Re: BUG #18812: Conditional rule: inconsistent check for statement

Hi Tom,

thanks for the fastest and exhaustive answer!

On 15/02/2025 01:55, Tom Lane wrote:
> We get variants of this complaint from time to time, but few of
> them present use-cases that seem compelling enough to justify the
> performance costs of not doing constant-folding.

I think it's the right decision! But...

On 15/02/2025 01:55, Tom Lane wrote:
 > PG Bug reporting form <noreply@postgresql.org> writes:
 >> I've two rules for a view - unconditional INSTEAD (skip) and conditional
 >> INSTEAD (always FALSE). But if I trying to insert a type mismatched 
data to
 >> the view, I've got a type constraint error.
 >
 > [ shrug... ]  The WHERE FALSE condition is evaluated later than it
 > would need to be to prevent this error.  If we use a value that
 > doesn't trigger the error:
 >
 > =# explain verbose INSERT INTO v (c) VALUES ('testtest');
 >                        QUERY PLAN
 > ------------------------------------------------------
 >   Insert on public.t  (cost=0.00..0.01 rows=0 width=0)
 >     ->  Result  (cost=0.00..0.01 rows=1 width=14)
 >           Output: 'testtest'::character varying(10)
 >           One-Time Filter: false
 > (4 rows)
 >
 > we can see that the "false" is actually applied at runtime, but the
 > value coercion happened during planner constant-folding.  In general
 > the order of application of WHERE clauses is not guaranteed, so
 > there's not a good argument that this outcome is wrong.

What do you think about adding the behavior described above (undefined 
behavior, generally) to the help?

---
WBR
Boris