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表连接

时间：2015-04-21 12:55:00      阅读：145      评论：0      收藏：0      [点我收藏+]

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主要的表连接类型：

  1. 嵌套循环
  2. 哈希连接
  3. 排序合并

各种连接类型的差异：

  1. 访问次数：
    1. 嵌套循环：驱动表查询n条记录，被驱动表访问n次。 
      SQL> create table t1(id number not null,n number,contents varchar2(4000));
Table created.
SQL> create table t2(id number not null,t1_id number not null, n number,contents varchar2(2000));
Table created.
SQL> execute dbms_random.seed(0);
PL/SQL procedure successfully completed.
SQL> insert into t1 select rownum,rownum,dbms_random.string(‘a‘,50) from dual connect by level<=100 order by dbms_random.random;
100 rows created.
SQL> insert into t2 select rownum,rownum,rownum,dbms_random.string(‘b‘,50) from dual connect by level<=100000 order by dbms_random.random;
100000 rows created.
SQL> commit;
Commit complete.
SQL> alter session set statistics_level =all;
SQL> select /*+ leading(t1) use_nl(t2)*/ * from t1,t2 where t1.id=t2.t1_id;
SQL> set linesize 1000
SQL> 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 |        |     100 |00:00:00.37 |    100K|
|   1 |  NESTED LOOPS       |      |     1 |    100 |     100 |00:00:00.37 |    100K|
|   2 |   TABLE ACCESS FULL| T1   |     1 |    100 |     100 |00:00:00.01 |     14 |
|*  3 |   TABLE ACCESS FULL| T2   |    100 |      1 |     100 |00:00:00.37 |    100K|
   3 - filter("T1"."ID"="T2"."T1_ID")
24 rows selected.
--注t1被访问一次，t2被访问100次。
    2. 哈希连接：驱动表和被驱动表都是最多被访问1次。 
      SQL> select /*+ leading(t1) use_hash(t2)*/ * from t1,t2 where t1.id=t2.t1_id;
SQL> select * from table(dbms_xplan.display_cursor(null,null,‘allstats last‘));

select /*+ leading(t1) use_hash(t2)*/ * from t1,t2 where t1.id=t2.t1_id
| Id  | Operation       | Name | Starts | E-Rows | A-Rows |     A-Time   | Buffers |  OMem |  1Mem | Used-Mem |
----------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |     1 |        |     100 |00:00:00.05 |    1019 |        |        |           |
|*  1 |  HASH JOIN       |      |     1 |    100 |     100 |00:00:00.05 |    1019 |    821K|    821K| 1244K (0)|
|   2 |   TABLE ACCESS FULL| T1   |     1 |    100 |     100 |00:00:00.01 |      7 |        |        |           |
|   3 |   TABLE ACCESS FULL| T2   |     1 |  93984 |     100K|00:00:00.01 |    1012 |        |        |           |
----------------------------------------------------------------------------------------------------------------
   1 - access("T1"."ID"="T2"."T1_ID")
-- 两个表最多访问1次
    3. 排序合并：两个表都是最多被访问1次，而且没有驱动和被驱动的概念。 
      SQL> select * from table(dbms_xplan.display_cursor(null,null,‘allstats last‘));

select /*+ordered use_merge(t2)*/ * from t1,t2 where t1.id=t2.t1_id

| Id  | Operation        | Name | Starts | E-Rows | A-Rows |   A-Time   | Buffers |    OMem |    1Mem | Used-Mem |
-----------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT    |       |      1 |         |      100 |00:00:00.05 |    1012 |         |         |        |
|   1 |  MERGE JOIN        |       |      1 |     100 |      100 |00:00:00.05 |    1012 |         |         |        |
|   2 |   SORT JOIN        |       |      1 |     100 |      100 |00:00:00.01 |       7 | 13312 | 13312 |12288  (0)|
|   3 |    TABLE ACCESS FULL| T1   |      1 |     100 |      100 |00:00:00.01 |       7 |         |         |        |
|*  4 |   SORT JOIN        |       |    100 |  93984 |      100 |00:00:00.05 |    1005 |    9762K|    1209K| 8677K (0)|
|   5 |    TABLE ACCESS FULL| T2   |      1 |  93984 |      100K|00:00:00.01 |    1005 |         |         |        |
-----------------------------------------------------------------------------------------------------------------

   4 - access("T1"."ID"="T2"."T1_ID")
       filter("T1"."ID"="T2"."T1_ID")
27 rows selected.
  2. 驱动顺序：
    1. 嵌套循环：有驱动顺序 
      SQL> select * from table(dbms_xplan.display_cursor(null,null,‘allstats last‘));
select /*+ leading(t2) use_nl(t1)*/ * from t1,t2 where t1.id=t2.t1_id
| Id  | Operation       | Name | Starts | E-Rows | A-Rows |     A-Time   | Buffers |
-------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |     1 |        |     100 |00:00:00.57 |    701K|
|   1 |  NESTED LOOPS       |      |     1 |    100 |     100 |00:00:00.57 |    701K|
|   2 |   TABLE ACCESS FULL| T2   |     1 |  93984 |     100K|00:00:00.01 |    1012 |
|*  3 |   TABLE ACCESS FULL| T1   |    100K|      1 |     100 |00:00:00.54 |    700K|

