是为每一行执行的 where 子句中的非相关存在（...）查询还是只执行一次？

Question

我有以下疑问：

SELECT *
FROM   t1, t2
WHERE  t1.some_id = t2.some_id
       and
       not exists(select true from some_table where some_column = true)

这里，not exists(select true from some_table where ...) 不引用主查询中的t1 或t2。

not exists(select true from some_table where ...) 是只执行一次还是对t1 和t2 之间的乘积中的每个元组 执行，其中t1.some_id = t2.some_id 为真？ p>

也就是说，如果只执行一次，如果结果为假，可以立即返回一个空表。从逻辑上讲，我们已经写了：

if (not exists(select true from some_table where some_column = true))
    SELECT *
    FROM   t1, t2
    WHERE  t1.some_id = t2.some_id
else
    empty table

Answer 1

首先，学会使用正确的join语法：

SELECT *
FROM t1 JOIN
     t2
     ON t1.some_id = t2.some_id
WHERE not exists(select true from some_table where some_column = true);

子查询应该只执行一次。然而，最终这取决于 Postgres 优化器。您可以使用LEFT JOIN 保证代码只执行一次：

SELECT t1.*, t2.*
FROM t1 JOIN
     t2
     ON t1.some_id = t2.some_id LEFT JOIN
     some_table st
     ON st.some_column = true
WHERE st.some_column IS NULL;

Answer 2

您甚至可以将 EXISTS() 条件包装在 pure SQL 函数中，它只会执行一次：

---

-- \i tmp.sql

CREATE TABLE omg
        ( id integer NOT NULL PRIMARY KEY
        , must_pay integer NOT NULL
        );
INSERT INTO omg(id, must_pay) VALUES(1,0);

CREATE FUNCTION owe_money() RETURNS BOOLEAN AS
$func$

SELECT EXISTS(SELECT 1
        FROM omg o
        WHERE o.must_pay > 0
        );
$func$
-- language sql;
language sql STABLE;

EXPLAIN
SELECT owe_money();

INSERT INTO omg(id, must_pay) VALUES(2,100);

EXPLAIN
SELECT owe_money();

EXPLAIN
SELECT * FROM omg
WHERE owe_money();

---

如果您将关键字STABLE 添加到函数定义中，DBMS 将知道返回值将不会更改（在同一事务/语句中）。如果没有STABLE，该函数将每行调用一次。

---

                                           QUERY PLAN                                            
-------------------------------------------------------------------------------------------------
 Seq Scan on omg  (cost=0.00..566.40 rows=713 width=8) (actual time=0.303..0.331 rows=2 loops=1)
   Filter: owe_money()
 Total runtime: 0.384 ms
(3 rows)

---

STABLE 函数的结果是一次性过滤器：

---

                                             QUERY PLAN                                               
-------------------------------------------------------------------------------------------------------
 Result  (cost=0.25..31.65 rows=2140 width=8) (actual time=0.304..0.316 rows=2 loops=1)
   One-Time Filter: owe_money()
   ->  Seq Scan on omg  (cost=0.25..31.65 rows=2140 width=8) (actual time=0.004..0.009 rows=2 loops=1)
 Total runtime: 0.379 ms

（4 行）

---

与不相关子查询相同的条件本质上产生与 STABLE 函数相同的计划（现在子查询执行一次，其结果 ($0) 在一次性过滤器中进行测试）：

---

EXPLAIN ANALYZE
SELECT * FROM omg
WHERE EXISTS (
        SELECT 1
        FROM omg o
        WHERE o.must_pay > 0
        );

                                                QUERY PLAN                                                
----------------------------------------------------------------------------------------------------------
 Result  (cost=0.05..31.45 rows=2140 width=8) (actual time=0.022..0.034 rows=2 loops=1)
   One-Time Filter: $0
   InitPlan 1 (returns $0)
     ->  Seq Scan on omg o  (cost=0.00..36.75 rows=713 width=0) (actual time=0.011..0.011 rows=1 loops=1)
           Filter: (must_pay > 0)
           Rows Removed by Filter: 1
   ->  Seq Scan on omg  (cost=0.00..31.40 rows=2140 width=8) (actual time=0.003..0.008 rows=2 loops=1)
 Total runtime: 0.081 ms
(8 rows)

---

但是我们不是还在扫描表格，如果该函数返回 false，则基本上执行 where false 每一行，或者如果该函数返回 true，则执行 where true？

---

EXPLAIN ANALYZE
SELECT * FROM omg
WHERE EXISTS (
        SELECT 1
        FROM omg o
        WHERE o.id < 0
        );

                                                         QUERY PLAN                                                          
-----------------------------------------------------------------------------------------------------------------------------
 Result  (cost=1.66..3.66 rows=100 width=8) (actual time=0.011..0.011 rows=0 loops=1)
   One-Time Filter: $0
   InitPlan 1 (returns $0)
     ->  Index Only Scan using omg_pkey on omg o  (cost=0.14..1.66 rows=1 width=0) (actual time=0.006..0.006 rows=0 loops=1)
           Index Cond: (id < 0)
           Heap Fetches: 0
   ->  Seq Scan on omg  (cost=0.00..2.00 rows=100 width=8) (never executed)
 Total runtime: 0.063 ms
(8 rows)

---

从未执行过说明了一切。