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From: Tom Lane <[email protected]>
To: Greg Rychlewski <[email protected]>
Cc: [email protected]
Subject: Re: nested loop joins
Date: Tue, 26 Oct 2021 14:56:12 -0400
Message-ID: <[email protected]> (raw)
In-Reply-To: <CAKemG7Xi_3eS+wT97ENWbzAmpwoVHJ5=YbQQcnswo5G5qHaNMw@mail.gmail.com>
References: <CAKemG7Xi_3eS+wT97ENWbzAmpwoVHJ5=YbQQcnswo5G5qHaNMw@mail.gmail.com>

Greg Rychlewski <[email protected]> writes:
> I'm looking to understand how nested loop joins with limits work. Say I
> have this query

> SELECT *
> FROM a  INNER JOIN b ON a.id = b.id
> LIMIT 10;

> Say this was done as a nested loop join where the rows of "a" are the outer
> loop.

> In this scenario, would all the rows of "a" be materialized or are they
> lazily evaluated?

Evaluation of the join will stop as soon as it's produced 10 rows.
Only as much of "a" will be read as necessary to get that.
(Potentially, not all of "b" would be read either, if 10 matches
to the first "a" row exist in "b".)

			regards, tom lane

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