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
<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
<rodrigoburgosmella@xxxxxxxxx <mailto:rodrigoburgosmella@xxxxxxxxx>> 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
(<davidaventimiglia@xxxxxxxxx <mailto:davidaventimiglia@xxxxxxxxx>>)
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.klaver@xxxxxxxxxxx
