[20180713]关于hash join 测试中一个疑问.txt

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[20180713]关于hash join 测试中一个疑问.txt --//上个星期做的测试,链接: http://blog.itpub.net/267265/viewspace-2157424/ --//前几天在家里12c上重复测试,才发现自己没注意细节问题. 1.环境: SCOTT@test01p> @ ver1 PORT_STRING VERSION BANNER CON_ID ------------------------------ -------------- -------------------------------------------------------------------------------- ---------- IBMPC/WIN_NT64-9.1.0 12.1.0.1.0 Oracle Database 12c Enterprise Edition Release 12.1.0.1.0 - 64bit Production 0 create table t1 as select level id ,'t1'||to_char(level) name from dual connect by level<=4; create table t2 as select level+1 id ,'t2'||to_char(level) name from dual connect by level<=4; insert into t1 values (null,'t1null'); insert into t2 values (null,'t2null'); commit ; --//分析略. insert into t2 select rownum+4 ,'t2'||to_char(rownum+4) from dual connect by level<=10000; commit; SCOTT@test01p> select rowid,t2.* from t2 where id<=4 or id is null; ROWID ID NAME ------------------ ---------- -------------------- AAAaT5AAJAAAADLAAA 2 t21 AAAaT5AAJAAAADLAAB 3 t22 AAAaT5AAJAAAADLAAC 4 t23 AAAaT5AAJAAAADOAAA t2null --//我自己一直以为执行insert into t2 values (null,'t2null');应该插入的数据块与id=2的数据块一样,实际情况不同. SCOTT@test01p> @ rowid AAAaT5AAJAAAADLAAA OBJECT FILE BLOCK ROW ROWID_DBA DBA TEXT ---------- ---------- ---------- ---------- -------------------- -------------------- ---------------------------------------- 107769 9 203 0 0x24000CB 9,203 alter system dump datafile 9 block 203 ; SCOTT@test01p> @ rowid AAAaT5AAJAAAADOAAA OBJECT FILE BLOCK ROW ROWID_DBA DBA TEXT ---------- ---------- ---------- ---------- -------------------- -------------------- ---------------------------------------- 107769 9 206 0 0x24000CE 9,206 alter system dump datafile 9 block 206 ; --//实际上ctas插入的第一块紧接着表段HEADER_BLOCK. SCOTT@test01p> select SEGMENT_NAME,SEGMENT_TYPE,HEADER_FILE,HEADER_BLOCK from dba_segments where owner=user and segment_name='T2'; SEGMENT_NAME SEGMENT_TYPE HEADER_FILE HEADER_BLOCK -------------------- -------------------- ----------- ------------ T2 TABLE 9 202 2.这样就很好解析我前面遇到的情况: SCOTT@test01p> alter session set statistics_level=all; Session altered. SCOTT@test01p> select * from t1 where id not in (select id from t2 ) and id is not null; no rows selected SCOTT@test01p> @ dpc '' '' PLAN_TABLE_OUTPUT ------------------------------------- SQL_ID 86bz316n141w9, child number 0 ------------------------------------- select * from t1 where id not in (select id from t2 ) and id is not null Plan hash value: 1275484728 ------------------------------------------------------------------------------------------------------------------------------------------------ | Id | Operation | Name | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem | ------------------------------------------------------------------------------------------------------------------------------------------------ | 0 | SELECT STATEMENT | | 1 | | | 14 (100)| | 0 |00:00:00.01 | 14 | | | | |* 1 | HASH JOIN ANTI NA | | 1 | 1 | 12 | 14 (0)| 00:00:01 | 0 |00:00:00.01 | 14 | 1888K| 1888K| 1090K (0)| |* 2 | TABLE ACCESS FULL| T1 | 1 | 4 | 32 | 4 (0)| 00:00:01 | 4 |00:00:00.01 | 7 | | | | | 3 | TABLE ACCESS FULL| T2 | 1 | 10005 | 40020 | 10 (0)| 00:00:01 | 956 |00:00:00.01 | 7 | | | | ------------------------------------------------------------------------------------------------------------------------------------------------ --//表T2做全表扫描buffers=7.而实际全表扫描逻辑读31.而且实际读T2记录数是956. SCOTT@test01p> select count(*) from t2 ; COUNT(*) ---------- 10005 SCOTT@test01p> @ dpc '' '' PLAN_TABLE_OUTPUT ------------------------------------- SQL_ID 34jwra7jk76u5, child number 0 ------------------------------------- select count(*) from t2 Plan hash value: 3321871023 ------------------------------------------------------------------------------------------------------------- | Id | Operation | Name | Starts | E-Rows | Cost (%CPU)| E-Time | A-Rows | A-Time | Buffers | ------------------------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | | 10 (100)| | 1 |00:00:00.01 | 31 | | 1 | SORT AGGREGATE | | 1 | 1 | | | 1 |00:00:00.01 | 31 | | 2 | TABLE ACCESS FULL| T2 | 1 | 10005 | 10 (0)| 00:00:01 | 10005 |00:00:00.01 | 31 | ------------------------------------------------------------------------------------------------------------- SCOTT@test01p> select count(*) from t2 where DBMS_ROWID.ROWID_BLOCK_NUMBER(rowid) between 203 and 205; COUNT(*) ---------- 955 --//这样扫描dba=9,203到9,205的记录数是955,加上dba=9.206第1条记录是id is NULL,因为存在Null 记录,查询就停止扫描T2. --//而如果交换表连接顺序: SCOTT@test01p> select /*+ SWAP_JOIN_INPUTS(@"SEL$5DA710D3" "T2"@"SEL$2") */ * from t1 where id not in (select id from t2 ) ; no rows selected SCOTT@test01p> @ dpc '' '' PLAN_TABLE_OUTPUT ------------------------------------- SQL_ID aqy7dusnfb5gm, child number 0 ------------------------------------- select /*+ SWAP_JOIN_INPUTS(@"SEL$5DA710D3" "T2"@"SEL$2") */ * from t1 where id not in (select id from t2 ) Plan hash value: 2739594415 ----------------------------------------------------------------------------------------------------------------------------------------------------- | Id | Operation | Name | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem | ----------------------------------------------------------------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | | | 14 (100)| | 0 |00:00:00.01 | 7 | | | | |* 1 | HASH JOIN RIGHT ANTI NA| | 1 | 2 | 24 | 14 (0)| 00:00:01 | 0 |00:00:00.01 | 7 | 1753K| 1753K| 1482K (0)| | 2 | TABLE ACCESS FULL | T2 | 1 | 10005 | 40020 | 10 (0)| 00:00:01 | 956 |00:00:00.01 | 7 | | | | | 3 | TABLE ACCESS FULL | T1 | 0 | 5 | 40 | 4 (0)| 00:00:01 | 0 |00:00:00.01 | 0 | | | | ----------------------------------------------------------------------------------------------------------------------------------------------------- Query Block Name / Object Alias (identified by operation id): ------------------------------------------------------------- 1 - SEL$5DA710D3 2 - SEL$5DA710D3 / T2@SEL$2 3 - SEL$5DA710D3 / T1@SEL$1 Predicate Information (identified by operation id): --------------------------------------------------- 1 - access("ID"="ID") --//这样T2表扫描到id is null时就停止.而T1表根本不做全表扫描操作.starts=0.