oracle 表连接 - nested loop 嵌套循环联接

oracle 表连接 - nested loop 嵌套循环连接
一. nested loop 原理

nested loop 连接(循环嵌套连接)指的是两个表连接时, 通过两层嵌套循环来进行依次的匹配, 最后得到返回结果集的表连接方法.

假如下面的 sql 语句中表 T1 和 T2 的连接方式是循环嵌套连接, T1 是驱动表

select *
from T1, T2
where T1.id = T2.id and T1.name = 'David';

那么将上述 sql 语句翻译为伪码应该如下所示:.

for each row in (select * from T1 where name = 'David') loop
for (select * from T2 where T2.id = outer.id) loop
If match then pass the row on to the next step
If no match then discard the row
end loop
end loop

具体来说, 如果上述 sql 语句执行循环嵌套连接的话, 那么实际的执行过程应该如下所示:
(1) 首先 oracle 会根据一定的规则(根据统计信息的成本计算或者 hint 强制)决定哪个表是驱动表, 哪个表是被驱动表 (假设 T1 是驱动表)
(2) 查询驱动表 "select * from T1 where name = 'David'" 然后得到驱动结果集 Q1
(3) 遍历驱动结果集 Q1 以及被驱动表 T2, 从驱动结果集 Q1 中取出一条记录, 接着遍历 T2 并按照连接条件 T2.id = T1.id 去判断 T2 中是否存在匹配的记录, 如果能够匹配则保留, 不能匹配则忽略此行, 然后再从 Q1 中取出下一条记录, 接着遍历 T2 进行匹配, 如此下去直到取完 Q1 中的所有记录

二. nested loop 特性

嵌套循环连接有以下特性:

(1) 通常 sql 语句中驱动表只访问一次, 被驱动表访问多次
(2) 不必等待处理完成所有行前可以先返回部分已经处理完成的数据
(3) 在限制条件以及连接条件列上建立索引, 能够提高执行效率
(4) 支持所有类型的连接 (等值连接, 非等值连接, like 等)

构造试验数据

SQL> CREATE TABLE t1 (
  2    id NUMBER NOT NULL,
  3    n NUMBER,
  4    pad VARCHAR2(4000),
  5    CONSTRAINT t1_pk PRIMARY KEY(id)
  6  );

Table created.

SQL> CREATE TABLE t2 (
  2    id NUMBER NOT NULL,
  3    t1_id NUMBER NOT NULL,
  4    n NUMBER,
  5    pad VARCHAR2(4000),
  6    CONSTRAINT t2_pk PRIMARY KEY(id),
  7    CONSTRAINT t2_t1_fk FOREIGN KEY (t1_id) REFERENCES t1
  8  );

Table created.

SQL> CREATE TABLE t3 (
  2    id NUMBER NOT NULL,
  3    t2_id NUMBER NOT NULL,
  4    n NUMBER,
  5    pad VARCHAR2(4000),
  6    CONSTRAINT t3_pk PRIMARY KEY(id),
  7    CONSTRAINT t3_t2_fk FOREIGN KEY (t2_id) REFERENCES t2
  8  );

Table created.
SQL> CREATE TABLE t4 (
  2    id NUMBER NOT NULL,
  3    t3_id NUMBER NOT NULL,
  4    n NUMBER,
  5    pad VARCHAR2(4000),
  6    CONSTRAINT t4_pk PRIMARY KEY(id),
  7    CONSTRAINT t4_t3_fk FOREIGN KEY (t3_id) REFERENCES t3
  8  );


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 <= 10 ORDER BY dbms_random.random;

10 rows created.

SQL> INSERT INTO t2 SELECT 100+rownum, t1.id, 100+rownum, t1.pad FROM t1, t1 dummy ORDER BY dbms_random.random;

100 rows created.

SQL> INSERT INTO t3 SELECT 1000+rownum, t2.id, 1000+rownum, t2.pad FROM t2, t1 dummy ORDER BY dbms_random.random;

1000 rows created.

