Add TOAST statistics columns to pg_stat_all_tables

Hi hackers,

I am proposing a patch that adds four columns to pg_stat_all_tables
(toast_relid, toast_n_dead_tup, toast_last_autovacuum, and
toast_autovacuum_count), so that the state of a table's TOAST table
can be observed from the owning table's row.

Motivation:

Statistics for TOAST tables already exist, but they are hard to reach
when troubleshooting. They appear only as separate rows in the
pg_toast schema, pg_stat_user_tables filters that schema out entirely,
and autovacuum processes a TOAST table independently of its main
table. As a result, a table can look perfectly healthy (n_dead_tup =
0, recently autovacuumed) while most of the dead space is in its TOAST
table, and nothing in the standard views hints that one should join
pg_class.reltoastrelid by hand. The typical symptom is that
pg_table_size() keeps growing while n_dead_tup stays near zero.

Design:

pg_statio_all_tables already exposes TOAST activity on the owning
table's row (toast_blks_read and toast_blks_hit) by joining pg_class
on reltoastrelid, so this patch applies the same pattern to
pg_stat_all_tables. The new columns call the existing statistics
functions on the TOAST relation's OID, so no new counters or C
functions are introduced, and tables without a TOAST table show NULLs.
The TOAST values are deliberately not folded into the table's own
counters, because TOAST tuples are chunks of long values and mixing
the units would make the numbers meaningless.

The patch is attached.

Thoughts?

--
Best regards,
Shinya Kato
NTT OSS Center