Re: Should we document the cost of pg_database_size()? Alternatives?

FS-based sizing isn't really enough
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Asking users to monitor at the filesystem level works, kind-of, but
it'll lead to confusion due to WAL and temp files in simple installs.
To get decent results they will need to have a separate dedicated
volume for pg_wal. And which temp files are counted will differ; IIRC
pg_database_size() does not count extents created by an in-progress
REINDEX etc, but DOES count temp table sizes, for example. FS-based
monitoring will also include things like spilled pg_replslot spilled
reorder buffers, which can be considerable and aren't reasonably
considered part of the "database size" or included in
pg_database_size(). And of course it can see only the sum of all
database sizes on a multi-database postgres instance unless the user
has one volume per database using distinct tablespaces. So
filesystem-based monitoring is not really a proper replacement.