RE: [HACKERS] CONSTRAINTS...

> -----Original Message-----
> Generic triggers in C that are argument driven would be
> possible. But the drawback is that those triggers have to be
> very smart to use saved SPI plans (one for every different
> argument set). And it must be row level triggers, so for an
> update to a 2 meg row table they will be fired 2 million
> times and run their queries inside - will take some time.
>
> More painful in the 2 meg row situation is that trigger
> invocation has to be delayed until COMMIT if the constraint
> is deferred. I think we cannot remember 2 million OLD plus 2
> million NEW tuples if one tuple can have up to 8K (will be
> 32GB to remember plus overhead), so we need to remember at
> least the CTID's of OLD and NEW and refetch them for the
> trigger invocation. OUTCH - the OLD ones are at the head and
> all the NEW ones are at the end of the tables file!
Agreed scrap the triggers idea.

> > Maybe let's go with the rewrite system, because it works, and is
> > flexible and strangely designed for this type of problem.
> Similar to
> > how we use the rewrite system for views.
>
> And the other changes I've planned for the rewrite system
> will improve this much more.
>
> 1. Change pg_rewrite.ev_attr into an int28. This would be
> useful for ON UPDATE rules so the rewrite system can
> easily check if a rule has to be applied or not. If none
> of named attributes gets something different assigned
> than it's own OLD value, they aren't updated so the rule
> could never result in an action and can be omitted
> completely.
Would we add a system table for constraints? I believe that the
DEFERRABLE option means we need special information about all
constraints. (ie. is_deferrable boolean, is_deferred boolean).

> 2. Create cross reference catalog that lists all relations
> used in a rule (rangetable). If we have a DELETE CASCADE
> constraint, the rule is triggered on the key table and
> the action is a DELETE from the referencing table. If now
> the referencing table is dropped, the rule get's
> corrupted because the resulting querytree isn't plannable
> any longer (the relations in the rules rangetable are
> identified by the OID in pg_class, not by relname). You
> can see the effect if you create a view and drop one of
> the base tables.
*Bleck* We can't place cascading or restriction information into the
table definition. Don't know if you had thought about it, but you could
only create them in reverse order (child<-parent), very messy when you
have many 'child' tables to cascade.

> Well, we need to define what to do if a table is dropped
> that occurs in the crossref. First of all, the rules have
> to be dropped too, but in the case of a view rule, maybe
> the whole view too?
I agree that we should drop rules defined on the dropped table, but not
those that simply access the dropped table, including views. If I need
to drop a table which has a view defined for it simply to rearrange
column order, I'd like not to have to recreate the view as well and/or
any RI-constraints that reference that table as well.

> And in the case where a key table to which another one
> has a CHECK reference is dropped? The rule action will
> allways abort, so it isn't useful any more. But I
> wouldn't like to silently drop it, because someone might
> want to drop and recreate the key table and this would
> silently result in that all the constraints have been
> lost.
This is what I mean about views and RI-constraints.

> Maybe we should change the rulesystem at all so that the
> rangetable entries in the rule actions etc. are updated
> with a lookup from pg_class at rewrite time. Must be done
> carefully because someone might drop a table and recreate
> it with a different schema corrupting the parsetree of
> the rule actions though.
Could we allow a 'recompile' of the rules? Which would update the rule
actions and report which rules have been corrupted. And possibly a way
to simple drop all invalid rules. We could simply mark the rules that
need to be recompiled on a DROP or ALTER TABLE (to allow for dropping a
column, or changing a column type) from the crossref.

> 3. Allow an unlimited number of rules on a relation.
> Currently there is a hard coded limit on the number of
> rules the relation can hold in it's slots.
I agree with this one without reference, especially if we actually
implement RI/constraints using rewrite.

> > I am basically asking for a reason _not_ to use the rewrite
> system for
> > this. I can't think of one myself.
>
> It might interfere with the new MVCC code. The rule actions
> must see exactly the OLD tuples that where used in the
> original statements. Not only those in the updated table
> itself, think of an INSERT...SELECT or an UPDATE where the
> TLE or qual expressions are values from other tables.
Shouldn't be too hard to accomplish. We simply need to be sure that the
values are either cached or re-cacheable/re-producible (if there are
memory concerns) until the end of the transaction. So, either:
1. cache all the values
2. store the part of the parsetree needed to recreate the values and
have MVCC mark any needed rows off limits to vacuum
I know that there are many other subtleties that I'm not addressing such
as indexes, but the above will get you the answer.
FWIW, RI and CONSTRAINTS would be affected by this.

> Not a real reason, just something to have in mind and maybe
> switching silently to another MVCC isolation level if
> constraint rules get applied, so all tables read from now on
> will get a read lock applied and cannot get updated
> concurrently until COMMIT.
Not needed, see the above. We just need to mark the rows with the tid,
or some-such, because dropped/updated rows will hang around until a
vacuum.

> And it's a problem I've came across just writing this note
> where MVCC already could have broken rewrite rule system
> semantics.
Maybe, but I know you'll straighten it all out. I have faith in you,
Jan.

> Jan
-DEJ

P.S. I love the discussion that my simple little request has sparked.