AW: [HACKERS] triggers, views and rules (not instead) - Mailing list pgsql-hackers

Since we have so little documentation on the rules, I think we should save
every
little word describing them, so could you simply put the following into a
rules.readme
(undigested is still better than not adding it)

> > 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.
>
I fought that like a windmill, I guess it would be better to kill the
CURRENT keyword
with this meaning alltogether, since it only has the same meaning as the
tablename itself.
I have already crossed it out of my mind and don't miss anything.
I think there should instead be an OLD and NEW keyword
like in triggers:
    referencing old as <oldname> new as <newname>
that only reference the tuples in memory.

>     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 :-(
>
Absolutely !    I did cry up when that was done, but nobody responded :-(
Well to be honest Vadim did respond with the trigger code, which made me
feel comfortable again.

>     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
>
Ok, to sum it up:
    1. We need and want the select part of the rewrite rules.
    2. for the insert/update/delete rules the old instance rules system
        was much more appropriate. TODO: dig up the old code
        and merge it with the current trigger Implementation
            it must be pretty much the wanted functionality (it
supported sql)
        3. the CURRENT keyword in the i/u/d rewrite rules is stupid
and should be disabled
           destroyed and burned in hell
        4. To stick to the mainstream we should enhance the trigger
syntax,
            and forget the rule stuff for i/u/d

        create trigger passwd_utr
        ..........
        referencing old as o new as n
          for each row (statement, statement, statement, procedure,
...... all PL/pgSQL syntax allowed );
        -- with a syntax to modify the new tuple in memory


Andreas