I need to generate unused random id with format is ID[0-9]{4} so i write below query but it seems to be too slow
SELECT * FROM ( SELECT user_id FROM ( SELECT 'ID' || LPAD(r, 4, '0') AS user_id FROM generate_series(1, 9999) as r) AS s EXCEPT SELECT user_id FROM account ) AS t ORDER BY random() LIMIT 1 and I execute explain analyze query. ------------------------------------------------------------------------------------------------------------------------------------------------------------------ Limit (cost=318.17..318.17 rows=1 width=32) (actual time=731.703..731.707 rows=1 loops=1) -> Sort (cost=318.17..318.95 rows=312 width=32) (actual time=731.693..731.693 rows=1 loops=1) Sort Key: random() -> Subquery Scan t (cost=285.79..305.24 rows=312 width=32) (actual time=424.299..659.193 rows=9999 loops=1) -> SetOp Except (cost=285.79..301.35 rows=311 width=16) (actual time=424.266..566.254 rows=9999 loops=1) -> Sort (cost=285.79..293.57 rows=3112 width=16) (actual time=424.139..470.529 rows=12111 loops=1) Sort Key: user_id -> Append (cost=0.00..105.24 rows=3112 width=16) (actual time=5.572..276.485 rows=12111 loops=1) -> Subquery Scan "*SELECT* 1" (cost=0.00..30.00 rows=1000 width=4) (actual time=5.565..149.615 rows=9999 loops=1) -> Function Scan on generate_series r (cost=0.00..20.00 rows=1000 width=4) (actual time=5.553..63.224 rows=9999 loops=1) -> Subquery Scan "*SELECT* 2" (cost=0.00..75.24 rows=2112 width=16) (actual time=0.030..28.473 rows=2112 loops=1) -> Seq Scan on account (cost=0.00..54.12 rows=2112 width=16) (actual time=0.019..10.155 rows=2112 loops=1) Total runtime: 738.809 ms do you have any idea for optimize? -- Choe, Cheng-Dae(최정대) Blog: http://www.comdongin.com/ ---------------------------(end of broadcast)--------------------------- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq