MAX and MIN are single-parameter aggregate functions. LEAST and GREATEST are two-parameter (though in postgres they could be defined for 3 and more parameters) scalar functions.
If LEAST and GREATEST can accept any number of parameters, wouldn't it make sense to code it like the way COALESCE works, rather than defining a function for it? This way we don't need define all the various functions with different types.
e.g.
SELECT greatest(a, b, c) FROM bar;
becomes
SELECT greatest(a, greatest(b, c)) from bar;
becomes
SELECT
CASE WHEN b < c
THEN
CASE WHEN c < a
THEN a
ELSE c
END
ELSE
CASE WHEN b < a
THEN a
ELSE b
END
END
FROM bar;From the docs:
COALESCE and NULLIF are just shorthand for CASE expressions. They are actually converted into CASE expressions at a very early stage of processing, and subsequent processing thinks it is dealing with CASE. Thus an incorrect COALESCE or NULLIF usage may draw an error message that refers to CASE.
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