public inbox for [email protected]  

help / color / mirror / Atom feed

From: Tom Lane <[email protected]>
To: WU Yan <[email protected]>
Cc: [email protected]
Subject: Re: Wasteful nested loop join when there is `limit` in the query
Date: Mon, 17 Feb 2025 02:01:41 -0500
Message-ID: <[email protected]> (raw)
In-Reply-To: <CAAdwFAwm6HwXM_cuPWZBxrxX4E7pBdVg=KcVDSP6q9ume3hYpQ@mail.gmail.com>
References: <CAAdwFAwm6HwXM_cuPWZBxrxX4E7pBdVg=KcVDSP6q9ume3hYpQ@mail.gmail.com>

WU Yan <[email protected]> writes:
> Hello everyone, I am still learning postgres planner and performance
> optimization, so please kindly point out if I missed something obvious.

An index on employee.name would likely help here.  Even if we had
an optimization for pushing LIMIT down through a join (which you
are right, we don't) it could not push the LIMIT through a sort step.
So you need presorted output from the scan of "employee".  I think
this example would behave better with that.  You may also need to
test with non-toy amounts of data to get the plan you think is
better: an example with only half a dozen rows is going to be
swamped by startup costs.

			regards, tom lane

view thread (2+ messages)

reply

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Reply to all the recipients using the --to and --cc options:
  reply via email

  To: [email protected]
  Cc: [email protected], [email protected], [email protected]
  Subject: Re: Wasteful nested loop join when there is `limit` in the query
  In-Reply-To: <[email protected]>

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

This inbox is served by agora; see mirroring instructions
for how to clone and mirror all data and code used for this inbox