Re: Parentheses in FROM clause and evaluation order. - Mailing list pgsql-sql
| From | Dario Bahena Tapia |
|---|---|
| Subject | Re: Parentheses in FROM clause and evaluation order. |
| Date | |
| Msg-id | [email protected] Whole thread Raw |
| In response to | Re: Parentheses in FROM clause and evaluation order. (Tom Lane <[email protected]>) |
| List | pgsql-sql |
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 <[email protected]>: > Stephan Szabo <[email protected]> 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 >