Re: [SQL] Order by

Herouth Maoz wrote:

>
> At 13:45 +0300 on 02/06/1999, Jan Wieck wrote:
>
>
> > That's all right and I'm not going to allow this just for the
> > ORDER BY. It will be enabled as kind of a side effect.
> > Because view's can have a GROUP BY clause, the
> > rewriter/planner must be able to produce plans where
> > different left-/righttrees have their own grouping. Grouping
> > requires a sort and thus sorting of subplans must be
> > supported.
> >
> > Why should we deny ORDER BY on views explicitly?
>
> The SQL92 standard, if I read it right, defines the view by a "query
> expression", which in its turn relies on "query specification", which in
> turn relies on "table expression", which does not include ORDER BY.
>
> In short, ORDER BY is not part of the query syntax used in the definition
> of views, because order is meaningless in views.

Right now, the parser accepts ORDER BY in CREATE VIEW because
the grammar includes the full SelectStmt rule for the view's
query expression. The LIMIT, DISTINCT and ORDER BY clauses
are rejected later in rewriteDefine.c.

>
> The basic idea is that we should get the same results if we use standard
> SQL whether we run on PostgreSQL or on Oracle. So, one may write an
> application which selects the data from the view, and relies on the order
> in the view for something (like grouping). The same program won't work in
> any other database system, because you simply can't make that assumption
> without adding 'ORDER BY' to your actual query.

GROUP BY must be possible in a view's query expression.
Without that views with aggregate columns could never work
properly.

If I now read your above statements right, "query expression"
-> "query specification" -> "table expression" also forbid's
GROUP BY, because it cannot occur in a table expression -
right? That means that a fully SQL92 compliant DB cannot
support aggregates in view definitions?

Oh what a perfect world - I'm stressing my brain to find a
way how they could work and the real solution is to forbid
them :-)

Jan

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