Re: Parentheses in FROM clause and evaluation order.

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(at)sss(dot)pgh(dot)pa(dot)us>:
> Stephan Szabo <sszabo(at)megazone(dot)bigpanda(dot)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?
>
> > 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 and
> > another from A joined with that table.
>
> 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).
>
> For example, SQL92 3.3.4.4 says
>
> A conforming implementation is not required to perform the exact
> sequence of actions defined in the General Rules, but shall achieve
> the same effect on SQL-data and schemas as that sequence.
>
>
> regards, tom lane
>