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?.

select str, array_to_string((string_to_array(str, '~'))[4:], '~')
from (values ('1~2~3~4~5~6~7~8'), ('7~8~22~99~1'), ('1~2')) v(str)

str
array_to_string

1~2~3~4~5~6~7~8
4~5~6~7~8

7~8~22~99~1
99~1

1~2

select str, regexp_replace(str, '^([^~]*[~]){3}(.*)$', '\2')
from (values ('1~2~3~4~5~6~7~8'), ('7~8~22~99~1'), ('1~2')) v(str)

str
regexp_replace

1~2~3~4~5~6~7~8
4~5~6~7~8

7~8~22~99~1
99~1

1~2
1~2

select str, regexp_replace(str, '^([^~]*[~]){3}', '')
from (values ('1~2~3~4~5~6~7~8'), ('7~8~22~99~1'), ('1~2')) v(str)

str
regexp_replace

1~2~3~4~5~6~7~8
4~5~6~7~8

7~8~22~99~1
99~1

1~2
1~2

select str, (regexp_matches(str, '^([^~]*[~]){3}(.*)$'))[2] as retval
from (values ('1~2~3~4~5~6~7~8'), ('7~8~22~99~1'), ('1~2')) v(str)

str
retval

1~2~3~4~5~6~7~8
4~5~6~7~8

7~8~22~99~1
99~1