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?.

SELECT
    '2021-01-01'::date AS month
GROUP BY
    rollup(month)
ORDER BY
    month NULLS FIRST;

month

2021-01-01

null

EXPLAIN ANALYZE
SELECT
    '2021-01-01'::date AS month
GROUP BY
    rollup(month)
ORDER BY
    1 NULLS FIRST;

QUERY PLAN

GroupAggregate  (cost=0.00..0.03 rows=2 width=0) (actual time=0.003..0.003 rows=2 loops=1)

  Group Key: '2021-01-01'::date

  Group Key: ()

  ->  Result  (cost=0.00..0.01 rows=1 width=0) (actual time=0.000..0.000 rows=1 loops=1)

Planning time: 0.017 ms

Execution time: 0.027 ms

WITH cte AS (
SELECT
    '2021-01-01'::date AS month
GROUP BY
    rollup(month)
)

SELECT * FROM cte ORDER BY month NULLS FIRST;

month

null

2021-01-01