   3 - filter("T1"."ID"="T2"."T1_ID")
24 rows selected.
    2. 哈希连接：有驱动顺序 
      SQL> select * from table(dbms_xplan.display_cursor(null,null,‘allstats last‘));
select /*+ leading(t2) use_hash(t1)*/ * from t1,t2 where t1.id=t2.t1_id
| Id  | Operation       | Name | Starts | E-Rows | A-Rows |     A-Time   | Buffers |  OMem |  1Mem | Used-Mem |
----------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |     1 |        |     100 |00:00:00.07 |    1019 |        |        |           |
|*  1 |  HASH JOIN       |      |     1 |    100 |     100 |00:00:00.07 |    1019 |     10M|  2143K|    16M (0)|
|   2 |   TABLE ACCESS FULL| T2   |     1 |  93984 |     100K|00:00:00.01 |    1005 |        |        |           |
|   3 |   TABLE ACCESS FULL| T1   |     1 |    100 |     100 |00:00:00.01 |     14 |        |        |           |
----------------------------------------------------------------------------------------------------------------
24 rows selected.
    3. 排序合并：无驱动顺序，可以看到所有的数据都是一样的没有差别 
      SQL> select * from table(dbms_xplan.display_cursor(null,null,‘allstats last‘));
select /*+leading(t1) use_merge(t2)*/ * from t1,t2 where t1.id=t2.t1_id

| Id  | Operation        | Name | Starts | E-Rows | A-Rows |   A-Time   | Buffers |    OMem |    1Mem | Used-Mem |
-----------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT    |       |      1 |         |      100 |00:00:00.04 |    1012 |         |         |        |
|   1 |  MERGE JOIN        |       |      1 |     100 |      100 |00:00:00.04 |    1012 |         |         |        |
|   2 |   SORT JOIN        |       |      1 |     100 |      100 |00:00:00.01 |       7 | 13312 | 13312 |12288  (0)|
|   3 |    TABLE ACCESS FULL| T1   |      1 |     100 |      100 |00:00:00.01 |       7 |         |         |        |
|*  4 |   SORT JOIN        |       |    100 |  93984 |      100 |00:00:00.04 |    1005 |    9762K|    1209K| 8677K (0)|
|   5 |    TABLE ACCESS FULL| T2   |      1 |  93984 |      100K|00:00:00.01 |    1005 |         |         |        |
   4 - access("T1"."ID"="T2"."T1_ID")
       filter("T1"."ID"="T2"."T1_ID")
27 rows selected.
SQL> select * from table(dbms_xplan.display_cursor(null,null,‘allstats last‘));
select /*+leading(t2) use_merge(t1)*/ * from t1,t2 where t1.id=t2.t1_id
| Id  | Operation        | Name | Starts | E-Rows | A-Rows |   A-Time   | Buffers |    OMem |    1Mem | Used-Mem |
-----------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT    |       |      1 |         |      100 |00:00:00.05 |    1012 |         |         |        |
|   1 |  MERGE JOIN        |       |      1 |     100 |      100 |00:00:00.05 |    1012 |         |         |        |
|   2 |   SORT JOIN        |       |      1 |  93984 |      101 |00:00:00.05 |    1005 |    9762K|    1209K| 8677K (0)|
|   3 |    TABLE ACCESS FULL| T2   |      1 |  93984 |      100K|00:00:00.01 |    1005 |         |         |        |
|*  4 |   SORT JOIN        |       |    101 |     100 |      100 |00:00:00.01 |       7 | 13312 | 13312 |12288  (0)|
|   5 |    TABLE ACCESS FULL| T1   |      1 |     100 |      100 |00:00:00.01 |       7 |         |         |        |
27 rows selected.
  3. 排序差异：
    1. 嵌套循环：无需排序
    2. 哈希连接：无须排序（hash_area_size），消耗的资源是用来建立哈希表。
    3. 排序合并：需要排序(sort_area_size)
  4. 限制场景：
    1. 嵌套循环:无任何限制
    2. 哈希连接：连接条件是<>><或like导致无法使用
    3. 排序合并：连接条件是><或like导致无法使用

