Re: Many left outer joins with limit performance

Gerhard Wiesinger <lists(at)wiesinger(dot)com> writes:
> FROM
> log l
> -- Order is relevant here
> LEFT OUTER JOIN key_description k1 ON k1.description = 'Raumsolltemperatur'
> LEFT OUTER JOIN log_details d1 ON l.id = d1.fk_id AND d1.fk_keyid = k1.keyid

Surely this query is just plain broken? You're forming a cross product
of the relevant log lines with the k1 rows having description =
'Raumsolltemperatur' (I assume this isn't unique, else it's not clear
what the point is) and then the subsequent left join cannot get rid of
anything. I think probably you meant something different, like

FROM
log l
LEFT OUTER JOIN log_details d1 ON l.id = d1.fk_id
LEFT OUTER JOIN key_description k1 ON k1.description = 'Raumsolltemperatur' AND d1.fk_keyid = k1.keyid

regards, tom lane