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

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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