Re: Should the optimizer optimize "current_date - interval '1 days'" (fwd)

Ron Mayer <ron@intervideo.com> writes:
>     where dat > (current_date - interval '1 days');
> never uses the index I have on "fact".

I suppose dat is of type date?

> Should it treat my current_dat... expression as a constant and use
> the index?  Or is there a good reason it doesn't?

You will never get an indexscan out of that because the expression
seen by the planner is

    where timestamp(dat) > timestamp-expression

which is not compatible with an index of datatype date.  You should
write something that yields a date, not a timestamp, for example

    where dat > (current_date - 1)

This should be indexable (and is, in current development sources)
but in 7.2 and before you have to do additional pushups because
the planner doesn't understand that current_date can be treated
as a constant for the duration of a single indexscan.  The standard
workaround is to create a function of a signature like
"days_ago(int) returns date" and mark it isCachable.  This is a cheat
but it works fine in interactive queries.  See past discussions in
the archives.

            regards, tom lane