Re: How to redirect output from PostgreSQL pg_recvlogical to a file or a pipe?

[David Ventimiglia]

Whoops!  Wrong SO link.  Here's the correct SO link:

https://stackoverflow.com/questions/77808615/how-to-use-logical-decoding-with-pg-recvlogical-to-pass-changes-through-a-non-tr

On Tue, Jan 16, 2024 at 1:15 PM David Ventimiglia <
davidaventimig...@hasura.io> wrote:

> Thanks for the reply, Jim.  No, I'm afraid that's not the missing piece.
> I knew enough to use jq to transform the JSON output into SQL statements.
> What I didn't know enough was about jq.  No, the missing piece turned out
> not to have anything to do with PostgreSQL or pg_recvlogical (I guessed
> incorrectly that it might), but rather with jq itself.  I didn't realize
> that jq buffers its input and it turns out all I had to do was use its
> --unbuffered switch.  The full chapter-and-verse is described in this
> Stack Overflow question and answer
> 
> .
>
> Cheers,
> David
>
> On Tue, Jan 16, 2024 at 12:57 PM Jim Nasby  wrote:
>
>> On 1/13/24 3:34 PM, David Ventimiglia wrote:
>> > The business problem I'm trying to solve is:
>> >
>> > "How do I capture logical decoding events with the wal2json output
>> > encoder, filter them with jq, and pipe them to psql, using
>> pg_recvlogical?"
>>
>> I think the missing piece here is that you can't simply pipe JSON into
>> psql and expect anything useful to happen. Are you using jq to turn the
>> JSON into actual SQL statements? What does some of your jq output look
>> like?
>> --
>> Jim Nasby, Data Architect, Austin TX
>>
>>

Re: How to redirect output from PostgreSQL pg_recvlogical to a file or a pipe?

[David Ventimiglia]

Thanks for the reply, Jim.  No, I'm afraid that's not the missing piece.  I
knew enough to use jq to transform the JSON output into SQL statements.
What I didn't know enough was about jq.  No, the missing piece turned out
not to have anything to do with PostgreSQL or pg_recvlogical (I guessed
incorrectly that it might), but rather with jq itself.  I didn't realize
that jq buffers its input and it turns out all I had to do was use its
--unbuffered switch.  The full chapter-and-verse is described in this Stack
Overflow question and answer

.

Cheers,
David

On Tue, Jan 16, 2024 at 12:57 PM Jim Nasby  wrote:

> On 1/13/24 3:34 PM, David Ventimiglia wrote:
> > The business problem I'm trying to solve is:
> >
> > "How do I capture logical decoding events with the wal2json output
> > encoder, filter them with jq, and pipe them to psql, using
> pg_recvlogical?"
>
> I think the missing piece here is that you can't simply pipe JSON into
> psql and expect anything useful to happen. Are you using jq to turn the
> JSON into actual SQL statements? What does some of your jq output look
> like?
> --
> Jim Nasby, Data Architect, Austin TX
>
>

Re: How to redirect output from PostgreSQL pg_recvlogical to a file or a pipe?

[Jim Nasby]

On 1/13/24 3:34 PM, David Ventimiglia wrote:

The business problem I'm trying to solve is:

"How do I capture logical decoding events with the wal2json output 
encoder, filter them with jq, and pipe them to psql, using pg_recvlogical?"


I think the missing piece here is that you can't simply pipe JSON into 
psql and expect anything useful to happen. Are you using jq to turn the 
JSON into actual SQL statements? What does some of your jq output look like?

--
Jim Nasby, Data Architect, Austin TX

Re: How to redirect output from PostgreSQL pg_recvlogical to a file or a pipe?

[David Ventimiglia]

I'm asking a question about technology. It has an answer. Whatever that
answer is, it's independent of anyone's purpose.

On Sat, Jan 13, 2024, 4:53 PM Ron Johnson  wrote:

> *No,* that's a technology problem.  What is the purpose of storing them
> back in the database using psql?
>
> On Sat, Jan 13, 2024 at 4:34 PM David Ventimiglia <
> davidaventimig...@hasura.io> wrote:
>
>> The business problem I'm trying to solve is:
>>
>> "How do I capture logical decoding events with the wal2json output
>> encoder, filter them with jq, and pipe them to psql, using pg_recvlogical?"
>>
>>>
>

Re: How to redirect output from PostgreSQL pg_recvlogical to a file or a pipe?

