planner, *_collapse_limit

От: Marcus Engene
Тема: planner, *_collapse_limit
Дата: ,
Msg-id: 50115756.6000109@engene.se
(см: обсуждение, исходный текст)
Ответы: Re: planner, *_collapse_limit  (Merlin Moncure)
Список: pgsql-performance

Hi,

I've read a little bit about join_collapse_limit and from_collapse_limit
and I see their reason to exist.

A stupid question: in algorithms 101 you're usually told to make a chess
program and then you usually do a width first min max tree. A low level
opponent would interrupt this possibly infinite traversal early, thus
returning a possibly bad move, and if it's on a higher level it's
allowed to work longer and it will likely present a better path in the tree.

I understood it as that the *_collapse_limits are to stop a worst case
join making the optimizer going haywire, but it feels sad that trivial
big joins are cut off even if they're not too nasty.

Why would it not make some sense to have some time/space constraint on
the join heuristics instead of/in combination to how the limit presently
work? If we hit the ceiling, the best produced plan so far is used. The
chess analogy would obviously be a handful chess pieces left but the
min-max-tree traversal constraint is on a low depth (rather than
time/memory) so it would quickly traverse the few options and then be
constrained.

Best regards,
Marcus



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