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From: Tom Lane <[email protected]>
To: Chao Li <[email protected]>
Cc: [email protected]
Subject: Re: Improve hash join's handling of tuples with null join keys
Date: Fri, 15 Aug 2025 12:52:06 -0400
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
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Chao Li <[email protected]> writes:
> With this patch, “isnull” now becomes true because of the change of strict op. Then the outer null join key tuple must be stored in a tuplestore. When an outer table contains a lot of null join key tuples, then the tuplestore could bump to very large, in that case, it would be hard to say this patch really benefits.

What's your point?  If we don't divert those tuples into the
tuplestore, then they will end up in the main hash table instead,
and the consequences of bloat there are far worse.

> Based on this patch, if we are doing a left join, and outer table is empty, then all tuples from the inner table should be returned. In that case, we can skip building a hash table, instead, we can put all inner table tuples into hashtable.innerNullTupleStore. Building a tuplestore should be cheaper than building a hash table, so this way makes a little bit more performance improvement.

I think that would make the logic completely unintelligible.  Also,
a totally-empty input relation is not a common situation.  We try to
optimize such cases when it's simple to do so, but we shouldn't let
that drive the fundamental design.

			regards, tom lane

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