Postgres 13

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CREATE TABLE tags (id serial, series int, index int, page int);

INSERT INTO tags (series, index, page)
SELECT
    ceil(random() * 10),
    ceil(random() * 100),
    ceil(random() * 1000)
FROM generate_series(1, 100000);

CREATE TABLE

INSERT 0 100000

CREATE INDEX ON tags (page, series, index);

CREATE INDEX

EXPLAIN ANALYZE
SELECT DISTINCT ON (page, series) page, series, index
FROM tags ORDER BY page, series, index;

QUERY PLAN

Unique  (cost=0.42..5680.42 rows=10000 width=12) (actual time=0.053..58.569 rows=10000 loops=1)

  ->  Index Only Scan using tags_page_series_index_idx on tags  (cost=0.42..5180.42 rows=100000 width=12) (actual time=0.051..48.703 rows=100000 loops=1)

        Heap Fetches: 100000

Planning Time: 0.291 ms

Execution Time: 59.083 ms

EXPLAIN

EXPLAIN ANALYZE
SELECT tags.page, tags.series, MIN(tags.index)
FROM tags GROUP BY tags.page, tags.series;

QUERY PLAN

HashAggregate  (cost=2291.00..2391.00 rows=10000 width=12) (actual time=32.026..33.447 rows=10000 loops=1)

  Group Key: page, series

  Batches: 1  Memory Usage: 1425kB

  ->  Seq Scan on tags  (cost=0.00..1541.00 rows=100000 width=12) (actual time=0.012..8.323 rows=100000 loops=1)

Planning Time: 0.203 ms

Execution Time: 33.906 ms

EXPLAIN