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Date: Fri, 14 Feb 2025 11:59:48 +0100
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Subject: Re: Virtual generated columns
To: jian he <jian.universality@gmail.com>,
 Richard Guo <guofenglinux@gmail.com>
Cc: Zhang Mingli <zmlpostgres@gmail.com>,
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From: Peter Eisentraut <peter@eisentraut.org>
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On 13.02.25 14:06, jian he wrote:
> I didn't solve the out join semantic issue.
> i am wondering, can we do the virtual generated column expansion in
> the rewrite stage as is,
> and wrap the expressions in PHVs if the virtual generated
> columns come from the nullable side of an outer join.

PlaceHolderVar looks like a fitting mechanism for this.  But it's so far 
a planner node, so it might take some additional consideration if we 
want to expand where it's used.

Maybe a short-term fix would be to error out if we find ourselves about 
to expand a Var with varnullingrels != NULL.  That would mean you 
couldn't use a virtual generated column on the nullable output side of 
an outer join, which is annoying but not fatal, and we could fix it 
incrementally later.