Re: Should use MERGE use BulkInsertState ?

From: Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>
To: Justin Pryzby <pryzby(at)telsasoft(dot)com>
Cc: Peter Eisentraut <peter(dot)eisentraut(at)enterprisedb(dot)com>, Amit Langote <amitlangote09(at)gmail(dot)com>, Japin Li <japinli(at)hotmail(dot)com>, Zhihong Yu <zyu(at)yugabyte(dot)com>, Simon Riggs <simon(dot)riggs(at)enterprisedb(dot)com>, pgsql-hackers(at)lists(dot)postgresql(dot)org, Tomas Vondra <tomas(dot)vondra(at)enterprisedb(dot)com>, Daniel Westermann <dwe(at)dbi-services(dot)com>, Erik Rijkers <er(at)xs4all(dot)nl>, Jaime Casanova <jcasanov(at)systemguards(dot)com(dot)ec>, Andres Freund <andres(at)anarazel(dot)de>
Subject: Re: Should use MERGE use BulkInsertState ?
Date: 2022-05-11 15:23:27
Message-ID: 202205111523.rhf6veb6i6c5@alvherre.pgsql
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On 2022-May-11, Justin Pryzby wrote:

> Should it use it ?
> It occured to me to ask when reading Bruce's release notes, which say:
> | [MERGE] is similar to INSERT ... ON CONFLICT but more batch-oriented.
> Currently, INSERT *never* uses bistate - even INSERT SELECT.
> INSERTing 10k tuples will dirty 10k buffers - not limited to the size of a
> strategy/ring buffer. Currently, MERGE will do the same.

As I understand it, the point of using a ring is to throttle performance
for bulk operations such as vacuum. I'm not sure why we would want to
throttle either MERGE or INSERT; it seems to me that we should want them
to go as fast as possible.

If MERGE were to use a ring buffer, why wouldn't UPDATE do the same?
There are some comments to that effect in src/backend/buffer/README --
they do mention UPDATE/DELETE and not INSERT. It seems to me that these
three commands (MERGE/UPDATE/DELETE) should be handled in similar ways,
so I don't think we need to consider lack of MERGE using a ring buffer
an open item for pg15.

COPY uses a ring buffer too.

Álvaro Herrera PostgreSQL Developer —

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