[David Ventimiglia]

It satisfies business constraints.

On Sat, Jan 13, 2024, 5:01 PM Karsten Hilbert 
wrote:

> Am Sat, Jan 13, 2024 at 05:53:14PM -0500 schrieb Ron Johnson:
>
> > *No,* that's a technology problem.  What is the purpose of storing them
> > back in the database using psql?
>
> Or even the end goal to be achieved by that ?
>
> Karsten
> --
> GPG  40BE 5B0E C98E 1713 AFA6  5BC0 3BEA AC80 7D4F C89B
>
>
>

Re: How to redirect output from PostgreSQL pg_recvlogical to a file or a pipe?

[Karsten Hilbert]

Am Sat, Jan 13, 2024 at 05:53:14PM -0500 schrieb Ron Johnson:

> *No,* that's a technology problem.  What is the purpose of storing them
> back in the database using psql?

Or even the end goal to be achieved by that ?

Karsten
--
GPG  40BE 5B0E C98E 1713 AFA6  5BC0 3BEA AC80 7D4F C89B

Re: How to redirect output from PostgreSQL pg_recvlogical to a file or a pipe?

[Ron Johnson]

*No,* that's a technology problem.  What is the purpose of storing them
back in the database using psql?

On Sat, Jan 13, 2024 at 4:34 PM David Ventimiglia <
davidaventimig...@hasura.io> wrote:

> The business problem I'm trying to solve is:
>
> "How do I capture logical decoding events with the wal2json output
> encoder, filter them with jq, and pipe them to psql, using pg_recvlogical?"
>
>>

Re: How to redirect output from PostgreSQL pg_recvlogical to a file or a pipe?

[David Ventimiglia]

The business problem I'm trying to solve is:

"How do I capture logical decoding events with the wal2json output encoder,
filter them with jq, and pipe them to psql, using pg_recvlogical?"

On Sat, Jan 13, 2024, 1:04 PM Ron Johnson  wrote:

