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Re: Why is PostgreSQL 7.0 SQL semantics different from Oracle's?

From: JanWieck(at)t-online(dot)de (Jan Wieck)
To: Thomas Holmgren <thm(at)cs(dot)auc(dot)dk>
Cc: pgsql-sql(at)postgresql(dot)org
Subject: Re: Why is PostgreSQL 7.0 SQL semantics different from Oracle's?
Date: 2000-05-24 21:04:06
Message-ID: 200005242104.XAA18271@hot.jw.home (view raw or flat)
Thread:
Lists: pgsql-sql
Thomas Holmgren wrote:
>
> Hello everyone! :)
>
> This little problem is bothering me a lot! It seems that PostgreSQL 7.0
> uses different semantics than Oracle when evaluting SQL?!

    Not that much, but ...

> [...]
>
> I have defined two views, viewA and viewB. They are defined as follow:
>
> CREATE VIEW viewA AS SELECT number, sum(amount) AS amount
> FROM A GROUP BY number;
>
> CREATE VIEW viewB AS SELECT number, sum(amount) AS amount
> FROM B FROUP BY number;

    here   the  problems  start.  PostgreSQL  has  (since  epoch)
    problems with aggregates, GROUP BY  clauses  and  some  other
    things when used in views.  We know exactly what causes these
    problems, but fixing them requires some huge  changes  across
    the  entire  backend.  This  work  is  scheduled  for the 7.2
    release.

> BUT WHEN I'm doing the EXACT SAME THING in PostgreSQL 7.0 I get a
> different result! It seems that Postgres executes the natural join in the
> query BEFORE performing the sum() in the definition of the views thus
> giving me a wrong result.

    Close, due to the fact that after applying the rewrite  rules
    for  the  views,  the entire thing is one join, but with only
    one (and thus  wrong)  gouping  step  on  the  toplevel.  The
    groupings  must be done on deeper levels per view, but theres
    no way to tell that in the querytree from the rewriter.

> How can I fix that??
> How come PostgreSQL uses different semantics when evaluating SQL
> expressions than other BDMSs?

    You can help us doing the huge changes in a couple of months.
    Even  if  you cannot help coding it, you might penetrate what
    we do with all those complicated schemas.

    Stay tuned.


Jan

--

#======================================================================#
# It's easier to get forgiveness for being wrong than for being right. #
# Let's break this rule - forgive me.                                  #
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