Global Hints v12

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Thus far, hints have been applied directly to tables that are referenced in the SQL command. It is also possible to apply hints to tables that appear in a view when the view is referenced in the SQL command. The hint does not appear in the view, itself, but rather in the SQL command that references the view.

When specifying a hint that is to apply to a table within a view, the view and table names are given in dot notation within the hint argument list.

Synopsis

<hint>(<view>.<table>)

Parameters

hint

Any of the hints in table Access Method Hints, Joining Relations Hints.

view

The name of the view containing table.

table

The table on which the hint is to be applied.

Examples

A view named, tx, is created from the three-table join of pgbench_history, pgbench_branches, and pgbench_accounts shown in the final example of Joining Relations Hints.

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;

The query plan produced by selecting from this view is shown below:

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)

The same hints that were applied to this join at the end of Joining Relations Hints can be applied to the view as follows:

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)

In addition to applying hints to tables within stored views, hints can be applied to tables within subqueries as illustrated by the following example. In this query on the sample application emp table, employees and their managers are listed by joining the emp table with a subquery of the emp table identified by the alias, b.

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)

The plan chosen by the query planner is shown below:

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)

A hint can be applied to the emp table within the subquery to perform an index scan on index, emp_pk, instead of a table scan. Note the difference in the query plans.

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)

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