Oracle 的SQL文，可以强制指定各种 hint。
但是在PostgreSQL中是不支持的。
其wiki 是这样说的：
http://wiki.postgresql.org/wiki/OptimizerHintsDiscussion
We are not interested in implementing hints in the exact ways they are commonly implemented on other databases. Proposals based on "because they've got them" will not be welcomed. If you have an idea that avoids the problems that have been observed with other hint systems, that could lead to valuable discussion.
但是可以通过如下的postgresql.conf参数来调节：
# - Planner Method Configuration - #enable_bitmapscan = on #enable_hashagg = on enable_hashjoin = on #enable_indexscan = on #enable_indexonlyscan = on #enable_material = on enable_mergejoin = on enable_nestloop = on #enable_seqscan = on #enable_sort = on #enable_tidscan = on
对于我的查询：
explain select dept.no_emps,emp.age from dept,emp where emp.name = dept.mgr and dept.dept_name = 'shoe';
如果 enable_hashjoin = on，其他也为on，则执行计划是：
postgres=# explain select dept.no_emps,emp.age from dept,emp where emp.name = dept.mgr and dept.dept_name = 'shoe'; QUERY PLAN ------------------------------------------------------------------ Hash Join (cost=19.30..45.07 rows=23 width=8) Hash Cond: ((emp.name)::text = (dept.mgr)::text) -> Seq Scan on emp (cost=0.00..21.30 rows=1130 width=42) -> Hash (cost=19.25..19.25 rows=4 width=42) -> Seq Scan on dept (cost=0.00..19.25 rows=4 width=42) Filter: ((dept_name)::text = 'shoe'::text) (6 rows) postgres=#
如果 enable_hashjoin=off，其他为on,则执行计划是：
postgres=# explain select dept.no_emps,emp.age from dept,emp where emp.name = dept.mgr and dept.dept_name = 'shoe'; QUERY PLAN -------------------------------------------------------------------- Merge Join (cost=97.89..103.79 rows=23 width=8) Merge Cond: ((dept.mgr)::text = (emp.name)::text) -> Sort (cost=19.29..19.30 rows=4 width=42) Sort Key: dept.mgr -> Seq Scan on dept (cost=0.00..19.25 rows=4 width=42) Filter: ((dept_name)::text = 'shoe'::text) -> Sort (cost=78.60..81.43 rows=1130 width=42) Sort Key: emp.name -> Seq Scan on emp (cost=0.00..21.30 rows=1130 width=42) (9 rows) postgres=#
如果enable_hashjoin 为 off，而 enable_mergejoin也为 off，则执行计划为：
postgres=# explain select dept.no_emps,emp.age from dept,emp where emp.name = dept.mgr and dept.dept_name = 'shoe'; QUERY PLAN ------------------------------------------------------------------ Nested Loop (cost=0.00..108.36 rows=23 width=8) Join Filter: ((dept.mgr)::text = (emp.name)::text) -> Seq Scan on emp (cost=0.00..21.30 rows=1130 width=42) -> Materialize (cost=0.00..19.27 rows=4 width=42) -> Seq Scan on dept (cost=0.00..19.25 rows=4 width=42) Filter: ((dept_name)::text = 'shoe'::text) (6 rows) postgres=#
