Re: Avoid full GIN index scan when possible

Julien Rouhaud <rjuju123(at)gmail(dot)com> writes:
> On Fri, Jun 28, 2019 at 6:10 PM Tomas Vondra
> <tomas(dot)vondra(at)2ndquadrant(dot)com> wrote:
>> 2) I'm not sure it's a good idea to add dependency on a specific AM type
>> into indxpath.c. At the moment there are only two places, both referring
>> to BTREE_AM_OID, do we really hard-code another OID here?
>>
>> I wonder if this could be generalized to another support proc in the
>> inde AM API, with just GIN implementing it.

> Yes, this patch was more a POC than anything, to discuss the approach
> before spending too much time on infrastructure. I considered another
> support function, but I'm still unclear of how useful it'd be for
> custom AM (as I don't see any use for that for the vanilla one I
> think), or whether if this should be opclass specific or not.

I just spent a lot of sweat to get rid of (most of) indxpath.c's knowledge
about specific AMs' capabilities; I'd be very sad if we started to put any
back. Aside from being a modularity violation, it's going to fall foul
of the principle that if index AM X wants something, some index AM Y is
going to want it too, eventually.

Also, I'm quite unhappy about including index_selfuncs.h into indxpath.c
at all, never mind whether you got the alphabetical ordering right.
I have doubts still about how we ought to refactor the mess that is
*selfuncs.c, but this isn't going in the right direction.

>> 3) selfuncs.c is hardly the right place for gin_get_optimizable_quals,
>> as it's only for functions computing selectivity estimates (and funcs
>> directly related to that). And the new function is not related to that
>> at all, so why not to define it in indxpath.c directly?

I not only don't want that function in indxpath.c, I don't even want
it to be known/called from there. If we need the ability for the index
AM to editorialize on the list of indexable quals (which I'm not very
convinced of yet), let's make an AM interface function to do it.

BTW, I have no idea what you think you're doing here by messing with
outer_relids, but it's almost certainly wrong, and if it isn't wrong
then it needs a comment explaining itself.

regards, tom lane