驱动表的限制条件有索引
/*
结论： Nested Loops Join连接优化，驱动表的限制条件有索引！
*/
--环境构造
--研究Nested Loops Join访问次数前准备工作
DROP TABLE t1 CASCADE CONSTRAINTS PURGE;
DROP TABLE t2 CASCADE CONSTRAINTS PURGE;
CREATE TABLE t1 (
id NUMBER NOT NULL,
n NUMBER,
contents VARCHAR2(4000)
)
;
CREATE TABLE t2 (
id NUMBER NOT NULL,
t1_id NUMBER NOT NULL,
n NUMBER,
contents VARCHAR2(4000)
)
;
execute dbms_random.seed(0);
INSERT INTO t1
SELECT rownum, rownum, dbms_random.string('a', 50)
FROM dual
CONNECT BY level <= 100
ORDER BY dbms_random.random;
INSERT INTO t2 SELECT rownum, rownum, rownum, dbms_random.string('b', 50) FROM dual CONNECT BY level <= 100000
ORDER BY dbms_random.random;
COMMIT;
select count(*) from t1;
select count(*) from t2;
--Nested Loops Join两表无索引试验
set linesize 1000
set autotrace off
alter session set statistics_level=all ;
SELECT /*+ leading(t1) use_nl(t2) */ *
FROM t1, t2
WHERE t1.id = t2.t1_id
AND t1.n = 19;
select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));
-------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers |
-------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 1 |00:00:00.01 | 1014 |
| 1 | NESTED LOOPS | | 1 | 1 | 1 |00:00:00.01 | 1014 |
|* 2 | TABLE ACCESS FULL| T1 | 1 | 1 | 1 |00:00:00.01 | 8 |
|* 3 | TABLE ACCESS FULL| T2 | 1 | 1 | 1 |00:00:00.01 | 1006 |
-------------------------------------------------------------------------------------
2 - filter("T1"."N"=19)
3 - filter("T1"."ID"="T2"."T1_ID")
----两表无索引场合如果不用HINT，一般走Hash Join
alter session set statistics_level=all ;
SELECT *
FROM t1, t2
WHERE t1.id = t2.t1_id
AND t1.n = 19;
select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));
----------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
----------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 1 |00:00:00.06 | 1013 | | | |
|* 1 | HASH JOIN | | 1 | 1 | 1 |00:00:00.06 | 1013 | 742K| 742K| 376K (0)|
|* 2 | TABLE ACCESS FULL| T1 | 1 | 1 | 1 |00:00:00.01 | 7 | | | |
| 3 | TABLE ACCESS FULL| T2 | 1 | 100K| 100K|00:00:00.02 | 1006 | | | |
----------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("T1"."ID"="T2"."T1_ID")
2 - filter("T1"."N"=19)
---对t1表的限制条件建索引
CREATE INDEX t1_n ON t1 (n);
---有了限制条件的索引，Nested Loops Join性能略有提升
set linesize 1000
alter session set statistics_level=all ;
SELECT /*+ leading(t1) use_nl(t2) */ *
FROM t1, t2
WHERE t1.id = t2.t1_id
AND t1.n = 19;
select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));
--------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | Reads |
--------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 1 |00:00:00.06 | 1009 | 1007 |
| 1 | NESTED LOOPS | | 1 | 1 | 1 |00:00:00.06 | 1009 | 1007 |
| 2 | TABLE ACCESS BY INDEX ROWID| T1 | 1 | 1 | 1 |00:00:00.01 | 3 | 6 |
|* 3 | INDEX RANGE SCAN | T1_N | 1 | 1 | 1 |00:00:00.01 | 2 | 1 |
|* 4 | TABLE ACCESS FULL | T2 | 1 | 1 | 1 |00:00:00.05 | 1006 | 1001 |
--------------------------------------------------------------------------------------------------------
3 - access("T1"."N"=19)
4 - filter("T1"."ID"="T2"."T1_ID")
---不过发现，增加了索引后Oracle不用HINT，还是走HASH连接。
alter session set statistics_level=all ;
SELECT *
FROM t1, t2
WHERE t1.id = t2.t1_id
AND t1.n = 19;
select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));
--------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
