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_remote
SELECT
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,'*'),
rownum, rownum, rpad('*',50,'*')
FROM dual
CONNECT 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_local
SELECT
rownum, rownum, rpad('*',50,'*'),
rownum, rownum, rpad('*',50,'*')
FROM dual
CONNECT 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.col25
from t_local l, t_remote@dblink_remote r
where 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.col01
Plan 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.col25
from t_local l, t_remote@dblink_remote r
where 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.col20
Plan 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.col25
from t_local l, t_remote@dblink_remote r
where 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.col02
Plan 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.col25
from t_local l, t_remote@dblink_remote r
where 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.col20
Plan 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.col01
Plan 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.col20
Plan 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.col02
Plan 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.col20
Plan 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.col01
Plan 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.col20 Plan 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.col02
Plan 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.col20 Plan 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.col01
Plan 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.col20
Plan 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 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.col02
Plan 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 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.col20
Plan 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>
所以我们看到，索引rebuild，是不能起到重新“唤醒”索引的作用。
（九）我们尝试 drop and recreate 第2个索引。
drop index t_remote_i02; create index t_remote_i02 on t_remote (col02); exec dbms_stats.gather_table_stats(user,'T_REMOTE');
（十）重复上面的测试3和测试4：
测试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.col02
Plan 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>
测试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.col20
Plan 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>
此时，其实我们可以预测，远程表此时col03上的索引是用不到的，我们来测试验证一下：
测试5：
PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------------------------------
SQL_ID bhkczcfrhvsuw, 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.col03=r.col03
Plan hash value: 830255788
-----------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Inst |IN-OUT|
-----------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 157 (100)| | | |
|* 1 | HASH JOIN | | 500K| 89M| 157 (1)| 00:00:01 | | |
| 2 | TABLE ACCESS FULL| T_LOCAL | 50 | 5400 | 3 (0)| 00:00:01 | | |
| 3 | REMOTE | T_REMOTE | 10000 | 781K| 153 (0)| 00:00:01 | DBLIN~ | R->S |
-----------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("L"."COL03"="R"."COL03")
Remote SQL Information (identified by operation id):
----------------------------------------------------
3 - SELECT "COL03","COL25","COL26","COL27" FROM "T_REMOTE" "R" (accessing
'DBLINK_REMOTE' )
28 rows selected.
SQL>
我们可以看到，通过drop之后再重建，是可以“唤醒”第二个索引的。这也证明了我们20个索引识别的移动窗口，是按照索引的创建时间来移动的。
综上：
1. 对于通过dblink关联本地表和远程表，如果远程表的索引个数少于20个，那么不受影响。
2. 对于通过dblink关联本地表和远程表，如果远程表的索引个数增加到21个或以上，那么oracle在执行远程操作的时候，将忽略最早创建的那个索引，但是会以20个为窗口移动，最新建立的索引会被意识到。此时如果查询的关联条件中，使用到最早创建的那个索引的字段，由于忽略了索引，会走全表扫描。
3. 要“唤醒”对原来索引的意识，rebuild索引无效，需要drop & create索引。
4. 在本地表数据量比较少，远程表的数据量很大，而索引数量超过20个，且关联条件的字段时最早索引的情况下，可以考虑使用DRIVING_SITE的hint，将本地表的数据全量到远程中，此时远程的关联查询可以意识到那个索引。可见文末的例子。是否使用hint，需要评估本地表数据全量推送到远程的成本，和远程表使用全表扫的成本。
附：在22个索引的情况下，尝试采用DRIVING_SITE的hint：
SQL> select l.col06,l.col05,l.col04,r.col27, r.col26,r.col25
2 from t_local l, t_remote@dblink_remote r
3 where l.col02=r.col02
4 ;
50 rows selected.
Elapsed: 00:00:00.03
Execution Plan
----------------------------------------------------------
Plan hash value: 830255788
-----------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Inst |IN-OUT|
-----------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 50 | 6300 | 156 (0)| 00:00:01 | | |
|* 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' )
Statistics
----------------------------------------------------------
151 recursive calls
0 db block gets
246 consistent gets
26 physical reads
0 redo size
2539 bytes sent via SQL*Net to client
641 bytes received via SQL*Net from client
5 SQL*Net roundtrips to/from client
10 sorts (memory)
0 sorts (disk)
50 rows processed
SQL>
--可以看到远程表示走全表扫。
SQL> select /*+DRIVING_SITE(r)*/ l.col06,l.col05,l.col04,r.col27, r.col26,r.col25
2 from t_local l, t_remote@dblink_remote r
3 where l.col02=r.col02
4 ;
50 rows selected.
Elapsed: 00:00:00.03
Execution Plan
----------------------------------------------------------
Plan hash value: 1716516160
-------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Inst |IN-OUT|
-------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT REMOTE | | 50 | 6450 | 103 (0)| 00:00:01 | | |
| 1 | NESTED LOOPS | | 50 | 6450 | 103 (0)| 00:00:01 | | |
| 2 | NESTED LOOPS | | 50 | 6450 | 103 (0)| 00:00:01 | | |
| 3 | REMOTE | T_LOCAL | 50 | 3300 | 3 (0)| 00:00:01 | ! | R->S |
|* 4 | INDEX RANGE SCAN | T_REMOTE_I02 | 1 | | 1 (0)| 00:00:01 | ORA12C | |
| 5 | TABLE ACCESS BY INDEX ROWID| T_REMOTE | 1 | 63 | 2 (0)| 00:00:01 | ORA12C | |
-------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
4 - access("A2"."COL02"="A1"."COL02")
Remote SQL Information (identified by operation id):
----------------------------------------------------
3 - SELECT "COL02","COL04","COL05","COL06" FROM "T_LOCAL" "A2" (accessing '!' )
Note
-----
- fully remote statement
- this is an adaptive plan
Statistics
----------------------------------------------------------
137 recursive calls
0 db block gets
213 consistent gets
25 physical reads
0 redo size
2940 bytes sent via SQL*Net to client
641 bytes received via SQL*Net from client
5 SQL*Net roundtrips to/from client
10 sorts (memory)
0 sorts (disk)
50 rows processed
SQL>
--可以看到本地表是走全表扫，但是远程表使用了第2个字段的索引。
总结
以上就是本文关于远程数据库的表超过20个索引的影响详细解析的全部内容，希望对大家有所帮助。感兴趣的朋友可以继续参阅本站：SQL提取数据库表名及字段名等信息代码示例、MySQL数据库表分区注意事项大全【推荐】等，有什么问题可以直接留言，小编会及时回复大家的。感谢朋友们对本站的支持！
原创文章，作者：306829225，如若转载，请注明出处：https://blog.ytso.com/236809.html