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Help with an interesting Postgres question: Why isn't an Index Only Scan used on a partition accessed via the parent table?.
create schema def;
create table t1(a smallint, b int, c double precision, d numeric, e text, f bigint);

insert into t1
select
floor(random()*100)::int,
floor(random()*100)::int,
random()*100,
random()*100,
repeat(gen_random_uuid()::text,floor(random()*5+1)::int),
floor(random()*9223372036854775807)::bigint
from generate_series(1,300);

select * from t1 limit 10;
CREATE SCHEMA

CREATE TABLE

INSERT 0 300
a b c d e f
23 71 1.1721947768125185 16.7528977262813 faf7cfd8-7e57-4d8f-9f00-e984c438a0adfaf7cfd8-7e57-4d8f-9f00-e984c438a0adfaf7cfd8-7e57-4d8f-9f00-e984c438a0adfaf7cfd8-7e57-4d8f-9f00-e984c438a0ad 2197393462332635136
72 29 39.01667117618446 93.9129676126963 a26742a9-b10a-4d87-87a4-bdd84ce9234ea26742a9-b10a-4d87-87a4-bdd84ce9234ea26742a9-b10a-4d87-87a4-bdd84ce9234ea26742a9-b10a-4d87-87a4-bdd84ce9234ea26742a9-b10a-4d87-87a4-bdd84ce9234e 5833481516626774016
81 80 58.70090848363101 42.8630097743508 36d0f0bd-e4c8-4b6a-bb3f-06c390f3f10a36d0f0bd-e4c8-4b6a-bb3f-06c390f3f10a36d0f0bd-e4c8-4b6a-bb3f-06c390f3f10a36d0f0bd-e4c8-4b6a-bb3f-06c390f3f10a 3033801512388929536
97 33 86.72729390270628 7.83294175433189 104dfce4-96c7-40f4-ad91-e8098e09cfa3104dfce4-96c7-40f4-ad91-e8098e09cfa3104dfce4-96c7-40f4-ad91-e8098e09cfa3 4900319619640123392
24 76 50.437690834132695 85.1301868094659 877a91dd-7e79-4967-bc58-f90c5804d010877a91dd-7e79-4967-bc58-f90c5804d010 7976803432216248320
93 64 11.57090610979723 16.6931001402385 ccf7f439-02b7-419b-ba50-54356e6bafffccf7f439-02b7-419b-ba50-54356e6bafffccf7f439-02b7-419b-ba50-54356e6bafff 5839714898056380416
76 37 24.573926476861008 97.3345531773341 88b0719b-4486-449f-86ab-d1b923229fd088b0719b-4486-449f-86ab-d1b923229fd088b0719b-4486-449f-86ab-d1b923229fd088b0719b-4486-449f-86ab-d1b923229fd088b0719b-4486-449f-86ab-d1b923229fd0 495360858684590080
99 51 32.70351635877666 7.55685924071043 f3d3b592-43e9-412b-b313-15f81c23d951f3d3b592-43e9-412b-b313-15f81c23d951 6106671529291317248
59 73 84.49272004157666 26.4332677307878 ce899bbc-e5da-4628-8b53-439dbbc9c894ce899bbc-e5da-4628-8b53-439dbbc9c894 1479129413091627008
98 51 38.189378995409044 66.0282166594723 b036df75-f457-4783-bf5a-2de6395baa34b036df75-f457-4783-bf5a-2de6395baa34b036df75-f457-4783-bf5a-2de6395baa34 8988141981055608832 
SELECT 10
select sum(pg_column_size( bks.* )) from t1 AS bks;
select sum(pg_column_size((a,b,c,d,e,f))) from t1 AS bks;
sum
52944 
SELECT 1
sum
52944 
SELECT 1
create or replace function column_sizes(target_table regclass)
returns table (column_name text, column_size bigint)
language plpgsql stable parallel restricted as $function$
declare
rec record;
parsed_target_table text[]:=parse_ident(target_table::text);
schema_name text:=case when parsed_target_table[2] is not null then
parsed_target_table[1] end;
table_name text:=parsed_target_table[array_upper(parsed_target_table,1)];
begin
for rec in execute format(' select column_name
from information_schema.columns
where (%1$L is null or table_schema = %1$L)
and (%2$L is null or table_name = %2$L)',
schema_name,
table_name ) loop
return query execute format(
'select %1$L,sum(pg_column_size(%1$I))::bigint from %s',
rec.column_name,
target_table);
end loop;
end $function$;
CREATE FUNCTION
select * from column_sizes('public.t1');
column_name column_size
a 600
b 1200
c 2400
d 3840
f 2400
e 33360 
SELECT 6
select format('select pg_column_size(%I) from %s','abc','public.t1'::regclass);
alter table t1 set schema def;
select format('select pg_column_size(%I) from %s','abc','def.t1'::regclass);
format
select pg_column_size(abc) from t1 
SELECT 1

ALTER TABLE
format
select pg_column_size(abc) from def.t1 
SELECT 1