欢迎您访问程序员文章站本站旨在为大家提供分享程序员计算机编程知识!
您现在的位置是: 首页  >  IT编程

[20180713]关于hash join 测试中一个疑问.txt

程序员文章站 2023-01-02 14:07:20
[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> @ ......

[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.