BUG #17123: ALTER TABLE ... NO INHERIT leaves orphaned statistics behind

The following bug has been logged on the website:

Bug reference:      17123
Logged by:          Christoph Berg
Email address:      christoph.berg@credativ.de
PostgreSQL version: 10.16
Operating system:   Any
Description:

Removing the last/only inheritance child from a table using ALTER TABLE NO
INHERIT leaves orphaned statistics behind that ANALYZE doesn't remove.
Original customer complaint on 10.16, but 15devel has the same issue.

psql -ef inherit.sql 
create table log (ts timestamptz, data text);
CREATE TABLE
Zeit: 2,378 ms
insert into log values ('2021-07-01', '1');
INSERT 0 1
Zeit: 0,457 ms
insert into log values ('2021-08-01', '2');
INSERT 0 1
Zeit: 0,197 ms
create table log2 (like log) inherits (log);
psql:inherit.sql:4: HINWEIS:  00000: Spalte »ts« wird mit geerbter
Definition zusammengeführt
ORT:  MergeAttributes, tablecmds.c:2837
psql:inherit.sql:4: HINWEIS:  00000: Spalte »data« wird mit geerbter
Definition zusammengeführt
ORT:  MergeAttributes, tablecmds.c:2837
CREATE TABLE
Zeit: 1,396 ms
insert into log2 values ('2021-01-01', '3');
INSERT 0 1
Zeit: 0,351 ms
insert into log2 values ('2021-02-01', '4');
INSERT 0 1
Zeit: 0,174 ms
analyze log;
ANALYZE
Zeit: 1,093 ms
select * from pg_stats where tablename = 'log' and attname = 'ts' 
─[ RECORD 1
]──────────┬──────────────────────────────────────────────────────────────────────────────────────────────────────
schemaname             │ public
tablename              │ log
attname                │ ts
inherited              │ f
null_frac              │ 0
avg_width              │ 8
n_distinct             │ -1
most_common_vals       │ ∅
most_common_freqs      │ ∅
histogram_bounds       │ {"2021-07-01 00:00:00+02","2021-08-01
00:00:00+02"}
correlation            │ 1
most_common_elems      │ ∅
most_common_elem_freqs │ ∅
elem_count_histogram   │ ∅
─[ RECORD 2
]──────────┼──────────────────────────────────────────────────────────────────────────────────────────────────────
schemaname             │ public
tablename              │ log
attname                │ ts
inherited              │ t
null_frac              │ 0
avg_width              │ 8
n_distinct             │ -1
most_common_vals       │ ∅
most_common_freqs      │ ∅
histogram_bounds       │ {"2021-01-01 00:00:00+01","2021-02-01
00:00:00+01","2021-07-01 00:00:00+02","2021-08-01 00:00:00+02"}
correlation            │ -0.6
most_common_elems      │ ∅
most_common_elem_freqs │ ∅
elem_count_histogram   │ ∅

Zeit: 4,616 ms
select ctid, stainherit from pg_statistic where starelid = 'log'::regclass
and staattnum = 1;
  ctid   │ stainherit 
─────────┼────────────
 (18,17) │ f
 (18,19) │ t
(2 Zeilen)

Zeit: 0,856 ms
alter table log2 no inherit log;
ALTER TABLE
Zeit: 0,449 ms
analyze log;
ANALYZE
Zeit: 0,394 ms
select * from pg_stats where tablename = 'log' and attname = 'ts' 
─[ RECORD 1
]──────────┬──────────────────────────────────────────────────────────────────────────────────────────────────────
schemaname             │ public
tablename              │ log
attname                │ ts
inherited              │ f
null_frac              │ 0
avg_width              │ 8
n_distinct             │ -1
most_common_vals       │ ∅
most_common_freqs      │ ∅
histogram_bounds       │ {"2021-07-01 00:00:00+02","2021-08-01
00:00:00+02"}
correlation            │ 1
most_common_elems      │ ∅
most_common_elem_freqs │ ∅
elem_count_histogram   │ ∅
─[ RECORD 2
]──────────┼──────────────────────────────────────────────────────────────────────────────────────────────────────
schemaname             │ public
tablename              │ log
attname                │ ts
inherited              │ t          <-- not removed
null_frac              │ 0
avg_width              │ 8
n_distinct             │ -1
most_common_vals       │ ∅
most_common_freqs      │ ∅
histogram_bounds       │ {"2021-01-01 00:00:00+01","2021-02-01
00:00:00+01","2021-07-01 00:00:00+02","2021-08-01 00:00:00+02"}
correlation            │ -0.6
most_common_elems      │ ∅
most_common_elem_freqs │ ∅
elem_count_histogram   │ ∅

Zeit: 1,526 ms
select ctid, stainherit from pg_statistic where starelid = 'log'::regclass
and staattnum = 1;
  ctid   │ stainherit 
─────────┼────────────
 (18,21) │ f
 (18,19) │ t             <-- not touched
(2 Zeilen)

Zeit: 0,373 ms

cat inherit.sql
create table log (ts timestamptz, data text);
insert into log values ('2021-07-01', '1');
insert into log values ('2021-08-01', '2');
create table log2 (like log) inherits (log);
insert into log2 values ('2021-01-01', '3');
insert into log2 values ('2021-02-01', '4');

analyze log;
select * from pg_stats where tablename = 'log' and attname = 'ts' \gx
select ctid, stainherit from pg_statistic where starelid = 'log'::regclass
and staattnum = 1;

alter table log2 no inherit log;

analyze log;
select * from pg_stats where tablename = 'log' and attname = 'ts' \gx
select ctid, stainherit from pg_statistic where starelid = 'log'::regclass
and staattnum = 1;