Samengevoegde kolommen met extra (verschillende) filters

Het geaggregeerde FILTER clausule in Postgres 9,4 of nieuwer is korter en sneller:

SELECT u.name
     , count(*) FILTER (WHERE g.winner_id  > 0)    AS played
     , count(*) FILTER (WHERE g.winner_id  = u.id) AS won
     , count(*) FILTER (WHERE g.winner_id <> u.id) AS lost
FROM   games g
JOIN   users u ON u.id IN (g.player_1_id, g.player_2_id)
GROUP  BY u.name;

• De handleiding
• Postgres Wiki
• Depesz-blogbericht

In Postgres 9.3 (of elke versie) is dit nog steeds korter en sneller dan geneste subselecties of CASE uitdrukkingen:

SELECT u.name
     , count(g.winner_id  > 0 OR NULL)    AS played
     , count(g.winner_id  = u.id OR NULL) AS won
     , count(g.winner_id <> u.id OR NULL) AS lost
FROM   games g
JOIN   users u ON u.id IN (g.player_1_id, g.player_2_id)
GROUP  BY u.name;

Details:

• Voor absolute prestaties, is SUM sneller of COUNT?