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From: Greg Sabino Mullane <htamfids@gmail.com>
Date: Sun, 23 Feb 2025 10:21:14 -0500
Message-ID: <CAKAnmm+14y_3HyHOcZwcHkqAvmZedXLSm97uVZHVr8eWg__W+Q@mail.gmail.com>
Subject: Re: documentation question regarding REFRESH MATERIALIZED VIEW CONCURRENTLY
To: Tobias McNulty <tobias@caktusgroup.com>
Cc: pgsql-general@lists.postgresql.org
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On Sat, Feb 22, 2025 at 8:58=E2=80=AFPM Tobias McNulty <tobias@caktusgroup.=
com>
wrote:

> "Without this option a refresh which affects a lot of rows will tend to
> use fewer resources"

...

> either that (1) the refresh operation actually updates the contents of a
> lot of rows in the materialized view


This is the correct interpretation. A regular refresh simply runs the query
and replaces the old view, regardless of the number of rows that have
changed. A concurrent refresh runs the query and updates the rows in place,
so it is very sensitive as to how many of those rows have changed. This
also means that many concurrent refreshes can lead to table bloat. And it
will generate more WAL than a regular refresh.

My takeaway: use regular refresh when you can. Switch to concurrent if the
number of changes is very small, or if constant client access to the view
is very important.

--=20
Cheers,
Greg

--
Crunchy Data - https://www.crunchydata.com
Enterprise Postgres Software Products & Tech Support

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<div dir=3D"ltr"><div dir=3D"ltr">On Sat, Feb 22, 2025 at 8:58=E2=80=AFPM T=
obias McNulty &lt;<a href=3D"mailto:tobias@caktusgroup.com">tobias@caktusgr=
oup.com</a>&gt; wrote:</div><div class=3D"gmail_quote gmail_quote_container=
"><blockquote class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;borde=
r-left:1px solid rgb(204,204,204);padding-left:1ex">&quot;Without this opti=
on a refresh which affects a lot of rows will tend to use fewer resources&q=
uot;</blockquote><div>...=C2=A0</div><blockquote class=3D"gmail_quote" styl=
e=3D"margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);paddin=
g-left:1ex">either that (1) the refresh operation actually updates the cont=
ents of a lot of rows in the materialized view</blockquote><div><br></div><=
div>This is the correct interpretation. A regular refresh simply runs the q=
uery and replaces the old view, regardless of the number of rows that have =
changed. A concurrent refresh runs the query and updates the rows in place,=
 so it is very sensitive as to how many of those rows have changed. This al=
so means that many concurrent refreshes can lead to table bloat. And it wil=
l generate more WAL than a regular refresh.</div><div><br></div><div>My tak=
eaway: use regular refresh when you can. Switch to concurrent if the number=
 of changes is very small, or if constant client access to the view is very=
 important.</div><div><br></div></div><span class=3D"gmail_signature_prefix=
">-- </span><br><div dir=3D"ltr" class=3D"gmail_signature"><div dir=3D"ltr"=
><div>Cheers,</div><div>Greg</div><div><br></div><div>--</div><div>Crunchy =
Data - <a href=3D"https://www.crunchydata.com" target=3D"_blank">https://ww=
w.crunchydata.com</a></div><div>Enterprise Postgres Software Products &amp;=
 Tech Support</div><div><br></div></div></div></div>

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