SELECT和WHERE子句中的功能相同

初学者问题：

f(x, y)我的数据库表中的x和y两列都有一个昂贵的函数。

我想执行一个查询，该查询将函数的结果作为列提供给我，并对其施加约束，例如

SELECT *, f(x, y) AS func FROM table_name WHERE func < 10;

但是，这不起作用，所以我将不得不写一些类似的东西

SELECT *, f(x, y) AS func FROM table_name WHERE f(x, y) < 10;

这会运行两次昂贵的功能吗？最好的方法是什么？

让我们创建一个具有副作用的函数，以便我们可以看到执行了多少次：

CREATE OR REPLACE FUNCTION test.this_here(val integer)
    RETURNS numeric
    LANGUAGE plpgsql
AS $function$
BEGIN
    RAISE WARNING 'I am called with %', val;
    RETURN sqrt(val);
END;
$function$;

然后像您这样调用它：

SELECT this_here(i) FROM generate_series(1,10) AS t(i) WHERE this_here(i) < 2;

WARNING:  I am called with 1
WARNING:  I am called with 1
WARNING:  I am called with 2
WARNING:  I am called with 2
WARNING:  I am called with 3
WARNING:  I am called with 3
WARNING:  I am called with 4
WARNING:  I am called with 5
WARNING:  I am called with 6
WARNING:  I am called with 7
WARNING:  I am called with 8
WARNING:  I am called with 9
WARNING:  I am called with 10
    this_here     
──────────────────
                1
  1.4142135623731
 1.73205080756888
(3 rows)

如您所见，该函数至少被调用一次（从WHERE子句中），当条件为true时，再次调用该函数以产生输出。

为了避免第二次执行，您可以按照Edgar的建议进行操作 -即包装查询并过滤结果集：

SELECT * 
  FROM (SELECT this_here(i) AS val FROM generate_series(1,10) AS t(i)) x 
 WHERE x.val < 2;

WARNING:  I am called with 1
... every value only once ...
WARNING:  I am called with 10

要进一步检查它的工作原理，可以去那里pg_stat_user_functions检查calls（假设track_functions设置为“全部”）。

让我们尝试一些没有副作用的东西：

CREATE OR REPLACE FUNCTION test.simple(val numeric)
 RETURNS numeric
 LANGUAGE sql
AS $function$
SELECT sqrt(val);
$function$;

SELECT simple(i) AS v 
  FROM generate_series(1,10) AS t(i)
 WHERE simple(i) < 2;
-- output omitted

SELECT * FROM pg_stat_user_functions WHERE funcname = 'simple';
-- 0 rows

simple()实际上太简单了，因此可以内联，因此它不会出现在视图中。让我们使其无懈可击：

CREATE OR REPLACE FUNCTION test.other_one(val numeric)
 RETURNS numeric
 LANGUAGE sql
AS $function$
SELECT 1; -- to prevent inlining
SELECT sqrt(val);
$function$;

SELECT other_one(i) AS v
  FROM generate_series(1,10) AS t(i)
 WHERE other_one(i) < 2;

SELECT * FROM pg_stat_user_functions ;
 funcid │ schemaname │ funcname  │ calls │ total_time │ self_time 
────────┼────────────┼───────────┼───────┼────────────┼───────────
 124311 │ test       │ other_one │    13 │      0.218 │     0.218

SELECT *
  FROM (SELECT other_one(i) AS v FROM generate_series(1,10) AS t(i)) x 
 WHERE v < 2;

SELECT * FROM pg_stat_user_functions ;
 funcid │ schemaname │ funcname  │ calls │ total_time │ self_time 
────────┼────────────┼───────────┼───────┼────────────┼───────────
 124311 │ test       │ other_one │    23 │      0.293 │     0.293

从外观上看，无论有无副作用，图片都是相同的。

更改other_one()为，IMMUTABLE会使行为（可能令人惊讶）变得更糟，因为在两个查询中它将被调用13次。

尝试再次调用它：

SELECT
     *
FROM (
SELECT
     *,
     f(x, y) AS func
FROM table_name
) a
WHERE a.func < 10;