pgstattuple: Have pgstattuple_approx accept TOAST tables
TOAST tables have a visibility map and a free space map, so they can
be supported by pgstattuple_approx just fine.
Add test cases to show how various pgstattuple functions accept TOAST
tables. Also add similar tests to pg_visibility, which already
accepted TOAST tables correctly but had no test coverage for them.
Reviewed-by: Laurenz Albe <laurenz.albe@cybertec.at>
Discussion: https://www.postgresql.org/message-id/flat/
27c4496a-02b9-dc87-8f6f-
bddbef54e0fe@2ndquadrant.com