Re: Statistics Import and Export

* pg_set_relation_stats() needs to do an ACL check so you can't set the
stats on someone else's table. I suggest honoring the new MAINTAIN
privilege as well.

* If possible, reading from pg_stats (instead of pg_statistic) would be
ideal because pg_stats already does the right checks at read time, so a
non-superuser can export stats, too.

* If reading from pg_stats, should you change the signature of
pg_set_relation_stats() to have argument names matching the columns of
pg_stats (e.g. most_common_vals instead of stakind/stavalues)?

In other words, make this a slightly higher level: conceptually
exporting/importing pg_stats rather than pg_statistic. This may also
make the SQL export queries simpler.

Also, I'm wondering about error handling. Is some kind of error thrown
by pg_set_relation_stats() going to abort an entire restore? That might
be easy to prevent with pg_restore, because it can just omit the stats,
but harder if it's in a SQL file.