finding gaps in dates

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.

The only way I can think of to do it is to create a set returning
function that accepts a begin and end date and returns a
date_trunc('minute', ...) for each minute between then and then doing

Select ts from srf(now() - '30 days'::inteval, now()) where ts not in
(select distinct date_trunc('minute', ts) from table where ts between
now() - '30 days'::inteval and now());

Of course, that seems extremely tedious to me (not tedious to code,
but tedious for the db to process).

If anyone can suggest a better way I'd really appreciate it.

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Re: finding gaps in dates

> 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 anomalies every 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.

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