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From: Tom Lane <[email protected]>
To: Richard Guo <[email protected]>
Cc: PostgreSQL-development <[email protected]>
Subject: Re: Remove no-op PlaceHolderVars
Date: Thu, 15 Jan 2026 22:37:28 -0500
Message-ID: <[email protected]> (raw)
In-Reply-To: <CAMbWs4-_fDPOiYJWj330pJS++kKbTw_EfO5-pC8nPKHUAScMWw@mail.gmail.com>
References: <CAMbWs48biJp-vof82PNP_LzzFkURh0W+RKt4phoML-MyYavgdg@mail.gmail.com>
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Richard Guo <[email protected]> writes:
> While fixing some performance issues caused by PHVs recently, it
> struck me again that we should be removing "no-op" PHVs whenever
> possible, because PHVs can be optimization barriers in several cases.

My immediate reaction is "how sure are you that they're no-ops"?
I recall that there are places where we intentionally insert PHVs
to preserve the separate identity of the contained expression
(so that, for example, it can be matched to a subquery output
later).

> I still do not have a good idea for ensuring that removing the PHV
> wrapper does not break the expression tree invariants.  But maybe we
> can use a conservative approach: we only strip the PHV if the
> contained expression is known to be safe (for example, a simple Var).

Do we generate a PHV at all in that case?  Seems like we could
deal with that by adding to the Var's varnullingrels instead of
making a wrapper node.

			regards, tom lane

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