Should vacuum process config file reload more often

Hi,

Users may wish to speed up long-running vacuum of a large table by
decreasing autovacuum_vacuum_cost_delay/vacuum_cost_delay, however the
config file is only reloaded between tables (for autovacuum) or after
the statement (for explicit vacuum). This has been brought up for
autovacuum in [1].

Andres suggested that it might be possible to check ConfigReloadPending
in vacuum_delay_point(), so I thought I would draft a rough patch and
start a discussion.

Since vacuum_delay_point() is also called by analyze and we do not want
to reload the configuration file if we are in a user transaction, I
widened the scope of the in_outer_xact variable in vacuum() and allowed
analyze in a user transaction to default to the current configuration
file reload cadence in PostgresMain().

I don't think I can set and leave vac_in_outer_xact the way I am doing
it in this patch, since I use vac_in_outer_xact in vacuum_delay_point(),
which I believe is reachable from codepaths that would not have called
vacuum(). It seems that if a backend sets it, the outer transaction
commits, and then the backend ends up calling vacuum_delay_point() in a
different way later, it wouldn't be quite right.

Apart from this, one higher level question I have is if there are other
gucs whose modification would make reloading the configuration file
during vacuum/analyze unsafe.

- Melanie

[1] https://www.postgresql.org/message-id/flat/22CA91B4-D341-4075-BD3C-4BAB52AF1E80%40amazon.com#37f05e33d2ce43680f96332fa1c0f3d4