On 2018/04/11 10:44, Tom Lane wrote:
> Kyotaro HORIGUCHI <horiguchi.kyot...@lab.ntt.co.jp> writes:
>> At least partition bound *must* be a constant. Any expression
>> that can be reduced to a constant at parse time ought to be
>> accepted but must not be accepted if not.
> My point is that *any* expression can be reduced to a constant,
> we just have to do so.

Currently transformPartitionBoundValue() applies eval_const_expressions()
by way of calling expression_planner().  However passing to it, say, an
expression representing random() is unable to reduce it to a Const because
simplify_function/evaluate_function won't compute a mutable function like
random().  So, that currently results in an error like this (note that
this is after applying Horiguchi-san's latest patch that enables
specifying random() as a partition bound in the first place):

create table foo_part partition of foo for values in ((random())::int);
ERROR:  specified value cannot be cast to type integer for column "a"
LINE 1: ...table foo_random partition of foo for values in ((random()):...
DETAIL:  The cast requires a non-immutable conversion.
HINT:  Try putting the literal value in single quotes.

The error is output after the following if check in
transformPartitionBoundValue fails:

    /* Fail if we don't have a constant (i.e., non-immutable coercion) */
    if (!IsA(value, Const))

I think what Tom is proposing here, instead of bailing out upon
eval_const_expressions() failing to simplify the input expression to a
Const, is to *invoke the executor* to evaluate the expression, like the
optimizer does in evaluate_expr, and cook up a Const with whatever comes
out to store it into the catalog (that is, in relpartbound).


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