Re: [HACKERS] create rule changes table to view ? - Mailing list pgsql-hackers

Peter Eisentraut wrote:

>
> On Mon, 12 Jul 1999, Tom Lane wrote:
>
> > Ryan Bradetich <rbrad@hpb50023.boi.hp.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.
>
> Does anyone else see a problem with this? This sort of approach almost
> prevents views with distinct, union, order by, etc. from ever being
> implemented.

Pardon - YES and NO!

    After  all  I  think (even if it was a really great job) that
    Stonebraker was wrong. Views cannot be completely implemented
    by  rules.  That  would  make it impossibly complicated for a
    query planner.

    But I'm a YESBUTTER :-)

    But it really was a great  job!  In  the  actual  version  of
    PostgreSQL  you  can  define a view that's a join of 3 tables
    and then select from that view by joining it with  another  2
    tables.  The  result  will be a querytree that's exactly what
    you would have to type if there wouldn't be any view's at all
    -  a join over 5 tables. That (however complicated) querytree
    is handed to the optimizer.

    It is the optimizer's job to decide the best access path  for
    a 5 table join.

    YESBUT!

    Stonebraker  was  wrong - and must have been bacause today we
    want to get SQL92 compliant - and that  spec  didn't  existed
    when  he  designed  our  rule  sytem.   The  rule  system  is
    something we got from  the  good  old  v4.2  Postgres.   That
    wasn't an SQL database, the querylanguage was POSTQUEL. So it
    isn't surprising that the original rule system  spec's  don't
    meet today's SQL needs.

    For  thing's like aggregates, distinct/grouping and the like,
    we need to take a step backward and really do  some  kind  of
    view  materialization  (create  a real execution path for the
    view's definition). But don't force that to be done  whenever
    a view is used - that doesn't make things better.

>
> I don't know what other people use their views for but I use them to store
> complicated queries. So, in essence it would suffice to store the text of
> the query with a view rather than faking tables for it, thus confusing all
> sorts of utility programs.
>
> Then again, I'd be interested to know what to developers' idea of normal
> usage of a view is.

    It doesn't count what 95% of our users use view's for. A view
    is  a  relation  like  a  table,  and  if  appearing  in  the
    rangetable, it must be treated like a relation.

    Well - let's only store the "QUERY TEXT" of a view:

      CREATE VIEW v1 AS SELECT X.a, X.b, Y.b AS c
        FROM tab1 X, tab2 Y
        WHERE X.a = Y.a;

    Simple enough - O.K.?

    Now we execute some simple queries:

      SELECT * FROM vi;

      SELECT Z.a, V.b, V.c FROM tab3 Z, v1 V
        WHERE Z.a = V.a;

      SELECT Z.a, SUM(V.c) FROM tab3 Z, v1 V
        WHERE Z.a = V.a;

      INSERT INTO tab4 SELECT Z.a, SUM(V.c) FROM tab3 Z, v1 V
        WHERE Z.a = V.a
        AND V.b > 2;

      DELETE FROM tab5 WHERE aa = v1.a AND bb < v1.c;

    Simple enough? All valid SQL statements! Could you now simply
    explain  HOW  to  build  the  correct  final  statements   by
    incorporating   the   stored  "QUERY  TEXT"  into  the  above
    statements?

    I really mean HOW - not what the equivalent statements,  hand
    translated,  would  look  like  (I've  read  querytrees  like
    printed in debug level 3 several night's until  I  understood
    how rules should work - so I know how to rewrite the above by
    hand).  The way I know to express  this  in  C  is  the  rule
    system  you  find  in  rewrite_handler.c  and rewrite_manip.c
    (mostly). If you know an easier way, let me know.

    PLEASE DON'T READ THIS REPLY AS A SORT OF  A  FLAME.  I  KNOW
    THAT IT IS HARD TO UNDERSTAND THE RULE SYSTEM - I HAD TO TAKE
    THAT LEARNING CURVE MYSELF. AFTER  ALL  I  STILL  MIGHT  HAVE
    MISSED SOMETHING - THUS I THINK WE STILL NEED MATERIALIZATION
    OF VIEWS IN SOME CASES (yesbut only in few cases - not in all
    view cases).


Jan

--

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