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oracle表連接-nestedloop嵌套循環連接

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oracle表連接-nestedloop嵌套循環連接

一. nested loop 原理

nested loop 連接(循環嵌套連接)指的是兩個表連接時, 通過兩層嵌套循環來進行依次的匹配, 最後得到返回結果集的表連接方法.

假如下面的 sql 語句中表 T1 和 T2 的連接方式是循環嵌套連接, T1 是驅動表

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

那麼將上述 sql 語句翻譯為偽碼應該如下所示:.

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

具體來說, 如果上述 sql 語句執行循環嵌套連接的話, 那麼實際的執行過程應該如下所示:
(1) 首先 oracle 會根據一定的規則(根據統計信息的成本計算或者 hint 強制)決定哪個表是驅動表, 哪個表是被驅動表 (假設 T1 是驅動表)
(2) 查詢驅動表 "select * from T1 where name = 'David'" 然後得到驅動結果集 Q1
(3) 遍歷驅動結果集 Q1 以及被驅動表 T2, 從驅動結果集 Q1 中取出一條記錄, 接著遍歷 T2 並按照連接條件 T2.id = T1.id 去判斷 T2 中是否存在匹配的記錄, 如果能夠匹配則保留, 不能匹配則忽略此行, 然後再從 Q1 中取出下一條記錄, 接著遍歷 T2 進行匹配, 如此下去直到取完 Q1 中的所有記錄
二. nested loop 特性

嵌套循環連接有以下特性:

(1) 通常 sql 語句中驅動表只訪問一次, 被驅動表訪問多次
(2) 不必等待處理完成所有行前可以先返回部分已經處理完成的數據
(3) 在限制條件以及連接條件列上建立索引, 能夠提高執行效率
(4) 支持所有類型的連接 (等值連接, 非等值連接, like 等)

構造試驗數據

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 讓 sql 語句通過 nested loop 連接, 並且指定 t3 為驅動表

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")

在執行計劃中我們可以看到驅動表 T3 訪問一次, 因為驅動表上有謂詞條件 t3.n = 1100, 通過執行謂詞條件後驅動結果集的記錄數為 1, 所以 T4 也只訪問一次(starts 列)

使用 hint 讓 sql 語句通過 nested loop 連接, 並且指定 t4 為驅動表

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"))

在執行計劃中我們可以看到驅動表 T4 訪問一次, 因為驅動表上 T4 結果集的記錄數為 10000, 所以 T4 訪問了 10000 次, buffers 和 A-time(實際執行時間) 都比較高.

三. nested loop 優化

在 nested loop 被驅動表上的連接列上 (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")

在執行計劃中可以看到在被驅動表上的連接列上加上索引後, buffer 從 121 下降到了 29

在驅動表的謂詞條件列上 (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")

在執行計劃中可以看到在驅動表上的謂詞條件列上加上索引後, buffer 從 29 繼續下降到了 17

四. 小結

由此可見, 在 sql 調優時如果遇到表的連接方式是 nested loop：

首先，要確保結果集小的表為驅動表，結果集多的表為被驅動表。這不意味著記錄多的表不能作為驅動表, 只要通過謂詞條件過濾後得到的結果集比較小，也可以作為驅動表。

其次，在驅動表的謂詞條件列以及被驅動表的連接列上加上索引，能夠顯著的提高執行性能。

最後，如果要查詢的列都在索引中，避免回表查詢列信息時，又將進一步提高執行性能。