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From: Ron Johnson <ronljohnsonjr@gmail.com>
Date: Tue, 3 Feb 2026 11:16:30 -0500
Message-ID: 
 <CANzqJaAh+mPakfiEB2=GiNioMYRPef0eO7ex04-mmDLvoDPX0g@mail.gmail.com>
Subject: Re: Does pg_dump custom format only compress the table data?
To: Laurenz Albe <laurenz.albe@cybertec.at>
Cc: Holger Jakobs <holger@jakobs.com>, pgsql-admin@lists.postgresql.org
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On Tue, Feb 3, 2026 at 7:48=E2=80=AFAM Laurenz Albe <laurenz.albe@cybertec.=
at>
wrote:

> On Tue, 2026-02-03 at 08:51 +0100, Holger Jakobs wrote:
> > Am 03.02.26 um 07:06 schrieb Laurenz Albe:
> > > On Mon, 2026-02-02 at 10:55 -0500, Ron Johnson wrote:
> > > > Out of curiosity, I opened that dump file in vim, and saw not only
> DDL strings but cleartext table data.
> > > >
> > > > Why isn't it compressed?
> > > Are your DDL statement large enough that you would expect that
> compressing
> > > them would offer any real advantage?
> >
> > That wasn't the question. It was about why table DATA aren't compressed=
,
> > but in clear text.
>
> Ah, I see.
>
> By default, you wouldn't see any table data, because they get compressed.
>
> If a custom format dump is not compressed, the reason could be:
>
> - PostgreSQL was built without support for compression (--without-zlib)
>
> - pg_dump was called with --compress=3D0
>

I always install RPMs from postgresql.org, and in this case specified
--compress=3Dzstd:long.

$ psql -V
psql (PostgreSQL) 17.7

dba=3D# insert into do_not_delete (node_name) values ('foo');
INSERT 0 1
dba=3D# insert into do_not_delete (node_name) values ('bar');
INSERT 0 1
dba=3D# insert into do_not_delete (node_name) values ('blarge');
INSERT 0 1
dba=3D#

$ pg_dump -V
pg_dump (PostgreSQL) 17.7

db=3Ddba
$ pg_dump -Fc --compress=3Dzstd:long -v -d${db} -f ${db}.dump  2> ${db}.log
$ dir ${db}.dump
-rw-r--r-- 1 postgres postgres 2205102870 2026-02-03 11:07:34 dba.dump

$ strings ${db}.dump | grep -E 'foo|bar|blarge'
foo     \N
bar     \N
blarge  \N
\Afoo
barYD
[snip]


--=20
Death to <Redacted>, and butter sauce.
Don't boil me, I'm still alive.
<Redacted> lobster!

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<div dir=3D"ltr"><div dir=3D"ltr">On Tue, Feb 3, 2026 at 7:48=E2=80=AFAM La=
urenz Albe &lt;<a href=3D"mailto:laurenz.albe@cybertec.at">laurenz.albe@cyb=
ertec.at</a>&gt; wrote:</div><div class=3D"gmail_quote gmail_quote_containe=
r"><blockquote class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;bord=
er-left:1px solid rgb(204,204,204);padding-left:1ex">On Tue, 2026-02-03 at =
08:51 +0100, Holger Jakobs wrote:<br>
&gt; Am 03.02.26 um 07:06 schrieb Laurenz Albe:<br>
&gt; &gt; On Mon, 2026-02-02 at 10:55 -0500, Ron Johnson wrote:<br>
&gt; &gt; &gt; Out of curiosity, I opened that dump file in vim, and saw no=
t only DDL strings but cleartext table data.<br>
&gt; &gt; &gt; <br>
&gt; &gt; &gt; Why isn&#39;t it compressed?<br>
&gt; &gt; Are your DDL statement large enough that you would expect that co=
mpressing<br>
&gt; &gt; them would offer any real advantage?<br>
&gt; <br>
&gt; That wasn&#39;t the question. It was about why table DATA aren&#39;t c=
ompressed, <br>
&gt; but in clear text.<br>
<br>
Ah, I see.<br>
<br>
By default, you wouldn&#39;t see any table data, because they get compresse=
d.<br>
<br>
If a custom format dump is not compressed, the reason could be:<br>
<br>
- PostgreSQL was built without support for compression (--without-zlib)<br>
<br>
- pg_dump was called with --compress=3D0<br></blockquote><div><br></div><di=
v>I always install RPMs from <a href=3D"http://postgresql.org">postgresql.o=
rg</a>, and in this case specified --compress=3Dzstd:long.</div><div><br></=
div><div><font face=3D"monospace">$ psql -V<br>psql (PostgreSQL) 17.7<br><b=
r>dba=3D# insert into do_not_delete (node_name) values (&#39;foo&#39;);<br>=
INSERT 0 1<br>dba=3D# insert into do_not_delete (node_name) values (&#39;ba=
r&#39;);<br>INSERT 0 1<br>dba=3D# insert into do_not_delete (node_name) val=
ues (&#39;blarge&#39;);<br>INSERT 0 1<br>dba=3D# <br><br>$ pg_dump -V<br>pg=
_dump (PostgreSQL) 17.7<br><br>db=3Ddba<br>$ pg_dump -Fc --compress=3Dzstd:=
long -v -d${db} -f ${db}.dump =C2=A02&gt; ${db}.log<br>$ dir ${db}.dump<br>=
-rw-r--r-- 1 postgres postgres 2205102870 2026-02-03 11:07:34 dba.dump<br><=
br>$ strings ${db}.dump | grep -E &#39;foo|bar|blarge&#39;<br>foo =C2=A0 =
=C2=A0 \N<br>bar =C2=A0 =C2=A0 \N<br>blarge =C2=A0\N<br>\Afoo<br>barYD<br>[=
snip]</font><br></div><div><br></div></div><div><br></div><span class=3D"gm=
ail_signature_prefix">-- </span><br><div dir=3D"ltr" class=3D"gmail_signatu=
re"><div dir=3D"ltr">Death to &lt;Redacted&gt;, and butter sauce.<div>Don&#=
39;t boil me, I&#39;m still alive.<br><div><div>&lt;Redacted&gt; lobster!</=
div></div></div></div></div></div>

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