Postgres 10

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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 emp_data (
Value1 int , 
value2 int
);

CREATE TABLE


insert into emp_data (value1, value2) values (100, 200), (200,200), (300,150) 

INSERT 0 3


Select case when value1 > value2 then value2
when value1 < value2 then value1  else null end as Comparison, value1, value2
from emp_data


comparison
value1
value2


100
100
200

null
200
200

150
300
150



SELECT 3