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Message-ID: <3d104d6f0508151920fcc8a03@mail.gmail.com>
Date: Mon, 15 Aug 2005 21:20:35 -0500
From: Dario Bahena Tapia <dario.mx@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Subject: Re: Parentheses in FROM clause and evaluation order.
Cc: Stephan Szabo <sszabo@megazone.bigpanda.com>,
	pgsql-sql@postgresql.org
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Ok, thanks for the responses guys.

Then, in the case where the final result is the same, could we think
the parentheses in the FROM clause, as a tool to clarify the query to
the user? Since in the end, this order could be changed by the
implementation for performance reasons.

salu2
dario estepario ...

2005/8/15, Tom Lane <tgl@sss.pgh.pa.us>:
> Stephan Szabo <sszabo@megazone.bigpanda.com> writes:
> > On Mon, 15 Aug 2005, Dario Bahena Tapia wrote:
> >> The final result seems to be the same, I just was curious about the
> >> standard behavior. Does the SQl says something about this execution
> >> order?
>=20
> > I believe SQL defines the order to pay attention to parens, so A join (=
B
> > join C) style clauses result in a "table" being derived from B join C a=
nd
> > another from A joined with that table.
>=20
> SQL only constrains the results, though.  It does not forbid the
> implementation from doing the work in whatever way seems best to it,
> so long as the results are the same (and "same" does not consider
> row ordering).
>=20
> For example, SQL92 3.3.4.4 says
>=20
>          A conforming implementation is not required to perform the exact
>          sequence of actions defined in the General Rules, but shall achi=
eve
>          the same effect on SQL-data and schemas as that sequence.
>=20
>=20
>                         regards, tom lane
>