT "COL20","COL25","COL26","COL27" FROM "T_REMOTE" "R" WHERE :1="COL20";Explained.SQL> select * from table(dbms_xplan.display());PLAN_TABLE_OUTPUT------------------------------------------------------------------------------------------------------------Plan hash value: 3993494813----------------------------------------------------------------------------------------------------| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |----------------------------------------------------------------------------------------------------| 0 | SELECT STATEMENT | | 1 | 63 | 2 (0)| 00:00:01 || 1 | TABLE ACCESS BY INDEX ROWID BATCHED| T_REMOTE | 1 | 63 | 2 (0)| 00:00:01 ||* 2 | INDEX RANGE SCAN | T_REMOTE_I20 | 1 | | 1 (0)| 00:00:01 |----------------------------------------------------------------------------------------------------Predicate Information (identified by operation id):PLAN_TABLE_OUTPUT--------------------------------------------------------------------------------------------------------------------------------------------------------------- 2 - access("COL20"=TO_NUMBER(:1))14 rows selected.SQL>

我们可以看到，对于远程表的执行计划，如果关联条件是远程表的第20个字段，这第20个字段上的索引是没有被忽略的，执行计划是走索引。

测试场景3：

PLAN_TABLE_OUTPUT---------------------------------------------------------------------------------------------------------SQL_ID 81ctrx5huhfvq, child number 1-------------------------------------select l.col06,l.col05,l.col04,r.col27, r.col26,r.col25 from t_local l,t_remote@dblink_remote r where l.col02=r.col02Plan hash value: 631452043-----------------------------------------------------------------------------------------------| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Inst |IN-OUT|-----------------------------------------------------------------------------------------------| 0 | SELECT STATEMENT | | | | 103 (100)| | | || 1 | NESTED LOOPS | | 50 | 6300 | 103 (0)| 00:00:01 | | || 2 | TABLE ACCESS FULL| T_LOCAL | 50 | 3000 | 3 (0)| 00:00:01 | | || 3 | REMOTE | T_REMOTE | 1 | 66 | 2 (0)| 00:00:01 | DBLIN~ | R->S |-----------------------------------------------------------------------------------------------Remote SQL Information (identified by operation id):---------------------------------------------------- 3 - SELECT "COL02","COL25","COL26","COL27" FROM "T_REMOTE" "R" WHERE :1="COL02" (accessing 'DBLINK_REMOTE' ) 23 rows selected.SQL> 远程：SQL> explain plan for 2 SELECT "COL02","COL25","COL26","COL27" FROM "T_REMOTE" "R" WHERE :1="COL02";Explained.SQL> select * from table(dbms_xplan.display());PLAN_TABLE_OUTPUT------------------------------------------------------------------------------------------------------------Plan hash value: 2505594687----------------------------------------------------------------------------------------------------| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |----------------------------------------------------------------------------------------------------| 0 | SELECT STATEMENT | | 1 | 63 | 2 (0)| 00:00:01 || 1 | TABLE ACCESS BY INDEX ROWID BATCHED| T_REMOTE | 1 | 63 | 2 (0)| 00:00:01 ||* 2 | INDEX RANGE SCAN | T_REMOTE_I02 | 1 | | 1 (0)| 00:00:01 |----------------------------------------------------------------------------------------------------Predicate Information (identified by operation id):PLAN_TABLE_OUTPUT--------------------------------------------------------------------------------------------------------------------------------------------------------------- 2 - access("COL02"=TO_NUMBER(:1))14 rows selected.SQL>

我们可以看到，对于远程表的执行计划，如果关联条件是远程表的第2个字段，这第2个字段上的索引是没有被忽略的，执行计划是走索引。

测试场景4：

PLAN_TABLE_OUTPUT---------------------------------------------------------------------------------------------------------SQL_ID 407pxjh9mgbry, child number 1-------------------------------------select l.col06,l.col05,l.col04,r.col27, r.col26,r.col25 from t_local l,t_remote@dblink_remote r where l.col02=r.col20Plan hash value: 631452043-----------------------------------------------------------------------------------------------| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Inst |IN-OUT|-----------------------------------------------------------------------------------------------| 0 | SELECT STATEMENT | | | | 103 (100)| | | || 1 | NESTED LOOPS | | 50 | 6300 | 103 (0)| 00:00:01 | | || 2 | TABLE ACCESS FULL| T_LOCAL | 50 | 3000 | 3 (0)| 00:00:01 | | || 3 | REMOTE | T_REMOTE | 1 | 66 | 2 (0)| 00:00:01 | DBLIN~ | R->S |-----------------------------------------------------------------------------------------------Remote SQL Information (identified by operation id):---------------------------------------------------- 3 - SELECT "COL20","COL25","COL26","COL27" FROM "T_REMOTE" "R" WHERE :1="COL20" (accessing 'DBLINK_REMOTE' ) 23 rows selected.SQL> 远程：SQL> explain plan for 2 SELECT "COL20","COL25","COL26","COL27" FROM "T_REMOTE" "R" WHERE :1="COL20";Explained.SQL> select * from table(dbms_xplan.display());PLAN_TABLE_OUTPUT------------------------------------------------------------------------------------------------------------Plan hash value: 3993494813----------------------------------------------------------------------------------------------------| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |----------------------------------------------------------------------------------------------------| 0 | SELECT STATEMENT | | 1 | 63 | 2 (0)| 00:00:01 || 1 | TABLE ACCESS BY INDEX ROWID BATCHED| T_REMOTE | 1 | 63 | 2 (0)| 00:00:01 ||* 2 | INDEX RANGE SCAN | T_REMOTE_I20 | 1 | | 1 (0)| 00:00:01 |----------------------------------------------------------------------------------------------------Predicate Information (identified by operation id):PLAN_TABLE_OUTPUT--------------------------------------------------------------------------------------------------------------------------------------------------------------- 2 - access("COL20"=TO_NUMBER(:1))14 rows selected.SQL>

