Postgres 9.5

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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 tbl (val text);
insert into tbl values 
('0,11.2,2,5,1-2,1,SAMPLE CLOT,'',11.2 g/dl,14300,145,negative'),
('9,11.2,2,5,1-2,1,SAMPLE CLOT,'',11.2 g/dl,14300,145,negative'),
('3,11.2,2,5,1-2,1,SAMPLE CLOT,'',11.2 g/dl,14300,145,negative'),
('8,11.2,2,5,1-2,1,SAMPLE CLOT,'',11.2 g/dl,14300,145,negative');
4 rows affected

select (regexp_split_to_array(val, ','))[1] from   tbl

regexp_split_to_array

0

9

3

8

select *
from   tbl
where  (regexp_split_to_array(val, ','))[1]::int < 7

val

0,11.2,2,5,1-2,1,SAMPLE CLOT,',11.2 g/dl,14300,145,negative

3,11.2,2,5,1-2,1,SAMPLE CLOT,',11.2 g/dl,14300,145,negative