Re: On partitioning

On Tue, Dec 16, 2014 at 12:15 PM, Robert Haas <robertmhaas@gmail.com> wrote:
> <Langote_Amit_f8@lab.ntt.co.jp> wrote:
>> Robert wrote:
>>> On Sun, Dec 14, 2014 at 9:12 PM, Amit Langote
>>> <Langote_Amit_f8@lab.ntt.co.jp> wrote:
>>> > This means if a user puts arbitrary expressions in a partition definition, say,
>>> >
>>> > ... FOR VALUES  extract(month from current_date) TO extract(month from
>>> current_date + interval '3 months'),
>>> >
>>> > we make sure that those expressions are pre-computed to literal values.
>>>
>>> I would expect that to fail, just as it would fail if you tried to
>>> build an index using a volatile expression.
>>
>> Oops, wrong example, sorry. In case of an otherwise good expression?
>
> I'm not really sure what you are getting here.  An "otherwise-good
> expression" basically means a constant.  Index expressions have to be
> things that always produce the same result given the same input,
> because otherwise you might get a different result when searching the
> index than you did when building it, and then you would fail to find
> keys that are actually present.

I think the point is partitioning based on the result of an expression
over row columns. Or if it's not, it should be made anyway:

PARTITION BY LIST (extract(month from date_created) VALUES (1, 3, 6, 9, 12);

Or something like that.