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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?.
with raw_dataset as (
SELECT 123 as user_id, 1 as flag, 101 as updated_at
UNION ALL SELECT 123 as user_id, 0 as flag, 102 as updated_at
UNION ALL SELECT 123 as user_id, 1 as flag, 103 as updated_at
UNION ALL SELECT 123 as user_id, 1 as flag, 104 as updated_at
UNION ALL SELECT 123 as user_id, 1 as flag, 105 as updated_at
UNION ALL SELECT 124 as user_id, 0 as flag, 101 as updated_at
UNION ALL SELECT 124 as user_id, 0 as flag, 103 as updated_at
UNION ALL SELECT 124 as user_id, 0 as flag, 110 as updated_at
)
select rd.*,
max(case when prev_flag is null or prev_flag <> flag then updated_at end) over (
partition by user_id
order by updated_at
) as most_recent_updated_at
from (select rd.*,
lag(flag) over (partition by user_id order by updated_at) as prev_flag
from raw_dataset rd
) rd
| user_id | flag | updated_at | prev_flag | most_recent_updated_at |
|---|---|---|---|---|
| 123 | 1 | 101 | null | 101 |
| 123 | 0 | 102 | 1 | 102 |
| 123 | 1 | 103 | 0 | 103 |
| 123 | 1 | 104 | 1 | 103 |
| 123 | 1 | 105 | 1 | 103 |
| 124 | 0 | 101 | null | 101 |
| 124 | 0 | 103 | 0 | 101 |
| 124 | 0 | 110 | 0 | 101 |
SELECT 8