> I have a logging application that should produce an entry in the
> database every minute or so, give or take a few seconds.
>
> I'm interested in finding out
> a: what minutes don't have a record and
> b: periods where the gap exceeded a certain amount of time.
Is this not the same question ?
Answer to a:
If your script is set to run at every minute + 00 seconds, if it ever
runs one second earlier, timestamp-truncate will keep the previous minute
and you're screwed. A simple solution would be to have your script run
every minute + 30 seconds.
Answer to b:
If you can do the following : examine the records in chronological order,
every time computing the delay between record N and record N-1 ; if this
delay is not one minute +/- a few seconds, you have detected an anomaly.Problem : you need to scan the whole table for
anomaliesevery time.Solution : put an ON INSERT trigger on your log table which :- checks the current time for sanity
(ie.is it +/- a few seconds from
the expected time ?)This solves part of a)- looks at the timestamp of the latest row, computes the difference with
the inserted one, and if > than 1 minute + a few seconds, inserts a row in
an anomaly logging table.This solves the rest of a) and b)
It's just an additional SELECT x FROM table ORDER BY timestamp DESC LIMIT
1 which has a negligible performance impact compared to your insert.