Re: Sorting when LEFT JOINING to 2 same tables, even

On Thu, 11 Mar 2004, Octavio Alvarez wrote:

>
> Hello to everybody.
>
> I ask your help for a severe problem when doing a query that LEFT JOINs
> one table to another ON a field, and then LEFT JOINs again to another
> "instance" of a table ON another field which stores the same entity, but
> with different meaning.
>
> I include 3 EXPLAIN ANALYZEs:
> * The first one, the target (and problematic) query, which runs in 5 to 6
> minutes.
> * The second one, a variation with the second LEFT JOIN commented out,
> which runs in 175 to 450 ms.
> * The third one, a variation of the first one with ORDER BY removed, which
> gives me about 19 seconds.
>
> Therefore, I feel like there are two problems here the one that raises the
> clock to 6 minutes and one that raises it to 20 seconds. I expected a much
> lower time. I checked indexes and data types already, they are all fine.
> All relevant fields have BTREEs, all PKs have UNIQUE BTREE, and all id and
> ext_* fields have 'integer' as data type. Each ext_* has its corresponding
> REFERENCES contraint.
>
> I translated all the table and field names to make it easier to read. I
> made my best not to let any typo go through.
>
> I'd appreciate any help.

This join filter
> Join Filter: ("outer".ext_materia__equivalencia =
> "outer".id)

which I believe belongs to

> LEFT JOIN t_materias AS t_materias__equivalentes ON
> ext_materia__equivalencia = t_materias.id

seems wrong. Did you maybe mean = t_materias__equivalentes.id
there?