Re: how to remove a for-loop from programming language and put it into the query?

Pedro Zorzenon Neto <pedro2009@mandic.com.br> wrote:

> [...]
> So, I need to get a report of all diagnostics of all hardware on
> december 25th.

> (external programming language)
> for ($i = 1; $i < 500; $i++) {
>   // return me the "most recent" diag_value from a hardware_id $i
>   // at the desired timestamp
>   runquery("select diag_value from diagnose_logs where ts <= '2009-12-25
> 23:59:59' and hardware_id = $i order by ts desc limit 1");
> }

> Currently I have an index on diagnose_logs(ts,hardware_id)
> I have 3 milion registers of 500 different hardware_id.

> The time to run 500 times this query is long... about 1 minute. When I
> need a montly day-by-day report of 500 hardwares, it takes about half an
> hour.

> can I turn this for-loop into a single query to run in postgres?

Another month, another case for "DISTINCT ON":

| SELECT DISTINCT ON (hardware_id)
|   hardware_id, diag_value
|   FROM diagnose_logs
|   WHERE ts <= '2009-12-25 23:59:59'
|   ORDER BY hardware_id, ts DESC;

BTW, I'd prefer "WHERE ts < '2009-12-26'" as otherwise you
don't catch a timestamp '2009-12-25 23:59:59.5' (not to
speak of leap seconds).

Tim