SQL> INSERT INTO t4 SELECT 10000+rownum, t3.id, 10000+rownum, t3.pad FROM t3, t1 dummy ORDER BY dbms_random.random;

10000 rows created.

SQL> COMMIT;

Commit complete.

使用 hint 让 sql 语句通过 nested loop 连接, 并且指定 t3 为驱动表

SQL> select /*+ leading(t3) use_nl(t4) */ * from t3, t4
  2  where t3.id = t4.t3_id and t3.n = 1100;

10 rows selected.

SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));

PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------------------
---------------------------------------------------------------------------------------------

SQL_ID  89hnfwqakjghg, child number 0
-------------------------------------
select /*+ leading(t3) use_nl(t4) */ * from t3, t4 where t3.id =
t4.t3_id and t3.n = 1100

Plan hash value: 1907878852

-------------------------------------------------------------------------------------
| Id  | Operation          | Name | Starts | E-Rows | A-Rows |   A-Time   | Buffers |
-------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |      1 |        |     10 |00:00:00.01 |     121 |
|   1 |  NESTED LOOPS      |      |      1 |     10 |     10 |00:00:00.01 |     121 |
|*  2 |   TABLE ACCESS FULL| T3   |      1 |      1 |      1 |00:00:00.01 |      16 |
|*  3 |   TABLE ACCESS FULL| T4   |      1 |     10 |     10 |00:00:00.01 |     105 |
-------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - filter("T3"."N"=1100)
   3 - filter("T3"."ID"="T4"."T3_ID")

在执行计划中我们可以看到驱动表 T3 访问一次, 因为驱动表上有谓词条件 t3.n = 1100, 通过执行谓词条件后驱动结果集的记录数为 1, 所以 T4 也只访问一次(starts 列)

使用 hint 让 sql 语句通过 nested loop 连接, 并且指定 t4 为驱动表

SQL> select /*+ leading(t4) use_nl(t3) full(t4) full(t3) */ * from t3, t4
  2  where t3.id = t4.t3_id and t3.n = 1100;

SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));

PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------
----------------------------------------------------------------------------------------------------------
SQL_ID  0yxm1muqwrfq2, child number 0
-------------------------------------
select /*+ leading(t4) use_nl(t3) full(t4) full(t3) */ * from t3, t4
where t3.id = t4.t3_id and t3.n = 1100

Plan hash value: 3886808168

-------------------------------------------------------------------------------------
| Id  | Operation          | Name | Starts | E-Rows | A-Rows |   A-Time   | Buffers |
-------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |      1 |        |     10 |00:00:00.25 |     150K|
|   1 |  NESTED LOOPS      |      |      1 |     10 |     10 |00:00:00.25 |     150K|
|   2 |   TABLE ACCESS FULL| T4   |      1 |  10000 |  10000 |00:00:00.01 |     105 |
|*  3 |   TABLE ACCESS FULL| T3   |  10000 |      1 |     10 |00:00:00.21 |     150K|
-------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   3 - filter(("T3"."N"=1100 AND "T3"."ID"="T4"."T3_ID"))

在执行计划中我们可以看到驱动表 T4 访问一次, 因为驱动表上 T4 结果集的记录数为 10000, 所以 T4 访问了 10000 次, buffers 和 A-time(实际执行时间) 都比较高.

三. nested loop 优化

在 nested loop 被驱动表上的连接列上 (T4 表的 t3_id 列) 建立索引

SQL> CREATE INDEX t4_t3_id ON t4(t3_id);

Index created.

SQL> select /*+ leading(t3) use_nl(t4) */ * from t3, t4
  2  where t3.id = t4.t3_id and t3.n = 1100;

10 rows selected.

SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------
------------------------------------------------------------------------------------------------------------------------------------

SQL_ID  89hnfwqakjghg, child number 0
-------------------------------------
select /*+ leading(t3) use_nl(t4) */ * from t3, t4 where t3.id =
t4.t3_id and t3.n = 1100

Plan hash value: 2039660043

------------------------------------------------------------------------------------------------------------
| Id  | Operation                    | Name     | Starts | E-Rows | A-Rows |   A-Time   | Buffers | Reads  |
------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |          |      1 |        |     10 |00:00:00.01 |      29 |   1 |
|   1 |  NESTED LOOPS                |          |      1 |        |     10 |00:00:00.01 |      29 |   1 |
|   2 |   NESTED LOOPS               |          |      1 |     10 |     10 |00:00:00.01 |      19 |   1 |
|*  3 |    TABLE ACCESS FULL         | T3       |      1 |      1 |      1 |00:00:00.01 |      16 |   0 |
|*  4 |    INDEX RANGE SCAN          | T4_T3_ID |      1 |     10 |     10 |00:00:00.01 |       3 |   1 |
|   5 |   TABLE ACCESS BY INDEX ROWID| T4       |     10 |     10 |     10 |00:00:00.01 |      10 |   0 |
------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   3 - filter("T3"."N"=1100)
   4 - access("T3"."ID"="T4"."T3_ID")

在执行计划中可以看到在被驱动表上的连接列上加上索引后, buffer 从 121 下降到了 29

在驱动表的谓词条件列上 (T3 表的 n 列) 加上索引

SQL> create index t3_n on t3(n);

Index created.

SQL> select /*+ leading(t3) use_nl(t4) */ * from t3, t4
  2  where t3.id = t4.t3_id and t3.n = 1100;

10 rows selected.

SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));

PLAN_TABLE_OUTPUT
-------------------------------------------------------------------------------------------------------------------------------------
-------------------------------------------------------------------------------------------------------------------------------------

SQL_ID  89hnfwqakjghg, child number 0
-------------------------------------
select /*+ leading(t3) use_nl(t4) */ * from t3, t4 where t3.id =
t4.t3_id and t3.n = 1100

Plan hash value: 2304842513

-------------------------------------------------------------------------------------------------------------
| Id  | Operation                     | Name     | Starts | E-Rows | A-Rows |   A-Time   | Buffers | Reads  |
-------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT              |          |      1 |        |     10 |00:00:00.01 |      17 |   1 |
|   1 |  NESTED LOOPS                 |          |      1 |        |     10 |00:00:00.01 |      17 |   1 |
|   2 |   NESTED LOOPS                |          |      1 |     10 |     10 |00:00:00.01 |       7 |   1 |
|   3 |    TABLE ACCESS BY INDEX ROWID| T3       |      1 |      1 |      1 |00:00:00.01 |       4 |   1 |
|*  4 |     INDEX RANGE SCAN          | T3_N     |      1 |      1 |      1 |00:00:00.01 |       3 |   1 |
|*  5 |    INDEX RANGE SCAN           | T4_T3_ID |      1 |     10 |     10 |00:00:00.01 |       3 |   0 |
|   6 |   TABLE ACCESS BY INDEX ROWID | T4       |     10 |     10 |     10 |00:00:00.01 |      10 |   0 |
-------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   4 - access("T3"."N"=1100)
   5 - access("T3"."ID"="T4"."T3_ID")

在执行计划中可以看到在驱动表上的谓词条件列上加上索引后, buffer 从 29 继续下降到了 17

四. 小结

由此可见, 在 sql 调优时如果遇到表的连接方式是 nested loop：

首先，要确保结果集小的表为驱动表，结果集多的表为被驱动表。这不意味着记录多的表不能作为驱动表, 只要通过谓词条件过滤后得到的结果集比较小，也可以作为驱动表。

其次，在驱动表的谓词条件列以及被驱动表的连接列上加上索引，能够显著的提高执行性能。

最后，如果要查询的列都在索引中，避免回表查询列信息时，又将进一步提高执行性能。

oracle 表连接 - nested loop 嵌套循环联接

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