> I think this might be an A-B problem.  Tell us the "business problem" you
> are trying to solve, not the problem you're having with your solution to
> the "business problem".
>
> (If you've already mentioned it, please restate it.)
>
> On Sat, Jan 13, 2024 at 11:49 AM David Ventimiglia <
> davidaventimig...@hasura.io> wrote:
>
>> Thanks.  I'm aware of all of those other alternatives, but the thing is,
>> I'm not trying to answer this broader question:
>>
>> *"What are some options for capturing change events in PostgreSQL?"*
>>
>> Rather, I'm trying to answer a narrower question:
>>
>> *"How does one capture output from pg_recvlogical and pipe it back into
>> the database with psql?"*
>>
>> Best,
>> David
>>
>> On Sat, Jan 13, 2024 at 10:29 AM Adrian Klaver 
>> wrote:
>>
>>> On 1/12/24 21:23, David Ventimiglia wrote:
>>> > Let me just lay my cards on the table.  What I'm really trying to do
>>> is
>>> > capture change events with logical decoding and then send them back
>>> into
>>> > the database into a database table.  To do that, I believe I need to
>>> > process the event records into SQL insert statements somehow.  xargs
>>> is
>>> > one option.  jq is another.  My idea was to pipe the pg_recvlogical
>>> > output through a jq transform into psql, but that didn't work (neither
>>> > did earlier experiments with xargs).  Redirecting the output to an
>>> > intermediate file via stdout was just an attempt to reduce the problem
>>> > to a simpler problem.  I had /thought/ (incorrectly, as it turns out)
>>> > that I was unable even to redirect it to a file, but evidently that's
>>> > not the case.  I can redirect it to a file.  What I cannot seem to do
>>> is
>>> > run it through a jq filter and pipe it back into psql.  I can run it
>>> > through a jq filter and redirect it to a file, no problem.  But the
>>> > minute I change it to pipe to psql, it ceases to produce the desired
>>> result.
>>> >
>>> > I tried illustrating this in this screencast:
>>> >
>>> > https://asciinema.org/a/npzgcTN8DDjUdkaZlVyYJhZ5y
>>> > 
>>> >
>>> > Perhaps another way to put this is, how /does/ one capture output from
>>> > pg_recvlogical and pipe it back into the database (or if you like,
>>> some
>>> > other database) with psql.  When I set out to do this I didn't think
>>> > bash pipes and redirection would be the hard part, and yet here I am.
>>> > Maybe there's some other way, because I'm fresh out of ideas.
>>>
>>> This is going to depend a lot on what you define as a change event. Is
>>> that DDL changes or data changes or both?
>>>
>>> Some existing solutions that cover the above to a one degree or another:
>>>
>>> Event triggers:
>>>
>>> https://www.postgresql.org/docs/current/event-triggers.html
>>>
>>> PGAudit
>>>
>>> https://github.com/pgaudit/pgaudit/blob/master/README.md
>>>
>>> Or since you are part of the way there already just using logical
>>> replication entirely:
>>>
>>> https://www.postgresql.org/docs/current/logical-replication.html
>>>
>>>
>>> >
>>> > Best,
>>> > David
>>> >
>>> > On Fri, Jan 12, 2024 at 8:42 PM Juan Rodrigo Alejandro Burgos Mella
>>> > mailto:rodrigoburgosme...@gmail.com>>
>>> wrote:
>>> >
>>> >
>>> > try use the following syntax (yes, with a 2 before the greater
>>> sign)
>>> >
>>> > pg_recvlogical -d postgres --slot test --start -f - 2>>
>>> sample.jsonl
>>> >
>>> > Atte
>>> > JRBM
>>> >
>>> > El vie, 12 ene 2024 a las 16:35, David Ventimiglia
>>> > (mailto:davidaventimig...@hasura.io
>>> >>)
>>> > escribió:
>>> >
>>> > Hello! How do I redirect logical decoding output from the
>>> > PostgreSQL CLI tool |pg_recvlogical| either to a file or to
>>> > another command via a pipe? I ask because when I try the
>>> > obvious, no output is recorded or sent:
>>> >
>>> > |pg_recvlogical -d postgres --slot test --start -f - >>
>>> > sample.jsonl |
>>> >
>>> > Lest there be any confusion, I already created the slot in an
>>> > earlier step. Moreover, I can verify that if I omit the output
>>> > redirection |>> sample| then it does work, insofar as it emits
>>> > the expected change events when I perform DML in another
>>> > terminal window. When I include the redirection (or
>>> > alternatively, set up a pipeline), then nothing happens.
>>> >
>>> > Note that I am aware of the option to pass a filename to the -f
>>> > switch to write to a file.  That works, but it's not what I'm
>>> > after because it doesn't help update my mental model of how
>>> this
>>> > is supposed to work.  Based on my current (flawed) mental model
>>> > built up from command 

Re: How to redirect output from PostgreSQL pg_recvlogical to a file or a pipe?

[Ron Johnson]

I think this might be an A-B problem.  Tell us the "business problem" you
are trying to solve, not the problem you're having with your solution to
the "business problem".

(If you've already mentioned it, please restate it.)

On Sat, Jan 13, 2024 at 11:49 AM David Ventimiglia <
davidaventimig...@hasura.io> wrote:

