From: Tom Lane <tgl@sss.pgh.pa.us>
To: Paul Ramsey <pramsey@cleverelephant.ca>
cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: Allowing extensions to supply operator-/function-specific info
In-reply-to: 
 <CACowWR1ycbLx0t7WwT8uuJnK7E3U+tfzKXBRPaQrgHiCG+hvbA@mail.gmail.com>
References: <15193.1548028093@sss.pgh.pa.us> <6953.1548297558@sss.pgh.pa.us>
 <6044.1548524131@sss.pgh.pa.us>
 <CA+TgmoYkLjVcbZZ8PYyaCTMYZJCQ8eiod8MSRMDaczjBw=a0bQ@mail.gmail.com>
 <19993.1548697885@sss.pgh.pa.us>
 <CACowWR3jZ4Xo1atU5xTDpEoeeKSw1==sKPhCy3a29HtcE9oQVQ@mail.gmail.com>
 <7671.1551135674@sss.pgh.pa.us>
 <CACowWR3_RyPSYO6a8TVLZ7KnL_rDNh2ywkSTeqFmrAPpP4y6-g@mail.gmail.com>
 <11051.1551139751@sss.pgh.pa.us>
 <CACowWR1ycbLx0t7WwT8uuJnK7E3U+tfzKXBRPaQrgHiCG+hvbA@mail.gmail.com>
Comments: In-reply-to Paul Ramsey <pramsey@cleverelephant.ca>
	message dated "Tue, 26 Feb 2019 13:23:36 -0800"
MIME-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-ID: <14746.1551219541.1@sss.pgh.pa.us>
Date: Tue, 26 Feb 2019 17:19:01 -0500
Message-ID: <14747.1551219541@sss.pgh.pa.us>
Precedence: bulk

Paul Ramsey <pramsey@cleverelephant.ca> writes:
> New line of questioning: under what conditions will the support
> function be called in a T_SupportRequestIndexCondition mode?

It'll be called if the target function appears at top level of a
WHERE or JOIN condition and any one of the function's arguments
syntactically matches some column of an index.

If there's multiple arguments matching the same index column, say
index on "x" and we have "f(z, x, x)", you'll get one call and
it will tell you about the first match (req->indexarg == 1 in
this example).  Sorting out what to do in such a case is your
responsibility.

If there's arguments matching more than one index column, say
index declared on (x, y) and we have "f(x, y)", you'll get a
separate call for each index column.  Again, sorting out what
to do for each one is your responsibility.

In most cases, multiple matching arguments are going to lead to
failure to construct any useful index condition, because your
comparison value has to be a pseudoconstant (ie, not a variable
from the same table, so in both of the above examples there's
no function argument you could compare to).  But we don't prejudge
that, because it's possible that a function with 3 or more arguments
could produce something useful anyway.  For instance, if what we've
got is "f(x, y, constant)" then it's possible that the semantics of
the function are such that y can be ignored and we can make something
indexable like "x && constant".  All this is the support function's
job to know.

> I have
> created a table (foo) a geometry column (g) and an index (GIST on
> foo(g)) and am running a query against foo using a noop function with
> a support function bound to it.

> The support function is called, twice, once in
> T_SupportRequestSimplify mode and once in T_SupportRequestCost mode.

What's the query look like exactly?  The other two calls will occur
anyway, but SupportRequestIndexCondition depends on the function
call's placement.

			regards, tom lane