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Global Hints

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到目前为止,hint已直接应用于 SQL 命令中引用的表。在 SQL 命令中引用视图时,也可以对视图中显示的表应用hint。hint本身不会在视图中显示,而是在引用视图的 SQL 命令中显示。

指定要应用于视图中表的hint时,视图和表名称在提示参数列表中以点表示法指定。

概要

hint(view.table)

参数

参数

说明

hint

表一表二中的任何提示。

view

包含table的视图的名称。

table

要对其应用hint的表。

示例

名为 tx 的视图从 pgbench_history、pgbench_branches 和 pgbench_accounts 的三表联接创建,如Joining Relations Hints的最后一个示例中所示。

CREATE VIEW tx AS SELECT h.mtime, h.delta, b.bid, a.aid FROM pgbench_history h, pgbench_branches b, pgbench_accounts a WHERE h.bid = b.bid AND h.aid = a.aid;

通过从此视图中选择生成的查询计划如下所示:

EXPLAIN SELECT * FROM tx;

                                       QUERY PLAN
----------------------------------------------------------------------------------------
 Hash Join  (cost=86814.29..123103.29 rows=500000 width=20)
   Hash Cond: (h.aid = a.aid)
   ->  Hash Join  (cost=21.45..15081.45 rows=500000 width=20)
         Hash Cond: (h.bid = b.bid)
         ->  Seq Scan on pgbench_history h  (cost=0.00..8185.00 rows=500000 width=20)
         ->  Hash  (cost=21.20..21.20 rows=20 width=4)
               ->  Seq Scan on pgbench_branches b  (cost=0.00..21.20 rows=20 width=4)
   ->  Hash  (cost=53746.15..53746.15 rows=2014215 width=4)
         ->  Seq Scan on pgbench_accounts a  (cost=0.00..53746.15 rows=2014215 width=4)
(9 rows)

Joining Relations Hints末尾应用于此联接的相同hint可应用于该视图,如下所示:

EXPLAIN SELECT /*+ USE_MERGE(tx.h tx.b) USE_HASH(tx.a) */ * FROM tx;

                                            QUERY PLAN
--------------------------------------------------------------------------------------------------
 Hash Join  (cost=152583.39..182562.49 rows=500000 width=20)
   Hash Cond: (h.aid = a.aid)
   ->  Merge Join  (cost=65790.55..74540.65 rows=500000 width=20)
         Merge Cond: (b.bid = h.bid)
         ->  Sort  (cost=21.63..21.68 rows=20 width=4)
               Sort Key: b.bid
               ->  Seq Scan on pgbench_branches b  (cost=0.00..21.20 rows=20 width=4)
         ->  Materialize  (cost=65768.92..68268.92 rows=500000 width=20)
               ->  Sort  (cost=65768.92..67018.92 rows=500000 width=20)
                     Sort Key: h.bid
                     ->  Seq Scan on pgbench_history h  (cost=0.00..8185.00 rows=500000 width=20)
   ->  Hash  (cost=53746.15..53746.15 rows=2014215 width=4)
         ->  Seq Scan on pgbench_accounts a  (cost=0.00..53746.15 rows=2014215 width=4)
(13 rows)

除了向存储视图中的表应用hint以外,还可以将hint应用于子查询中的表,如以下示例所示。在示例应用程序 emp 表的此查询中,通过联接 emp 表与别名 b 标识的 emp 表的子查询,列出员工及其经理。

SELECT a.empno, a.ename, b.empno "mgr empno", b.ename "mgr ename" FROM emp a, (SELECT * FROM emp) b WHERE a.mgr = b.empno;

 empno | ename  | mgr empno | mgr ename
-------+--------+-----------+-----------
  7369 | SMITH  |      7902 | FORD
  7499 | ALLEN  |      7698 | BLAKE
  7521 | WARD   |      7698 | BLAKE
  7566 | JONES  |      7839 | KING
  7654 | MARTIN |      7698 | BLAKE
  7698 | BLAKE  |      7839 | KING
  7782 | CLARK  |      7839 | KING
  7788 | SCOTT  |      7566 | JONES
  7844 | TURNER |      7698 | BLAKE
  7876 | ADAMS  |      7788 | SCOTT
  7900 | JAMES  |      7698 | BLAKE
  7902 | FORD   |      7566 | JONES
  7934 | MILLER |      7782 | CLARK
(13 rows)

查询计划程序选择的计划如下所示:

EXPLAIN SELECT a.empno, a.ename, b.empno "mgr empno", b.ename "mgr ename" FROM emp a, (SELECT * FROM emp) b WHERE a.mgr = b.empno;

                           QUERY PLAN
-----------------------------------------------------------------
 Hash Join  (cost=1.32..2.64 rows=13 width=22)
   Hash Cond: (a.mgr = emp.empno)
   ->  Seq Scan on emp a  (cost=0.00..1.14 rows=14 width=16)
   ->  Hash  (cost=1.14..1.14 rows=14 width=11)
         ->  Seq Scan on emp  (cost=0.00..1.14 rows=14 width=11)
(5 rows)

hint可应用于子查询中的 emp 表,以对索引 emp_pk 执行索引扫描,而不是表扫描。记下查询计划中的差异。

EXPLAIN SELECT /*+ INDEX(b.emp emp_pk) */ a.empno, a.ename, b.empno "mgr empno", b.ename "mgr ename" FROM emp a, (SELECT * FROM emp) b WHERE a.mgr = b.empno;

                                QUERY PLAN
---------------------------------------------------------------------------
 Merge Join  (cost=4.17..13.11 rows=13 width=22)
   Merge Cond: (a.mgr = emp.empno)
   ->  Sort  (cost=1.41..1.44 rows=14 width=16)
         Sort Key: a.mgr
         ->  Seq Scan on emp a  (cost=0.00..1.14 rows=14 width=16)
   ->  Index Scan using emp_pk on emp  (cost=0.14..12.35 rows=14 width=11)
(6 rows)