> Thanks.  I'm aware of all of those other alternatives, but the thing is,
> I'm not trying to answer this broader question:
>
> *"What are some options for capturing change events in PostgreSQL?"*
>
> Rather, I'm trying to answer a narrower question:
>
> *"How does one capture output from pg_recvlogical and pipe it back into
> the database with psql?"*
>
> Best,
> David
>
> On Sat, Jan 13, 2024 at 10:29 AM Adrian Klaver 
> wrote:
>
>> On 1/12/24 21:23, David Ventimiglia wrote:
>> > Let me just lay my cards on the table.  What I'm really trying to do is
>> > capture change events with logical decoding and then send them back
>> into
>> > the database into a database table.  To do that, I believe I need to
>> > process the event records into SQL insert statements somehow.  xargs is
>> > one option.  jq is another.  My idea was to pipe the pg_recvlogical
>> > output through a jq transform into psql, but that didn't work (neither
>> > did earlier experiments with xargs).  Redirecting the output to an
>> > intermediate file via stdout was just an attempt to reduce the problem
>> > to a simpler problem.  I had /thought/ (incorrectly, as it turns out)
>> > that I was unable even to redirect it to a file, but evidently that's
>> > not the case.  I can redirect it to a file.  What I cannot seem to do
>> is
>> > run it through a jq filter and pipe it back into psql.  I can run it
>> > through a jq filter and redirect it to a file, no problem.  But the
>> > minute I change it to pipe to psql, it ceases to produce the desired
>> result.
>> >
>> > I tried illustrating this in this screencast:
>> >
>> > https://asciinema.org/a/npzgcTN8DDjUdkaZlVyYJhZ5y
>> > 
>> >
>> > Perhaps another way to put this is, how /does/ one capture output from
>> > pg_recvlogical and pipe it back into the database (or if you like, some
>> > other database) with psql.  When I set out to do this I didn't think
>> > bash pipes and redirection would be the hard part, and yet here I am.
>> > Maybe there's some other way, because I'm fresh out of ideas.
>>
>> This is going to depend a lot on what you define as a change event. Is
>> that DDL changes or data changes or both?
>>
>> Some existing solutions that cover the above to a one degree or another:
>>
>> Event triggers:
>>
>> https://www.postgresql.org/docs/current/event-triggers.html
>>
>> PGAudit
>>
>> https://github.com/pgaudit/pgaudit/blob/master/README.md
>>
>> Or since you are part of the way there already just using logical
>> replication entirely:
>>
>> https://www.postgresql.org/docs/current/logical-replication.html
>>
>>
>> >
>> > Best,
>> > David
>> >
>> > On Fri, Jan 12, 2024 at 8:42 PM Juan Rodrigo Alejandro Burgos Mella
>> > mailto:rodrigoburgosme...@gmail.com>>
>> wrote:
>> >
>> >
>> > try use the following syntax (yes, with a 2 before the greater sign)
>> >
>> > pg_recvlogical -d postgres --slot test --start -f - 2>> sample.jsonl
>> >
>> > Atte
>> > JRBM
>> >
>> > El vie, 12 ene 2024 a las 16:35, David Ventimiglia
>> > (mailto:davidaventimig...@hasura.io
>> >>)
>> > escribió:
>> >
>> > Hello! How do I redirect logical decoding output from the
>> > PostgreSQL CLI tool |pg_recvlogical| either to a file or to
>> > another command via a pipe? I ask because when I try the
>> > obvious, no output is recorded or sent:
>> >
>> > |pg_recvlogical -d postgres --slot test --start -f - >>
>> > sample.jsonl |
>> >
>> > Lest there be any confusion, I already created the slot in an
>> > earlier step. Moreover, I can verify that if I omit the output
>> > redirection |>> sample| then it does work, insofar as it emits
>> > the expected change events when I perform DML in another
>> > terminal window. When I include the redirection (or
>> > alternatively, set up a pipeline), then nothing happens.
>> >
>> > Note that I am aware of the option to pass a filename to the -f
>> > switch to write to a file.  That works, but it's not what I'm
>> > after because it doesn't help update my mental model of how this
>> > is supposed to work.  Based on my current (flawed) mental model
>> > built up from command line experience with other tools, this
>> > /should/ work.  I should be able to send the output to stdout
>> > and then redirect it to a file.  It surprises me that I cannot.
>> >
>> > Anyway, thanks!
>> >
>> > Best,
>> >
>> > David
>> >
>>
>> --
>> Adrian Klaver
>> adrian.kla...@aklaver.com
>>
>>

Re: How to redirect output from PostgreSQL pg_recvlogical to a file or a pipe?

[Adrian Klaver]

On 1/13/24 08:48, David Ventimiglia wrote:
Thanks.  I'm aware of all of those other alternatives, but the thing is, 
I'm not trying to answer this broader question:


/"What are some options for capturing change events in PostgreSQL?"/
/
/
Rather, I'm trying to answer a narrower question:

/"How does one capture output from pg_recvlogical and pipe it back into 
the database with psql?"/


I don't know. For those that might a self contained example of what you 
want to achieve would be a good start. I doubt that many will look at 
the screencast.



Best,
David



--
Adrian Klaver
adrian.kla...@aklaver.com

Re: How to redirect output from PostgreSQL pg_recvlogical to a file or a pipe?

