Postgres 12

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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 table test as
SELECT * FROM (
SELECT '01.00 01.12' AS c_
UNION ALL
SELECT '01.00^01.13' 
UNION ALL
SELECT '01.00 01.15') T1 ;
3 rows affected

select * from test order by 1 ;

c_

01.00 01.12

01.00^01.13

01.00 01.15

select c_, md5( c_ )
from test
order by 2 ;

c_
md5

01.00 01.12
335dca42dedecedc19ba65065a7777ec

01.00^01.13
4e1661df191eb006274be5552a998280

01.00 01.15
c1197d14549263a867fd9850f42b68b1

select c_, sha256( c_::bytea ) from test order by 2 ;

c_
sha256

01.00 01.12
\xa367f00f2ecd669aaf2241d22db5a794a862f8f375100e02460221a3b4fc6e99

01.00 01.15
\xa8a2796734c97616e35630d3ed60cddbeafc7846b6fdecc1d503fe02066df453

01.00^01.13
\xb913bc75e6d9541471f07c01ca43ac7238fff28cfac6bf6750f41863ed129386

c_	md5
01.00 01.12	335dca42dedecedc19ba65065a7777ec
01.00^01.13	4e1661df191eb006274be5552a998280
01.00 01.15	c1197d14549263a867fd9850f42b68b1

c_	sha256
01.00 01.12	\xa367f00f2ecd669aaf2241d22db5a794a862f8f375100e02460221a3b4fc6e99
01.00 01.15	\xa8a2796734c97616e35630d3ed60cddbeafc7846b6fdecc1d503fe02066df453
01.00^01.13	\xb913bc75e6d9541471f07c01ca43ac7238fff28cfac6bf6750f41863ed129386