尝试创建聚合函数：
create aggregate min (my_type) (
sfunc = least,stype = my_type
);
ERROR: Syntax error at or near "least"
LINE 2: sfunc = least,^
我错过了什么？
Although the manual calls least a function：
The GREATEST and LEAST functions select the largest or smallest value from a list of any number of expressions.
我找不到它了：
\dfS least
List of functions
Schema | Name | Result data type | Argument data types | Type
--------+------+------------------+---------------------+------
(0 rows)
解决方法
至少和最伟大的不是真正的功能;在内部,它们被解析为MinMaxExpr(参见src / include / nodes / primnodes.h).
你可以使用这样的通用函数实现你想要的：
CREATE FUNCTION my_least(anyelement,anyelement) RETURNS anyelement LANGUAGE sql IMMUTABLE CALLED ON NULL INPUT AS 'SELECT LEAST($1,$2)';
(感谢Erwin Brandstetter关于CALLED ON NULL INPUT以及使用LEAST的想法.)
然后,您可以创建聚合
CREATE AGGREGATE min(my_type) (sfunc = my_least,stype = my_type);