我们可以看到，对于远程表的执行计划，如果关联条件是远程表的第20个字段，这第20个字段上的索引是没有被忽略的，执行计划是走索引。

我们目前可以总结到，当远程表第21个索引建立的时候，通过dblink关联本地表和远程表，如果关联条件是远程表的第1个建立的索引的字段，那么这个索引将被忽略，从而走全表扫描。如果关联条件是远程表的第2个建立索引的字段，则不受影响。

似乎是有效索引的窗口是20个，当新建第21个，那么第1个就被无视了。

（五）建立第22个索引，我们在来看看上述猜测是否符合。

create index t_remote_i22 on t_remote (col22);exec dbms_stats.gather_table_stats(user,'T_REMOTE');

（六），目前远程表有22个索引，重复上面4个测试：

测试场景1：

PLAN_TABLE_OUTPUT---------------------------------------------------------------------------------------------------------SQL_ID 04schqc3d9rgm, child number 2-------------------------------------select l.col06,l.col05,l.col04,r.col27, r.col26,r.col25 from t_local l,t_remote@dblink_remote r where l.col01=r.col01Plan hash value: 830255788-----------------------------------------------------------------------------------------------| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Inst |IN-OUT|-----------------------------------------------------------------------------------------------| 0 | SELECT STATEMENT | | | | 156 (100)| | | ||* 1 | HASH JOIN | | 50 | 6300 | 156 (0)| 00:00:01 | | || 2 | TABLE ACCESS FULL| T_LOCAL | 50 | 3000 | 3 (0)| 00:00:01 | | || 3 | REMOTE | T_REMOTE | 10000 | 644K| 153 (0)| 00:00:01 | DBLIN~ | R->S |-----------------------------------------------------------------------------------------------Predicate Information (identified by operation id):--------------------------------------------------- 1 - access("L"."COL01"="R"."COL01")Remote SQL Information (identified by operation id):---------------------------------------------------- 3 - SELECT "COL01","COL25","COL26","COL27" FROM "T_REMOTE" "R" (accessing 'DBLINK_REMOTE' ) 28 rows selected.SQL>

测试场景2：

PLAN_TABLE_OUTPUT---------------------------------------------------------------------------------------------------------SQL_ID 5rwtbwcnv0tsm, child number 2-------------------------------------select l.col06,l.col05,l.col04,r.col27, r.col26,r.col25 from t_local l,t_remote@dblink_remote r where l.col01=r.col20Plan hash value: 631452043-----------------------------------------------------------------------------------------------| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Inst |IN-OUT|-----------------------------------------------------------------------------------------------| 0 | SELECT STATEMENT | | | | 103 (100)| | | || 1 | NESTED LOOPS | | 50 | 6300 | 103 (0)| 00:00:01 | | || 2 | TABLE ACCESS FULL| T_LOCAL | 50 | 3000 | 3 (0)| 00:00:01 | | || 3 | REMOTE | T_REMOTE | 1 | 66 | 2 (0)| 00:00:01 | DBLIN~ | R->S |-----------------------------------------------------------------------------------------------Remote SQL Information (identified by operation id):---------------------------------------------------- 3 - SELECT "COL20","COL25","COL26","COL27" FROM "T_REMOTE" "R" WHERE :1="COL20" (accessing 'DBLINK_REMOTE' )23 rows selected.SQL>