[David Ventimiglia]

Thanks.  I'm aware of all of those other alternatives, but the thing is,
I'm not trying to answer this broader question:

*"What are some options for capturing change events in PostgreSQL?"*

Rather, I'm trying to answer a narrower question:

*"How does one capture output from pg_recvlogical and pipe it back into the
database with psql?"*

Best,
David

On Sat, Jan 13, 2024 at 10:29 AM Adrian Klaver 
wrote:

> On 1/12/24 21:23, David Ventimiglia wrote:
> > Let me just lay my cards on the table.  What I'm really trying to do is
> > capture change events with logical decoding and then send them back into
> > the database into a database table.  To do that, I believe I need to
> > process the event records into SQL insert statements somehow.  xargs is
> > one option.  jq is another.  My idea was to pipe the pg_recvlogical
> > output through a jq transform into psql, but that didn't work (neither
> > did earlier experiments with xargs).  Redirecting the output to an
> > intermediate file via stdout was just an attempt to reduce the problem
> > to a simpler problem.  I had /thought/ (incorrectly, as it turns out)
> > that I was unable even to redirect it to a file, but evidently that's
> > not the case.  I can redirect it to a file.  What I cannot seem to do is
> > run it through a jq filter and pipe it back into psql.  I can run it
> > through a jq filter and redirect it to a file, no problem.  But the
> > minute I change it to pipe to psql, it ceases to produce the desired
> result.
> >
> > I tried illustrating this in this screencast:
> >
> > https://asciinema.org/a/npzgcTN8DDjUdkaZlVyYJhZ5y
> > 
> >
> > Perhaps another way to put this is, how /does/ one capture output from
> > pg_recvlogical and pipe it back into the database (or if you like, some
> > other database) with psql.  When I set out to do this I didn't think
> > bash pipes and redirection would be the hard part, and yet here I am.
> > Maybe there's some other way, because I'm fresh out of ideas.
>
> This is going to depend a lot on what you define as a change event. Is
> that DDL changes or data changes or both?
>
> Some existing solutions that cover the above to a one degree or another:
>
> Event triggers:
>
> https://www.postgresql.org/docs/current/event-triggers.html
>
> PGAudit
>
> https://github.com/pgaudit/pgaudit/blob/master/README.md
>
> Or since you are part of the way there already just using logical
> replication entirely:
>
> https://www.postgresql.org/docs/current/logical-replication.html
>
>
> >
> > Best,
> > David
> >
> > On Fri, Jan 12, 2024 at 8:42 PM Juan Rodrigo Alejandro Burgos Mella
> > mailto:rodrigoburgosme...@gmail.com>>
> wrote:
> >
> >
> > try use the following syntax (yes, with a 2 before the greater sign)
> >
> > pg_recvlogical -d postgres --slot test --start -f - 2>> sample.jsonl
> >
> > Atte
> > JRBM
> >
> > El vie, 12 ene 2024 a las 16:35, David Ventimiglia
> > (mailto:davidaventimig...@hasura.io>>)
> > escribió:
> >
> > Hello! How do I redirect logical decoding output from the
> > PostgreSQL CLI tool |pg_recvlogical| either to a file or to
> > another command via a pipe? I ask because when I try the
> > obvious, no output is recorded or sent:
> >
> > |pg_recvlogical -d postgres --slot test --start -f - >>
> > sample.jsonl |
> >
> > Lest there be any confusion, I already created the slot in an
> > earlier step. Moreover, I can verify that if I omit the output
> > redirection |>> sample| then it does work, insofar as it emits
> > the expected change events when I perform DML in another
> > terminal window. When I include the redirection (or
> > alternatively, set up a pipeline), then nothing happens.
> >
> > Note that I am aware of the option to pass a filename to the -f
> > switch to write to a file.  That works, but it's not what I'm
> > after because it doesn't help update my mental model of how this
> > is supposed to work.  Based on my current (flawed) mental model
> > built up from command line experience with other tools, this
> > /should/ work.  I should be able to send the output to stdout
> > and then redirect it to a file.  It surprises me that I cannot.
> >
> > Anyway, thanks!
> >
> > Best,
> >
> > David
> >
>
> --
> Adrian Klaver
> adrian.kla...@aklaver.com
>
>

Re: How to redirect output from PostgreSQL pg_recvlogical to a file or a pipe?

