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From: Tom Lane <[email protected]>
To: Alexander Pyhalov <[email protected]>
Cc: Ranier Vilela <[email protected]>
Cc: Zhihong Yu <[email protected]>
Cc: Pgsql Hackers <[email protected]>
Subject: Re: Push down time-related SQLValue functions to foreign server
Date: Tue, 18 Jan 2022 14:11:30 -0500
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
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Alexander Pyhalov <[email protected]> writes:
> Tom Lane писал 2022-01-18 19:53:
>> However, before we proceed any further with this patch, I think we
>> really ought to stop and think about the question I raised last
>> night: why are we building a one-off feature for SQLValueFunction?
>> Wouldn't the same parameter-substitution mechanism work for *any*
>> stable expression that doesn't contain remote Vars?  That would
>> subsume the now() case as well as plenty of others.

> This means we'll translate something like
> explain select * from t where d > now() - '1 day'::interval;
> to
> select * from t where d > $1;

Right.

> The question is how will we reliably determine its typmod (given that I 
> read in exprTypmod() comment
> "returns the type-specific modifier of the expression's result type, if 
> it can be determined.  In many cases, it can't".

I don't think we need to.  If exprTypmod() says the typmod is -1,
then that's what it is.

> What do we save if we don't ship whole expression as param, but only 
> stable functions? Allowing them seems to be more straightforward.

How so?  Right off the bat, you get rid of the need for your own
version of contain_mutable_function.  ISTM this approach can probably
be implemented in a patch that's noticeably smaller than what you
have now.  It'll likely be touching entirely different places in
postgres_fdw, of course.

			regards, tom lane

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