I mean this part describing track_function:
https://www.postgresql.org/docs/10/static/runtime-config-statistics.html
Enables tracking of function call counts and time used. Specify pl to track
only procedural-language functions, all to also track SQL and C language
functions. The default is none, which disables function statistics tracking.
Only superusers can change this setting.
Note
SQL-language functions that are simple enough to be “inlined” into the
calling query will not be tracked, regardless of this setting.
Only case described here, that exclude function from being tracked it's
inlining, not the time and not the place in the query.
So I would expect that pg_stat_user_function will show me that my function
was executed. Good that are other ways to do it, but changing
track_functions to 'all' I would expect all calls will be tracked...
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