On 5/24/16 9:56 PM, Craig Ringer wrote:
On 25 May 2016 at 06:56, Tom Lane <t...@sss.pgh.pa.us
<mailto:t...@sss.pgh.pa.us>> wrote:

    ilm...@ilmari.org <mailto:ilm...@ilmari.org> (Dagfinn Ilmari
    =?utf-8?Q?Manns=C3=A5ker?=) writes:
    > Tom Lane <t...@sss.pgh.pa.us <mailto:t...@sss.pgh.pa.us>> writes:
    >> ... and if the CHECK expression is immutable ...

    > Doesn't it have to be already?

    AFAIK we don't insist on that currently.  You could imagine useful
    that are not, for example CHECK(write_timestamp <= now()).

That seems like abuse of CHECK to me, and a job for a trigger. If anyone
proposed allowing that and it wasn't already allowed (or at least not
prohibited explicitly) it'd get shot down in flames.

Yeah, non-IMMUTABLE checks are a really bad idea, especially because they will only trip you up well after the fact (like when restoring from a dump).

If we wanted checks that apply only on row insert/update a CHECK WRITE
or similar would seem suitable; something that implies that it's an
_action_ taken on write and doesn't stop the constraint later becoming
violated by unrelated changes. Like a trigger. Such a check could be
allowed to use subqueries, reference other tables, call functions and
all the other fun stuff you're not meant to do in a CHECK constraint.
Like a trigger.

Or we could use triggers.

Rather than creating new CHECK syntax, I'd rather have a notion of "check triggers" that simply evaluate a boolean expression (and don't require defining a function).
Jim Nasby, Data Architect, Blue Treble Consulting, Austin TX
Experts in Analytics, Data Architecture and PostgreSQL
Data in Trouble? Get it in Treble! http://BlueTreble.com
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