索引与连接的经典优化

  1. 嵌套循环：驱动表限制条件有索引，被驱动表连接条件有索引： 
    select * from t1,t2 where t1.id=t2.t1_id and t1.n=19 and t2.n=2932
--这里 t1.id=t2.t1_id是连接条件； t1.n=19 and t2.n=2932是限制条件。索引要建在t1(n) 和t2(t1_id)上。
    1. 适用于：两表返回记录少；而且被驱动表仅仅匹配少量记录。就算大表，加上索引以后也能很快定位。
    2. 不等值查询，不得不用，因为其他索引失效。
  2. 哈希连接：索引列在表连接中无特殊要求，和单表情况无异
    1. 连接条件起不到什么作用，但是限制条件列需要加上索引。
    2. 在PGA中增大hash_area_size也是可行的，默认128k。
  3. 排序合并：连接条件上的索引可以消除排序 
    --开始没有建索引
SQL> 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 |         |      100 |00:00:00.06 |    1012 |         |         |        |
|   1 |  MERGE JOIN        |       |      1 |     100 |      100 |00:00:00.06 |    1012 |         |         |        |
|   2 |   SORT JOIN        |       |      1 |     100 |      100 |00:00:00.01 |       7 | 13312 | 13312 |12288  (0)|
|   3 |    TABLE ACCESS FULL| T1   |      1 |     100 |      100 |00:00:00.01 |       7 |         |         |        |
|*  4 |   SORT JOIN        |       |    100 |     100K|      100 |00:00:00.06 |    1005 |    9762K|    1209K| 8677K (0)|
|   5 |    TABLE ACCESS FULL| T2   |      1 |     100K|      100K|00:00:00.02 |    1005 |         |         |        |
--在t1上建索引
SQL> create index idx_1 on t1(id);
SQL> select * from table(dbms_xplan.display_cursor(null,null,‘allstats last‘));

select /*+leading(t1) use_merge(t2)*/ * from t1,t2 where t1.id=t2.t1_id

| Id  | Operation             | Name  | Starts | E-Rows | A-Rows |   A-Time   | Buffers |  OMem |  1Mem | Used-Mem |
---------------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT         |         |        1 |        |    100 |00:00:00.06 |      1021 |       |       |      |
|   1 |  MERGE JOIN             |         |        1 |    100 |    100 |00:00:00.06 |      1021 |       |       |      |
|   2 |   TABLE ACCESS BY INDEX ROWID| T1    |        1 |    100 |    100 |00:00:00.01 |        16 |       |       |      |
|   3 |    INDEX FULL SCAN         | IDX_1 |        1 |    100 |    100 |00:00:00.01 |         8 |       |       |      |
|*  4 |   SORT JOIN             |         |      100 |    100K|    100 |00:00:00.06 |      1005 |  9762K|  1209K| 8677K (0)|
|   5 |    TABLE ACCESS FULL         | T2    |        1 |    100K|    100K|00:00:00.02 |      1005 |       |       |      |
--在t2上建索引，发现没有起到作用。
SQL> select * from table(dbms_xplan.display_cursor(null,null,‘allstats last‘));
select /*+leading(t1) use_merge(t2)*/ * from t1,t2 where t1.id=t2.t1_id

| Id  | Operation             | Name  | Starts | E-Rows | A-Rows |   A-Time   | Buffers |  OMem |  1Mem | Used-Mem |
---------------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT         |         |        1 |        |    100 |00:00:00.06 |      1021 |       |       |      |
|   1 |  MERGE JOIN             |         |        1 |    100 |    100 |00:00:00.06 |      1021 |       |       |      |
|   2 |   TABLE ACCESS BY INDEX ROWID| T1    |        1 |    100 |    100 |00:00:00.01 |        16 |       |       |      |
|   3 |    INDEX FULL SCAN         | IDX_1 |        1 |    100 |    100 |00:00:00.01 |         8 |       |       |      |
|*  4 |   SORT JOIN             |         |      100 |    100K|    100 |00:00:00.06 |      1005 |  9762K|  1209K| 8677K (0)|
|   5 |    TABLE ACCESS FULL         | T2    |        1 |    100K|    100K|00:00:00.02 |      1005 |       |       |      |

    --将两张表次序对调，索引idx_1排序的功能彻底失效。看来还是有bug。

    SQL> select * from table(dbms_xplan.display_cursor(null,null,‘allstats last‘));
    select /*+leading(t2) use_merge(t1)*/ * from t1,t2 where t1.id=t2.t1_id
    | Id  | Operation        | Name | Starts | E-Rows | A-Rows |   A-Time   | Buffers |    OMem |    1Mem | Used-Mem |
    -----------------------------------------------------------------------------------------------------------------
    |   0 | SELECT STATEMENT    |       |      1 |         |      100 |00:00:00.07 |    1012 |         |         |        |
    |   1 |  MERGE JOIN        |       |      1 |     100 |      100 |00:00:00.07 |    1012 |         |         |        |
    |   2 |   SORT JOIN        |       |      1 |     100K|      101 |00:00:00.07 |    1005 |    9762K|    1209K| 8677K (0)|
    |   3 |    TABLE ACCESS FULL| T2   |      1 |     100K|      100K|00:00:00.01 |    1005 |         |         |        |
    |*  4 |   SORT JOIN        |       |    101 |     100 |      100 |00:00:00.01 |       7 | 13312 | 13312 |12288  (0)|
    |   5 |    TABLE ACCESS FULL| T1   |      1 |     100 |      100 |00:00:00.01 |       7 |         |         |        |

表连接

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原文地址：http://www.cnblogs.com/alexweng/p/4444033.html
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