测试场景3：

PLAN_TABLE_OUTPUT---------------------------------------------------------------------------------------------------------SQL_ID 81ctrx5huhfvq, child number 2-------------------------------------select l.col06,l.col05,l.col04,r.col27, r.col26,r.col25 from t_local l,t_remote@dblink_remote r where l.col02=r.col02Plan hash value: 830255788-----------------------------------------------------------------------------------------------| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Inst |IN-OUT|-----------------------------------------------------------------------------------------------| 0 | SELECT STATEMENT | | | | 156 (100)| | | ||* 1 | HASH JOIN | | 50 | 6300 | 156 (0)| 00:00:01 | | || 2 | TABLE ACCESS FULL| T_LOCAL | 50 | 3000 | 3 (0)| 00:00:01 | | || 3 | REMOTE | T_REMOTE | 10000 | 644K| 153 (0)| 00:00:01 | DBLIN~ | R->S |-----------------------------------------------------------------------------------------------Predicate Information (identified by operation id):--------------------------------------------------- 1 - access("L"."COL02"="R"."COL02")Remote SQL Information (identified by operation id):---------------------------------------------------- 3 - SELECT "COL02","COL25","COL26","COL27" FROM "T_REMOTE" "R" (accessing 'DBLINK_REMOTE' ) 28 rows selected.SQL>

测试场景4：

PLAN_TABLE_OUTPUT---------------------------------------------------------------------------------------------------------SQL_ID 407pxjh9mgbry, child number 2-------------------------------------select l.col06,l.col05,l.col04,r.col27, r.col26,r.col25 from t_local l,t_remote@dblink_remote r where l.col02=r.col20Plan hash value: 631452043-----------------------------------------------------------------------------------------------| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Inst |IN-OUT|-----------------------------------------------------------------------------------------------| 0 | SELECT STATEMENT | | | | 103 (100)| | | || 1 | NESTED LOOPS | | 50 | 6300 | 103 (0)| 00:00:01 | | || 2 | TABLE ACCESS FULL| T_LOCAL | 50 | 3000 | 3 (0)| 00:00:01 | | || 3 | REMOTE | T_REMOTE | 1 | 66 | 2 (0)| 00:00:01 | DBLIN~ | R->S |-----------------------------------------------------------------------------------------------Remote SQL Information (identified by operation id):---------------------------------------------------- 3 - SELECT "COL20","COL25","COL26","COL27" FROM "T_REMOTE" "R" WHERE :1="COL20" (accessing 'DBLINK_REMOTE' )23 rows selected.SQL>

上述的测试，其实是可以验证我们的猜测的。oracle对于通过dblink关联访问远程表，只是会意识到最近创建的20个索引的字段。这个意识到索引的窗口是20个，一旦建立了一个新索引，那么最旧的一个索引会被无视。

（七）我们尝试rebuild索引，看看有没有效果：

rebuild第2个索引

alter index t_remote_i02 rebuild;exec dbms_stats.gather_table_stats(user,'T_REMOTE');

（八）在第2个索引rebuild之后，重复上面4个测试：

--测试场景1：PLAN_TABLE_OUTPUT---------------------------------------------------------------------------------------------------------SQL_ID 04schqc3d9rgm, child number 0-------------------------------------select l.col06,l.col05,l.col04,r.col27, r.col26,r.col25 from t_local l,t_remote@dblink_remote r where l.col01=r.col01Plan hash value: 830255788-----------------------------------------------------------------------------------------------| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Inst |IN-OUT|-----------------------------------------------------------------------------------------------| 0 | SELECT STATEMENT | | | | 156 (100)| | | ||* 1 | HASH JOIN | | 50 | 6300 | 156 (0)| 00:00:01 | | || 2 | TABLE ACCESS FULL| T_LOCAL | 50 | 3000 | 3 (0)| 00:00:01 | | || 3 | REMOTE | T_REMOTE | 10000 | 644K| 153 (0)| 00:00:01 | DBLIN~ | R->S |-----------------------------------------------------------------------------------------------Predicate Information (identified by operation id):--------------------------------------------------- 1 - access("L"."COL01"="R"."COL01")Remote SQL Information (identified by operation id):---------------------------------------------------- 3 - SELECT "COL01","COL25","COL26","COL27" FROM "T_REMOTE" "R" (accessing 'DBLINK_REMOTE' )28 rows selected.SQL> --测试场景2：PLAN_TABLE_OUTPUT---------------------------------------------------------------------------------------------------------SQL_ID 5rwtbwcnv0tsm, child number 0-------------------------------------select l.col06,l.col05,l.col04,r.col27, r.col26,r.col25 from t_local l,t_remote@dblink_remote r where l.col01=r.col20Plan hash value: 631452043-----------------------------------------------------------------------------------------------| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Inst |IN-OUT|-----------------------------------------------------------------------------------------------| 0 | SELECT STATEMENT | | | | 103 (100)| | | || 1 | NESTED LOOPS | | 50 | 6300 | 103 (0)| 00:00:01 | | || 2 | TABLE ACCESS FULL| T_LOCAL | 50 | 3000 | 3 (0)| 00:00:01 | | || 3 | REMOTE | T_REMOTE | 1 | 66 | 2 (0)| 00:00:01 | DBLIN~ | R->S |-----------------------------------------------------------------------------------------------Remote SQL I