pgsql: pgstattuple: Have pgstattuple_approx accept TOAST tables

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(dot)albe(at)cybertec(dot)at>
Discussion: https://www.postgresql.org/message-id/flat/27c4496a-02b9-dc87-8f6f-bddbef54e0fe(at)2ndquadrant(dot)com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ee0202d552791f14bc407ce299628ce8d50eebe3

Modified Files
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contrib/pg_visibility/expected/pg_visibility.out | 17 ++++++++++++++--
contrib/pg_visibility/sql/pg_visibility.sql | 7 +++++--
contrib/pgstattuple/expected/pgstattuple.out | 25 +++++++++++++++++++++---
contrib/pgstattuple/pgstatapprox.c | 10 +++++-----
contrib/pgstattuple/sql/pgstattuple.sql | 5 +++++
5 files changed, 52 insertions(+), 12 deletions(-)