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 activity (
  id text,
  userid text,
  createdat timestamp,
  username text
);

INSERT INTO activity VALUES
   ('1d658a','4957f3','20161206 21:16:35'::timestamp,'Tom Jones')
 , ('3a86e3','684edf','20161203 21:16:35'::timestamp,'Harry Smith')
 , ('595756','582107','20161226 21:16:35'::timestamp,'William Hanson')
 , ('2c87fe','784723','20161207 21:16:35'::timestamp,'April Cordon')
 , ('32509a','4957f3','20161220 21:16:35'::timestamp,'Tom Jones')
 , ('72e703','582107','20170101 21:16:35'::timestamp,'William Hanson')
 , ('6d658a','582107','20161206 21:16:35'::timestamp,'William Hanson')
 , ('5c077c','5934c4','20161206 21:16:35'::timestamp,'Sandra Holmes')
 , ('92142b','57ea5c','20161215 21:16:35'::timestamp,'Lucy Lawless')
 , ('3dd0a6','5934c4','20161204 21:16:35'::timestamp,'Sandra Holmes')
 , ('43509a','4957f3','20161120 21:16:35'::timestamp,'Tom Jones')
 , ('85142b','57ea5c','20161115 21:16:35'::timestamp,'Lucy Lawless')
 , ('7c87fe','784723','20170107 21:16:35'::timestamp,'April Cordon')
 , ('9c87fe','784723','20170207 21:16:35'::timestamp,'April Cordo')
 ;

14 rows affected

month	users_this_month	users_previous_month
NOV 2016	2	null
DEC 2016	6	2
JAN 2017	2	6
FEB 2017	1	2