Re: [GENERAL] SQL query problem (many table in FROM statement and many LEFT JOIN's) - Mailing list pgsql-sql

On Friday 10 October 2003 08:53, Marek Lewczuk wrote:
> > > SELECT
> > >   _CON.con_id,
> >
> > Please make sure you get the quoting right regarding table
> > names. PostgreSQL will fold _CON into _con unless quoted
> > "_CON". So, it may be that you created the table with quotes
> > ("_CON"). Now, in your query you don't use quotes and thusly
> > it is looking for a _con table. The simple rule of thumb is
> > to either always or never use quotes.
>
> I don't think that this is the solution, becouse the query:
>
> SELECT
>   _CON.con_id,
>   _MOD.mod_ty,
>   _CON.dri_id,
>   _CON.man_cod,
>   _ENG.eng_pow
> FROM
>   db_data.mda_mod _MOD,
>   db_data.mda_mod_con _CON,
>   db_data.mda_mak_eng _ENG,
>   db_data.set_mda_fue _FUE
> WHERE
>   _MOD.mod_id = '283' AND
>   _CON.mod_id = _MOD.mod_id AND
>   _CON.psd <= NOW() AND
>   _CON.ped > NOW() AND
>   _ENG.eng_id = _CON.eng_id AND
>   _ENG.eng_fue = _FUE.fue_id
>
> ...is working fine. I belive that this some problem with LEFT JOIN and
> FROM statement. If I will rewrite this query:

I think what's happening here is the fact that if you use explicit joins in a
query that forces the order of the joins. You originally had:

  db_data.mda_mod _MOD,
  db_data.mda_mod_con _CON,
  db_data.mda_mak_eng _ENG,
  db_data.set_mda_fue _FUE
  LEFT JOIN db_data.mda_mod_ver _VER ON _VER.ver_id = _CON.ver_id
  LEFT JOIN db_data.mda_mod_yea _YEA ON _YEA.yea_id = _CON.yea_id
which was being parsed as something like:

( (_FUE LEFT JOIN _VER)
  LEFT JOIN _YEA ),
_MOD, _CON, ENG

Of course, if it tries to evaluate in this order it can't see _CON from the
innermost brackets.

I believe the "force planner order" is configurable in the forthcoming 7.4

--
  Richard Huxton
  Archonet Ltd