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CREATE TABLE purchases ( id serial -- PK constraint added below , customer_id int -- REFERENCES customer , total int -- could be amount of money in Cent , some_column text -- to make the row bigger, more realistic ); INSERT INTO purchases (customer_id, total, some_column) -- 200k rows SELECT (random() * 10000)::int AS customer_id -- 10k distinct customers , (random() * random() * 100000)::int AS total , 'note: ' || repeat('x', (random()^2 * random() * random() * 500)::int) FROM generate_series(1,200000) g; ALTER TABLE purchases ADD CONSTRAINT purchases_id_pkey PRIMARY KEY (id); DELETE FROM purchases WHERE random() > 0.9; -- some dead rows INSERT INTO purchases (customer_id, total, some_column) SELECT (random() * 10000)::int AS customer_id -- 10k customers , (random() * random() * 100000)::int AS total , 'note: ' || repeat('x', (random()^2 * random() * random() * 500)::int) FROM generate_series(1,20000) g; -- add 20k to make it ~ 200k CREATE INDEX purchases_3c_idx ON purchases (customer_id, total DESC, id); VACUUM ANALYZE purchases; CREATE TABLE customer AS SELECT customer_id, 'customer_' || customer_id AS customer FROM purchases GROUP BY 1 ORDER BY 1; ALTER TABLE customer ADD CONSTRAINT customer_customer_id_pkey PRIMARY KEY (customer_id); VACUUM ANALYZE customer; SELECT DISTINCT ON (customer_id) id, customer_id, total FROM purchases ORDER BY customer_id, total DESC, id;