Re: [HACKERS] create rule changes table to view ?

Tom Lane wrote:

>
> Ryan Bradetich <rbrad(at)hpb50023(dot)boi(dot)hp(dot)com> writes:
> > psql declares the the type to be view? if the relkind is a relation
> > and the relhasrules = true in pg_class for that entry. I will pull
> > the latest source and see if I can come up with a better way for
> > determining the type tomorrow, if someone else doesn't beat me to it
>
> The way Jan explained it to me, a view *is* a table that happens to
> have an "on select do instead" rule attached to it. If the table
> has data in it (which it normally wouldn't) you can't see that data
> anyway because of the select rule.

Right

>
> This is another example like SERIAL columns, UNIQUE columns, etc, where
> we are not really leaving enough information in the system tables to
> allow accurate reconstruction of what the user originally said. Was
> it a CREATE VIEW, or a CREATE TABLE and manual attachment of a rule?
> No way to tell. In one sense it doesn't matter a whole lot, but for
> psql displays and pg_dump it would be nice to know what happened.

Oh - but for VIEW's we leave enough information in the system
tables. Rules on event SELECT actually

1. must be INSTEAD

2. have exactly one action. This action must be another
SELECT which exactly produces a targetlist where all
attributes are in the order and of the types of the
tables schema

3. must be named "_RET<tablename>"

4. must be the only rule on event SELECT.

These restrictions clearly tell that if a table has an ON
SELECT rule, it IS A VIEW! There is absolutely no other
possibility.

Stonebraker originally planned to have other rules on the
SELECT case too, namely attribute rules which only rewrite a
single attribute of a table, and rules performing other
actions than a SELECT if someone scans that table. But AFAIK
these plans never materialized.

The problem on SELECT rules is that they have totally
different semantics than any other rules in that they must
get applied not only on SELECT. Instead we also rewrite
things like

INSERT ... SELECT

and

DELETE ... WHERE x = view.y AND view.z = ...

so views become usable in all kinds of statements.

When fixing the rewrite system for v6.4 I decided to simplify
the rewriting of SELECT rules by restricting them totally to
views. After that, I simply took out all that screwed up
code dealing with attribute rewriting and sent it down to the
bit recycling.

I don't plan to turn this wheel back. And if someone else
ever succeeds in doing so, we'll have another "ruleguru" :-)

So if you find an entry in pg_rewrite with ev_type=1 and
ev_class=<my_tables_oid>, then my_table is a view - end of
story.

Jan

--

#======================================================================#
# It's easier to get forgiveness for being wrong than for being right. #
# Let's break this rule - forgive me. #
#========================================= wieck(at)debis(dot)com (Jan Wieck) #