[Adrian Klaver]

On 1/12/24 21:23, David Ventimiglia wrote:
Let me just lay my cards on the table.  What I'm really trying to do is 
capture change events with logical decoding and then send them back into 
the database into a database table.  To do that, I believe I need to 
process the event records into SQL insert statements somehow.  xargs is 
one option.  jq is another.  My idea was to pipe the pg_recvlogical 
output through a jq transform into psql, but that didn't work (neither 
did earlier experiments with xargs).  Redirecting the output to an 
intermediate file via stdout was just an attempt to reduce the problem 
to a simpler problem.  I had /thought/ (incorrectly, as it turns out) 
that I was unable even to redirect it to a file, but evidently that's 
not the case.  I can redirect it to a file.  What I cannot seem to do is 
run it through a jq filter and pipe it back into psql.  I can run it 
through a jq filter and redirect it to a file, no problem.  But the 
minute I change it to pipe to psql, it ceases to produce the desired result.


I tried illustrating this in this screencast:

https://asciinema.org/a/npzgcTN8DDjUdkaZlVyYJhZ5y 



Perhaps another way to put this is, how /does/ one capture output from 
pg_recvlogical and pipe it back into the database (or if you like, some 
other database) with psql.  When I set out to do this I didn't think 
bash pipes and redirection would be the hard part, and yet here I am.  
Maybe there's some other way, because I'm fresh out of ideas.


This is going to depend a lot on what you define as a change event. Is 
that DDL changes or data changes or both?


Some existing solutions that cover the above to a one degree or another:

Event triggers:

https://www.postgresql.org/docs/current/event-triggers.html

PGAudit

https://github.com/pgaudit/pgaudit/blob/master/README.md

Or since you are part of the way there already just using logical 
replication entirely:


https://www.postgresql.org/docs/current/logical-replication.html




Best,
David

On Fri, Jan 12, 2024 at 8:42 PM Juan Rodrigo Alejandro Burgos Mella 
mailto:rodrigoburgosme...@gmail.com>> wrote:



try use the following syntax (yes, with a 2 before the greater sign)

pg_recvlogical -d postgres --slot test --start -f - 2>> sample.jsonl

Atte
JRBM

El vie, 12 ene 2024 a las 16:35, David Ventimiglia
(mailto:davidaventimig...@hasura.io>>)
escribió:

Hello! How do I redirect logical decoding output from the
PostgreSQL CLI tool |pg_recvlogical| either to a file or to
another command via a pipe? I ask because when I try the
obvious, no output is recorded or sent:

|pg_recvlogical -d postgres --slot test --start -f - >>
sample.jsonl |

Lest there be any confusion, I already created the slot in an
earlier step. Moreover, I can verify that if I omit the output
redirection |>> sample| then it does work, insofar as it emits
the expected change events when I perform DML in another
terminal window. When I include the redirection (or
alternatively, set up a pipeline), then nothing happens.

Note that I am aware of the option to pass a filename to the -f
switch to write to a file.  That works, but it's not what I'm
after because it doesn't help update my mental model of how this
is supposed to work.  Based on my current (flawed) mental model
built up from command line experience with other tools, this
/should/ work.  I should be able to send the output to stdout
and then redirect it to a file.  It surprises me that I cannot.

Anyway, thanks!

Best,

David



--
Adrian Klaver
adrian.kla...@aklaver.com

Re: How to redirect output from PostgreSQL pg_recvlogical to a file or a pipe?

[David Ventimiglia]

Let me just lay my cards on the table.  What I'm really trying to do is
capture change events with logical decoding and then send them back into
the database into a database table.  To do that, I believe I need to
process the event records into SQL insert statements somehow.  xargs is one
option.  jq is another.  My idea was to pipe the pg_recvlogical output
through a jq transform into psql, but that didn't work (neither did earlier
experiments with xargs).  Redirecting the output to an intermediate file
via stdout was just an attempt to reduce the problem to a simpler problem.
I had *thought* (incorrectly, as it turns out) that I was unable even to
redirect it to a file, but evidently that's not the case.  I can redirect
it to a file.  What I cannot seem to do is run it through a jq filter and
pipe it back into psql.  I can run it through a jq filter and redirect it
to a file, no problem.  But the minute I change it to pipe to psql, it
ceases to produce the desired result.

