Re: [HACKERS] triggers, views and rules (not instead)

Andreas wrote:
>
> Jan wrote:
>
> The only things not working for copy are rewrite rules. But I
> think we should restrict rules to the view handling in the
> future and move forward by implementing a pure and really
> powerful procedural language.
>
> Hm, it looks like you are not really a fan of the rewrite system,
> eventhough you seem to have the most insight in these matters. I wonder why?

Confusing - eh? Well I know much about the internals of the
postgres rule system and due to this I know where the limits
are. Especially in the case of qualifications it somtimes
gets totally confused about what to compare against what. Try
to add a delete rule on a view that is simply a select * from
another table and then delete some tuples :-)

>
> Why I like the rewrite system is:
> 1. select rewrite -- select trigger would be no good (optimizer)

Exactly that's what is done if you create a view. Postgres
creates a regular table (look at pg_class and into the
database directory) and then sets up a relation level instead
rewrite rule on select.

> 2. The client can be really dumb, like MS Access or some other
> standard ODBC tool
> which does not know anything about funcs procs and the like
> (even without using passthrough)

Yupp - the client must not know why and how and where the
data is left and coming from. But that's true in any case - a
trigger for each row on insert can do anything different and
push the data wherever it wants.

> 3. it is a lot more powerful than views

As said - views are only one special rule case in Postgres.

> 4. it allows the optimizer to get involved (this is where triggers
> fail per definition)
> 5. once understood it is very easy to use
> easier than trigger with c stored procedure at least

Optimizing again and again. If the rules aren't instead, the
querytree get's additional queries for every rule appended.
Have a table field that references an entry in another table
and this entry should have a refcount. So on update you must
decrease the refcount from the old ref and increase it on the
new. You create two rules so the UPDATE will result in 1
scan and 2 nestloops with scans inside - really optimized if
the referenced value doesn't change. And don't think that a
rule qual of NEW != CURRENT might help - that will result in
2 mergejoins where the scanned tuples are compared.

BTW, this sample doesn't work currently because the rules
queries are appended at the end of the querytree, thus the
decrement scan having the same qual will not find the old
tuple at all because it's already outdated
(command_counter_increment between processing the queries).
Referencing CURRENT in a rule is not what most people think
it is.

The old 4.2 postgres had a second, instance level rule system
(prs2 stubs) that fired the rules actions when actually the
old tuple and the new projected tuple where handy. There you
could have made also things like 'UPDATE NEW SET a = 4' that
really modified the in memory tuple in the executors
expression context. Who the hell removed all that? It was so
nice :-(

A really simple to write trigger can compare old != new and
only if send down the other two queries. This time they wont
be nestloops, they are simple scans. And the trigger can
arrange that the queries it uses are only parsed on it's
first of all calls and store the generated execution plans
permanently for quick execution (look at SPI_prepare).

For the stored C procedures you're totally right. I don't
like the C functions because it requires postgres superuser
rights to develop them and thus I created PL/Tcl where joe
user can hack around without having complete access to the
whole database (look at src/pl/tcl). And someday after 6.3
release I'll really start on a plain PL/pgSQL implementation
that would give a normal user the opportunity to create
functions and triggers on a high level. There is light at the
end of the tunnel - hope that it isn't the coming train :-)

>
> I guess if triggers could also trigger simple select statements, I could do
> most of what I want using triggers except of course the select stuff.
> But as I said I like the rules system very much, especially after your
> recent
> fixes Jan :-) So please stick to supporting all 3: triggers, views and
> rules. Wow :-)

Well - a trigger cannot build a view. The relation underlying
the view doesn't contain any tuples and a select trigger will
never be fired. As long as there is no possibility to return
tuple sets from non-SQL functions. But a trigger can do
things like the pg_hide_passwd stuff much more powerful. You
could define the trigger so that it checks if the user is a
superuser and overwrite the passwd value only in the case
where he/she isn't. If fired at the right place it would too
work for things like the copy command etc.

We must stay with all 3 features. And I will take a look at
the INSERT ... SELECT view problem really soon as it is a
rule system problem that breaks views. But this is only the
SELECT rewriting part of the rule system which I really like
(optimizable). The other areas (insert, update, delete) of
the rule system are dangerous and I really think a powerful
PL/pgSQL language could make them obsolete.

Jan

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