pg_stat_activity is system view that shows what is currently happening in the DB.
With newer pgs it's rather wide (22 columns as I write this). And, it's not always obvious what all this means. So let's try to figure it out.
On 17th of January 2021, Magnus Hagander committed patch:
Add pg_stat_database counters for sessions and session time This add counters for number of sessions, the different kind of session termination types, and timers for how much time is spent in active vs idle in a database to pg_stat_database. Internally this also renames the parameter "force" to disconnect. This was the only use-case for the parameter before, so repurposing it to this mroe narrow usecase makes things cleaner than inventing something new. Author: Laurenz Albe Reviewed-By: Magnus Hagander, Soumyadeep Chakraborty, Masahiro Ikeda Discussion: https://email@example.com
On 18th of December 2020, Fujii Masao committed patch:
pg_stat_statements: Track time at which all statistics were last reset. This commit adds "stats_reset" column into the pg_stat_statements_info view. This column indicates the time at which all statistics in the pg_stat_statements view were last reset. Per discussion, this commit also changes pg_stat_statements_info code so that "dealloc" column is reset at the same time as "stats_reset" is reset, i.e., whenever all pg_stat_statements entries are removed, for the sake of consistency. Previously "dealloc" was reset only when pg_stat_statements_reset(0, 0, 0) is called and was not reset when pg_stat_statements_reset() with non-zero value argument discards all entries. This was confusing. Author: Naoki Nakamichi, Yuki Seino Reviewed-by: Yuki Seino, Kyotaro Horiguchi, Li Japin, Fujii Masao Discussion: https://firstname.lastname@example.org
On 28th of January, Fujii Masao committed patch:
Add pg_stat_archiver statistics view. This view shows the statistics about the WAL archiver process's activity. Gabriele Bartolini, reviewed by Michael Paquier, refactored a bit by me.
On 4th of April, Tom Lane committed patch:
Add \watch [SEC] command to psql. This allows convenient re-execution of commands. Will Leinweber, reviewed by Peter Eisentraut, Daniel Farina, and Tom Lane
On 28th of November Tom Lane committed patch by Dave Page, which adds new capability to system monitoring and logging:
Log Message: ----------- Add support for an application_name parameter, which is displayed in pg_stat_activity and recorded in log entries. Dave Page, reviewed by Andres Freund