I tried illustrating this in this screencast:

https://asciinema.org/a/npzgcTN8DDjUdkaZlVyYJhZ5y

Perhaps another way to put this is, how *does* one capture output from
pg_recvlogical and pipe it back into the database (or if you like, some
other database) with psql.  When I set out to do this I didn't think bash
pipes and redirection would be the hard part, and yet here I am.  Maybe
there's some other way, because I'm fresh out of ideas.

Best,
David

On Fri, Jan 12, 2024 at 8:42 PM Juan Rodrigo Alejandro Burgos Mella <
rodrigoburgosme...@gmail.com> wrote:

>
> try use the following syntax (yes, with a 2 before the greater sign)
>
> pg_recvlogical -d postgres --slot test --start -f - 2>> sample.jsonl
>
> Atte
> JRBM
>
> El vie, 12 ene 2024 a las 16:35, David Ventimiglia (<
> davidaventimig...@hasura.io>) escribió:
>
>> Hello! How do I redirect logical decoding output from the PostgreSQL CLI
>> tool pg_recvlogical either to a file or to another command via a pipe? I
>> ask because when I try the obvious, no output is recorded or sent:
>>
>>
>> pg_recvlogical -d postgres --slot test --start -f - >> sample.jsonl
>>
>>
>> Lest there be any confusion, I already created the slot in an earlier
>> step. Moreover, I can verify that if I omit the output redirection >>
>> sample then it does work, insofar as it emits the expected change events
>> when I perform DML in another terminal window. When I include the
>> redirection (or alternatively, set up a pipeline), then nothing happens.
>>
>> Note that I am aware of the option to pass a filename to the -f switch to
>> write to a file.  That works, but it's not what I'm after because it
>> doesn't help update my mental model of how this is supposed to work.  Based
>> on my current (flawed) mental model built up from command line experience
>> with other tools, this *should* work.  I should be able to send the
>> output to stdout and then redirect it to a file.  It surprises me that I
>> cannot.
>>
>> Anyway, thanks!
>>
>> Best,
>>
>> David
>>
>

Re: How to redirect output from PostgreSQL pg_recvlogical to a file or a pipe?

[Adrian Klaver]

On 1/12/24 14:03, David Ventimiglia wrote:

Reply to list also.
Ccing list

Hi Adrian,

I left out the creation of the slot both from the SO question and from 
this mailing list question, because I believe it's a red herring.  I run 
into the same problem with the default output plugin as I do with the 
wal2json plugin.  However, the problem is a little different from what I 
described.  It turns out it's not output redirection from pg_recvlogical 
sending to stdout that fails.  Rather, it's output redirection from 
pg_recvlogical sending to stdout and then passed through a pipeline that 
fails.  Or something like that.


This works.  The sample.txt file is non-empty.

pg_recvlogical -d postgres --slot=test --create-slot
pg_recvlogical -d postgres -n --slot=test --start -f - > sample.txt


This does not work.  The sample.txt file is empty.

pg_recvlogical -d postgres --slot=test --create-slot
pg_recvlogical -d postgres -n --slot=test --start -f - | awk
'{print}' > sample.txt


Weirdly, this works.  The sample.txt file is non-empty.

pg_recvlogical -d postgres --slot=test --create-slot
pg_recvlogical -d postgres -n --slot=test --start -f - | cat >
sample.txt


FWIW, this is demonstrated in this screen-cast 
.  I suspect it has something to do with 
the flushing of buffers, as other people have suggested.


Thanks!
David




On Fri, Jan 12, 2024 at 1:48 PM Adrian Klaver > wrote:


On 1/12/24 11:34, David Ventimiglia wrote:
 > Hello! How do I redirect logical decoding output from the
PostgreSQL CLI
 > tool |pg_recvlogical| either to a file or to another command via
a pipe?
 > I ask because when I try the obvious, no output is recorded or sent:
 >
 > |pg_recvlogical -d postgres --slot test --start -f - >>
sample.jsonl |
 >
 > Lest there be any confusion, I already created the slot in an
earlier
 > step. Moreover, I can verify that if I omit the output
redirection |>>
 > sample| then it does work, insofar as it emits the expected change
 > events when I perform DML in another terminal window. When I
include the
 > redirection (or alternatively, set up a pipeline), then nothing
happens.

