Re: assertions and constraint triggers

On ons, 2010-08-11 at 10:47 -0400, Tom Lane wrote:
> > I thought the point of ASSERTIONs was that you could write a thing
> such as:
> > CREATE ASSERTION foo CHECK ((SELECT count(*) FROM tbl) = 4);
> > Enforcing that kind of constraints without true serializability
> seems 
> > impractical.
> 
> Enforcing that kind of constraint seems impractical with or without
> serializability.  You need some optimization method that avoids the
> need
> to do full-table scans after every update, or it's not going to be
> useful for any real-world situation.  Without a scheme that can do
> incremental checking for some useful class of assertion expressions,
> this isn't going to go far.

I'm not sure how great a use case there is for an assertion of the kind
"this table must contain at least 30 million rows".  But I think there
are many uses cases for checks like that on small and rarely changing
tables.