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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 event(event_id serial, ts timestamp);
INSERT INTO event (ts)
SELECT generate_series(timestamp '2018-05-01'
, timestamp '2018-05-08'
, interval '7 min') + random() * interval '7 min';
CREATE TABLE
INSERT 0 1441
-- for the whole table, time interval 17 min (LIMIT 5 for this demo only)
WITH grid AS (
SELECT start_time
, lead(start_time, 1, 'infinity') OVER (ORDER BY start_time) AS end_time
FROM (
SELECT generate_series(min(ts), max(ts), interval '17 min') AS start_time
FROM event
) sub
)
SELECT start_time, count(e.ts) AS events
FROM grid g
LEFT JOIN event e ON e.ts >= g.start_time
AND e.ts < g.end_time
GROUP BY start_time
ORDER BY start_time
LIMIT 5;
| start_time | events |
|---|---|
| 2018-05-01 00:03:30.345273 | 3 |
| 2018-05-01 00:20:30.345273 | 3 |
| 2018-05-01 00:37:30.345273 | 2 |
| 2018-05-01 00:54:30.345273 | 2 |
| 2018-05-01 01:11:30.345273 | 2 |
SELECT 5
-- minimal equivalent
SELECT start_time, count(e.ts) AS events
FROM (SELECT generate_series(min(ts), max(ts), interval '17 min') FROM event) g(start_time)
LEFT JOIN event e ON e.ts >= g.start_time
AND e.ts < g.start_time + interval '17 min'
GROUP BY 1
ORDER BY 1
LIMIT 5;
| start_time | events |
|---|---|
| 2018-05-01 00:03:30.345273 | 3 |
| 2018-05-01 00:20:30.345273 | 3 |
| 2018-05-01 00:37:30.345273 | 2 |
| 2018-05-01 00:54:30.345273 | 2 |
| 2018-05-01 01:11:30.345273 | 2 |
SELECT 5
-- every 15 minutes in the past week (demo data is soon outdated vs. now()!)
SELECT to_char(start_time, 'YYYY-MM-DD HH24:MI'), count(e.ts) AS events
FROM generate_series(date_trunc('day', localtimestamp - interval '7 days')
, localtimestamp
, interval '15 min') g(start_time)
LEFT JOIN event e ON e.ts >= g.start_time
AND e.ts < g.start_time + interval '15 min'
GROUP BY start_time
ORDER BY start_time
LIMIT 5;
| to_char | events |
|---|---|
| 2023-10-23 00:00 | 0 |
| 2023-10-23 00:15 | 0 |
| 2023-10-23 00:30 | 0 |
| 2023-10-23 00:45 | 0 |
| 2023-10-23 01:00 | 0 |
SELECT 5