--------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 1 |00:00:00.05 | 1008 | | | |
|* 1 | HASH JOIN | | 1 | 1 | 1 |00:00:00.05 | 1008 | 742K| 742K| 350K (0)|
| 2 | TABLE ACCESS BY INDEX ROWID| T1 | 1 | 1 | 1 |00:00:00.01 | 2 | | | |
|* 3 | INDEX RANGE SCAN | T1_N | 1 | 1 | 1 |00:00:00.01 | 1 | | | |
| 4 | TABLE ACCESS FULL | T2 | 1 | 89127 | 100K|00:00:00.02 | 1006 | | | |
--------------------------------------------------------------------------------------------------------------------------
1 - access("T1"."ID"="T2"."T1_ID")
3 - access("T1"."N"=19)
/*
原因在于，这个T1表总记录也不过100条，所以用索引效果并没有很明显，如果这个T1表记录有几十万上百万条，那检索一条记录出来
，用索引效果就非常明显了！
*/
被驱动表限制条件有索引
/*
结论： 给Nested Loops Join连接优化，被驱动表的连接条件有索引！
*/
--环境构造
--研究Nested Loops Join访问次数前准备工作
DROP TABLE t1 CASCADE CONSTRAINTS PURGE;
DROP TABLE t2 CASCADE CONSTRAINTS PURGE;
CREATE TABLE t1 (
id NUMBER NOT NULL,
n NUMBER,
contents VARCHAR2(4000)
)
;
CREATE TABLE t2 (
id NUMBER NOT NULL,
t1_id NUMBER NOT NULL,
n NUMBER,
contents VARCHAR2(4000)
)
;
execute dbms_random.seed(0);
INSERT INTO t1
SELECT rownum, rownum, dbms_random.string('a', 50)
FROM dual
CONNECT BY level <= 100
ORDER BY dbms_random.random;
INSERT INTO t2 SELECT rownum, rownum, rownum, dbms_random.string('b', 50) FROM dual CONNECT BY level <= 100000
ORDER BY dbms_random.random;
COMMIT;
select count(*) from t1;
select count(*) from t2;
--（对驱动表t1表的限制条件建索引），如下
CREATE INDEX t1_n ON t1 (n);
--（对被驱动表t2表的连接条件建索引），如下：
CREATE INDEX t2_t1_id ON t2(t1_id);
----这下表连接性能有了大幅度提升
alter session set statistics_level=all ;
SELECT /*+ leading(t1) use_nl(t2) */ *
FROM t1, t2
WHERE t1.id = t2.t1_id
AND t1.n = 19;
select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));
----------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | Reads|
-----------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 1 |00:00:00.01 | 7 | 4 |
| 1 | NESTED LOOPS | | 1 | | 1 |00:00:00.01 | 7 | 4 |
| 2 | NESTED LOOPS | | 1 | 1 | 1 |00:00:00.01 | 6 | 4 |
| 3 | TABLE ACCESS BY INDEX ROWID| T1 | 1 | 1 | 1 |00:00:00.01 | 3 | 0 |
|* 4 | INDEX RANGE SCAN | T1_N | 1 | 1 | 1 |00:00:00.01 | 2 | 0 |
|* 5 | INDEX RANGE SCAN | T2_T1_ID | 1 | 1 | 1 |00:00:00.01 | 3 | 4 |
| 6 | TABLE ACCESS BY INDEX ROWID | T2 | 1 | 1 | 1 |00:00:00.01 | 1 | 0 |
-----------------------------------------------------------------------------------------------------------
4 - access("T1"."N"=19)
5 - access("T1"."ID"="T2"."T1_ID")
--性能有了大幅度提升，BUFFERS居然只有7
---增加了索引后Oracle不用HINT，终于自己去选择Nested Loops Join
alter session set statistics_level=all ;
SELECT *
FROM t1, t2
WHERE t1.id = t2.t1_id
AND t1.n = 19;
select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));
----------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers |
----------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 1 |00:00:00.01 | 7 |
| 1 | NESTED LOOPS | | 1 | | 1 |00:00:00.01 | 7 |
| 2 | NESTED LOOPS | | 1 | 1 | 1 |00:00:00.01 | 6 |
| 3 | TABLE ACCESS BY INDEX ROWID| T1 | 1 | 1 | 1 |00:00:00.01 | 3 |
|* 4 | INDEX RANGE SCAN | T1_N | 1 | 1 | 1 |00:00:00.01 | 2 |
|* 5 | INDEX RANGE SCAN | T2_T1_ID | 1 | 1 | 1 |00:00:00.01 | 3 |
| 6 | TABLE ACCESS BY INDEX ROWID | T2 | 1 | 1 | 1 |00:00:00.01 | 1 |
----------------------------------------------------------------------------------------------------
4 - access("T1"."N"=19)
5 - access("T1"."ID"="T2"."T1_ID")