You left out the important part of the SO question:

pg_recvlogical -d postgres --slot test --create-slot -P wal2json

I can get:

pg_recvlogical -d postgres --slot test --start -f - >> sample.jsonl

to work when I do:

pg_recvlogical -d postgres --slot test --create-slot

wal2json seems to be the issue.

 >
 > Note that I am aware of the option to pass a filename to the -f
switch
 > to write to a file.  That works, but it's not what I'm after
because it
 > doesn't help update my mental model of how this is supposed to work.
 > Based on my current (flawed) mental model built up from command line
 > experience with other tools, this /should/ work.  I should be
able to
 > send the output to stdout and then redirect it to a file.  It
surprises
 > me that I cannot.
 >
 > Anyway, thanks!
 >
 > Best,
 >
 > David
 >

-- 
Adrian Klaver

adrian.kla...@aklaver.com 



--
Adrian Klaver
adrian.kla...@aklaver.com

Re: How to redirect output from PostgreSQL pg_recvlogical to a file or a pipe?

[Francisco Olarte]

David:

On Fri, 12 Jan 2024 at 20:35, David Ventimiglia
 wrote:
> pg_recvlogical -d postgres --slot test --start -f - >> sample.jsonl
>
> Lest there be any confusion, I already created the slot in an earlier step. 
> Moreover, I can verify that if I omit the output redirection >> sample then 
> it does work, insofar as it emits the expected change events when I perform 
> DML in another terminal window. When I include the redirection (or 
> alternatively, set up a pipeline), then nothing happens.

Have you tested the command as printed ( with -f - ) but without redirection?

Have you ruled out the usual suspect, stdout is line buffered when
going to a tty, full buffered when not ? ( by killing pg_revlogical
and/or insuring a long enough output is generated )

Francisco Olarte.

Re: How to redirect output from PostgreSQL pg_recvlogical to a file or a pipe?

[Adrian Klaver]

On 1/12/24 11:34, David Ventimiglia wrote:
Hello! How do I redirect logical decoding output from the PostgreSQL CLI 
tool |pg_recvlogical| either to a file or to another command via a pipe? 
I ask because when I try the obvious, no output is recorded or sent:


|pg_recvlogical -d postgres --slot test --start -f - >> sample.jsonl |

Lest there be any confusion, I already created the slot in an earlier 
step. Moreover, I can verify that if I omit the output redirection |>> 
sample| then it does work, insofar as it emits the expected change 
events when I perform DML in another terminal window. When I include the 
redirection (or alternatively, set up a pipeline), then nothing happens.


You left out the important part of the SO question:

pg_recvlogical -d postgres --slot test --create-slot -P wal2json

I can get:

pg_recvlogical -d postgres --slot test --start -f - >> sample.jsonl

to work when I do:

pg_recvlogical -d postgres --slot test --create-slot

wal2json seems to be the issue.



Note that I am aware of the option to pass a filename to the -f switch 
to write to a file.  That works, but it's not what I'm after because it 
doesn't help update my mental model of how this is supposed to work.  
Based on my current (flawed) mental model built up from command line 
experience with other tools, this /should/ work.  I should be able to 
send the output to stdout and then redirect it to a file.  It surprises 
me that I cannot.


Anyway, thanks!

Best,

David



--
Adrian Klaver
adrian.kla...@aklaver.com

How to redirect output from PostgreSQL pg_recvlogical to a file or a pipe?

[David Ventimiglia]

Hello! How do I redirect logical decoding output from the PostgreSQL CLI
tool pg_recvlogical either to a file or to another command via a pipe? I
ask because when I try the obvious, no output is recorded or sent:


pg_recvlogical -d postgres --slot test --start -f - >> sample.jsonl


Lest there be any confusion, I already created the slot in an earlier step.
Moreover, I can verify that if I omit the output redirection >> sample then
it does work, insofar as it emits the expected change events when I perform
DML in another terminal window. When I include the redirection (or
alternatively, set up a pipeline), then nothing happens.

Note that I am aware of the option to pass a filename to the -f switch to
write to a file.  That works, but it's not what I'm after because it
doesn't help update my mental model of how this is supposed to work.  Based
on my current (flawed) mental model built up from command line experience
with other tools, this *should* work.  I should be able to send the output
to stdout and then redirect it to a file.  It surprises me that I cannot.

Anyway, thanks!

Best,

David