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本篇内容介绍了“怎么正确使用PostgreSQL中的OR”的有关知识，在实际案例的操作过程中，不少人都会遇到这样的困境，接下来就让小编带领大家学习一下如何处理这些情况吧！希望大家仔细阅读，能够学有所成！
在SQL语句中，对OR使用不当可能会导致较差的查询效率。这并不意味着不能用OR而是在使用OR时需考虑可能存在的性能问题。
测试数据：
DROP TABLE a; CREATE TABLE a(id integer NOT NULL, a_val text NOT NULL); INSERT INTO a SELECT i, md5(i::text) FROM generate_series(1, 1000000) i; DROP TABLE b; CREATE TABLE b(id integer NOT NULL, b_val text NOT NULL); INSERT INTO b SELECT i, md5(i::text) FROM generate_series(1, 1000000) i; ALTER TABLE a ADD PRIMARY KEY (id); ALTER TABLE b ADD PRIMARY KEY (id); ALTER TABLE b ADD FOREIGN KEY (id) REFERENCES a; VACUUM (ANALYZE) a; VACUUM (ANALYZE) b;
OR vs IN
条件语句p1 OR p2，如可以考虑使用IN来改写，比如：
[local:/data/pg12]:5432 pg12@testdb=# EXPLAIN verbose
SELECT id FROM a
WHERE id = 42
OR id = 4711;
QUERY PLAN
---------------------------------------------------------------------------
Bitmap Heap Scan on public.a (cost=8.87..16.80 rows=2 width=4)
Output: id
Recheck Cond: ((a.id = 42) OR (a.id = 4711))
-> BitmapOr (cost=8.87..8.87 rows=2 width=0)
-> Bitmap Index Scan on a_pkey (cost=0.00..4.43 rows=1 width=0)
Index Cond: (a.id = 42)
-> Bitmap Index Scan on a_pkey (cost=0.00..4.43 rows=1 width=0)
Index Cond: (a.id = 4711)
(8 rows)
[local:/data/pg12]:5432 pg12@testdb=#
[local:/data/pg12]:5432 pg12@testdb=# EXPLAIN verbose
SELECT id FROM a
WHERE id in (42,4711);
QUERY PLAN
----------------------------------------------------------------------------
Index Only Scan using a_pkey on public.a (cost=0.42..8.88 rows=2 width=4)
Output: id
Index Cond: (a.id = ANY ('{42,4711}'::integer[]))
(3 rows)
[local:/data/pg12]:5432 pg12@testdb=#使用OR操作符，PG优化器走的是Bitmap Index Scan，使用IN，优化器选择的路径是Index Only Scan，相对于Bitmap Index Scan少了Bitmap的建立，成本自然要低不少。
OR and Join
在Join场景中，如果在参与join的表上都存在查询条件然后在where子句中应用OR关联，那么优化器会选择a和b连接然后使用Filter过滤，由于先进行join而没有进行谓词下推，因此为了得到1行而filter了999999行，代价巨大。
[local:/data/pg12]:5432 pg12@testdb=# EXPLAIN verbose SELECT id, a.a_val, b.b_val FROM a JOIN b USING (id) WHERE a.id = 42 OR b.id = 42; QUERY PLAN --------------------------------------------------------------------------------------------- Gather (cost=21965.00..45327.62 rows=2 width=70) Output: a.id, a.a_val, b.b_val Workers Planned: 2 -> Parallel Hash Join (cost=20965.00..44327.42 rows=1 width=70) Output: a.id, a.a_val, b.b_val Inner Unique: true Hash Cond: (a.id = b.id) Join Filter: ((a.id = 42) OR (b.id = 42)) -> Parallel Seq Scan on public.a (cost=0.00..12500.67 rows=416667 width=37) Output: a.id, a.a_val -> Parallel Hash (cost=12500.67..12500.67 rows=416667 width=37) Output: b.b_val, b.id -> Parallel Seq Scan on public.b (cost=0.00..12500.67 rows=416667 width=37) Output: b.b_val, b.id (14 rows)
在这种情况下，可以通过使用UNION来关联两个JOIN提升性能
[local:/data/pg12]:5432 pg12@testdb=# EXPLAIN verbose pg12@testdb-# SELECT id, a.a_val, b.b_val pg12@testdb-# FROM a JOIN b USING (id) pg12@testdb-# WHERE a.id = 42 pg12@testdb-# UNION pg12@testdb-# SELECT id, a.a_val, b.b_val pg12@testdb-# FROM a JOIN b USING (id) pg12@testdb-# WHERE b.id = 42 pg12@testdb-# ; QUERY PLAN ---------------------------------------------------------------------------------------------------- Unique (cost=33.83..33.85 rows=2 width=68) Output: a.id, a.a_val, b.b_val -> Sort (cost=33.83..33.84 rows=2 width=68) Output: a.id, a.a_val, b.b_val Sort Key: a.id, a.a_val, b.b_val -> Append (cost=0.85..33.82 rows=2 width=68) -> Nested Loop (cost=0.85..16.90 rows=1 width=70) Output: a.id, a.a_val, b.b_val -> Index Scan using a_pkey on public.a (cost=0.42..8.44 rows=1 width=37) Output: a.id, a.a_val Index Cond: (a.id = 42) -> Index Scan using b_pkey on public.b (cost=0.42..8.44 rows=1 width=37) Output: b.id, b.b_val Index Cond: (b.id = 42) -> Nested Loop (cost=0.85..16.90 rows=1 width=70) Output: a_1.id, a_1.a_val, b_1.b_val -> Index Scan using a_pkey on public.a a_1 (cost=0.42..8.44 rows=1 width=37) Output: a_1.id, a_1.a_val Index Cond: (a_1.id = 42) -> Index Scan using b_pkey on public.b b_1 (cost=0.42..8.44 rows=1 width=37) Output: b_1.id, b_1.b_val Index Cond: (b_1.id = 42) (22 rows) [local:/data/pg12]:5432 pg12@testdb=#
两个子连接选择了成本最低的NL join，总成本是原来SQL语句成本的0.1%都不到，差了3个数量级。
“怎么正确使用PostgreSQL中的OR”的内容就介绍到这里了，感谢大家的阅读。如果想了解更多行业相关的知识可以关注创新互联-成都网站建设公司网站，小编将为大家输出更多高质量的实用文章！
