last aggregate allows you to get the value of one column
as ordered by another. For example,
last(temperature, time) returns the
latest temperature value based on time within an aggregate group.
|ANY ELEMENT||The value to return|
|TIMESTAMP or INTEGER||The timestamp to use for comparison|
Get the temperature every 5 minutes for each device over the past day:
SELECT device_id, time_bucket('5 minutes', time) AS interval, last(temp, time) FROM metrics WHERE time > now () - INTERVAL '1 day' GROUP BY device_id, interval ORDER BY interval DESC;
firstcommands do not use indexes, and instead perform a sequential scan through their groups. They are primarily used for ordered selection within a
GROUP BYaggregate, and not as an alternative to an
ORDER BY time DESC LIMIT 1clause to find the latest value (which uses indexes).
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