[SOLVED] Selecting an average of records grouped by 5 minute periods

Selecting an average of records grouped by 5 minute periods

I'm having a slight issue. I have a PostgreSQL table with such format

time (datetime)     | players (int) | servers (int)
---------------------------------------------------
2013-12-06 13:40:01 | 80            | 20
2013-12-06 13:41:13 | 78            | 21
etc.

I would like to group them by 5 minute periods and get an average of the group as a single value, so there will be 20% of the records, each containing an average of ~5 numbers, with time set to the first time value in the group. I have no idea how to do this in PgSQL. So the result would be:

2013-12-06 13:40:01 | avg of players on :40, :41, :42, :43, :44 | same with servers
2013-12-06 13:45:05 | avg of players on :45, :46, :47, :48, :49 | same with servers
2013-12-06 13:50:09 | avg of players on :50, :51, :52, :53, :54 | same with servers
2013-12-06 13:55:12 | avg of players on :55, :56, :57, :58, :59 | same with servers

Solution

SELECT grid.t5
     , min(t."time") AS min_time
--   , array_agg(extract(min FROM t."time")) AS 'players_on'  -- optional
     , avg(t.players) AS avg_players
     , avg(t.servers) AS avg_servers
FROM  (
   SELECT generate_series(min("time")
                        , max("time")
                        , interval '5 min') AS t5
   FROM   tbl
   ) grid
LEFT   JOIN tbl t ON t."time" >= grid.t5
                 AND t."time" <  grid.t5 + interval '5 min'
GROUP  BY grid.t5
ORDER  BY grid.t5;

The subquery grid produces one row for every 5 minutes from minimum to maximum "time" in your table.

LEFT JOIN back to the table slicing data in 5-min intervals. Include the lower bound, exclude the upper border.
To drop 5-min slots where nothing happened, use JOIN instead of LEFT JOIN.

To have your grid-times start at 0:00, 5:00 etc, round down the min("time") in generate_series().

Aside: I wouldn't use "time" as identifier. It's a reserved word in standard SQL and a function / type name in Postgres.