This is interesting question with at least couple options. So, let's dig in…
On 3rd of April 2019, Alvaro Herrera committed patch:
Log all statements from a sample of transactions This is useful to obtain a view of the different transaction types in an application, regardless of the durations of the statements each runs. Author: Adrien Nayrat
When working with PostgreSQL you generally want to get information about slow queries. The usual approach is to set log_min_duration_statement to some low(ish) value, run your app, and then analyze logs.
But you can log to many places – flat file, flat file on another disk, local syslog, remote syslog. And – perhaps, instead of log_min_duration_statement – just use pg_stat_statements?
Well, I wondered about it, and decided to test.
On 29th of June, Andres Freund committed patch:
Add cluster_name GUC which is included in process titles if set. When running several postgres clusters on one OS instance it's often inconveniently hard to identify which "postgres" process belongs to which postgres instance. Add the cluster_name GUC, whose value will be included as part of the process titles if set. With that processes can more easily identified using tools like 'ps'. To avoid problems with encoding mismatches between postgresql.conf, consoles, and individual databases replace non-ASCII chars in the name with question marks. The length is limited to NAMEDATALEN to make it less likely to truncate important information at the end of the status. Thomas Munro, with some adjustments by me and review by a host of people.
update at the end of the post!
There obviously is one – after all, logging information has to be more expensive than not logging it. But how big is it? And more importantly, what is the difference between logging to stderr/file, csvlog and syslog? And what about syslog to remote machine?
One of the questions that pop up frequently on IRC is how to see queries are now executed on the server, and what queries were earlier.
Theoretically answer to this is simple – pg_stat_activity and log_min_duration_statement. Or log_statement. What is the difference? That's exactly why I'm writing this post.