--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;
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 |
-------------------------------------------------------------------------------------
| 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")
---Nested Loops Join的t2表先访问的情况
alter session set statistics_level=all;
SELECT /*+ leading(t2) use_nl(t1)*/ *
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 |00:00:01.46 | 701K|
| 1 | NESTED LOOPS | | 1 | 1 | 1 |00:00:01.46 | 701K|
| 2 | TABLE ACCESS FULL| T2 | 1 | 100K| 100K|00:00:00.02 | 1006 |
|* 3 | TABLE ACCESS FULL| T1 | 100K| 1 | 1 |00:00:01.40 | 700K|
-------------------------------------------------------------------------------------
3 - filter(("T1"."N"=19 AND "T1"."ID"="T2"."T1_ID"))
--发现性能有巨大差异！