Maciek Sakrejda <m.sakrejda@gmail.com> writes:
> The pg_stats.avg_width field is documented [1] as "Average width in
> bytes of column's entries" but it's not defined exactly what "entries"
> means here with respect to STORAGE (the underlying pg_statistic
> documentation doesn't clarify this either). I thought initially this
> was the "logical" size of the values, but I ran an experiment that
> suggests this interpretation is not right:
Nope. What that's meant to be is the size of value that would be passed
around inside the executor. In this case:
> maciek=# select avg_width from pg_stats where tablename = 'foo' and
> attname = 'a';
> avg_width
> -----------
> 18
what you are getting is the size of the TOAST pointer. That's correct
for the planner's purposes, because it'd be the TOAST pointer not the
detoasted value that would be passed through joins, sorts, hashes, etc.
And we really only care about how much space would be needed for
things like sort temp files.
regards, tom lane