Re: Using SUBSELECT in CHECK expressions

On Thursday, November 6, 2003, at 02:38 AM, Tom Lane wrote:

> Michael Glaesemann <grzm(at)myrealbox(dot)com> writes:
>> On Wednesday, November 5, 2003, at 06:26 AM, Tom Lane wrote:
>>> Just put the SELECT into a function that's called by the CHECK
>>> expression.
>
>> Tom, you've just provided the solution to something I've been thinking
>> about a lot. I really have to start thinking in terms of functions.
>> Are
>> there any restrictions on the function used in the CHECK, i.e., it
>> must
>> be STRICT or IMMUTABLE?
>
> IIRC we check that it is labeled IMMUTABLE. Whether it really is or
> not, we cannot check (and if it uses a SELECT on tables that could
> change, then of course it isn't; so you are going to need to tell a
> little white lie here).
>
> Because of that, you need to be careful that you pass at least one
> nonconstant argument to the function within the CHECK expression, else
> you risk having the call constant-folded too early. Normally you'd
> probably pass values from the checked row into the function, so this
> doesn't seem like it should be a problem, but I could see someone
> getting bit by it someday ...

Thanks for the clarification. Is this in the documentation somewhere?
I'm not quite used to the new documentation ordering yet.

Michael
grzm myrealbox com