On Wed, Apr 14, 2021 at 5:33 PM Brent Wood <pcreso@xxxxxxxxx> wrote:
>
> Thanks for your reply,
>
> The table is essentially:
> create table readings (timer timestamp primary key,
> readings hstore);
>
> the hstore comprises (<sensor_id> <reading>) key/value pairs for readings taken at the time specified in the timestamp.
>
> eg: "67" "-45.67436", "68" "176.5424" could be key/value pairs representing latitude & longitude, with a timestamp in the timer column.
>
> There would be several lat/lon hstore pairs in a given minute, the query I want would return the last one in the timeseries for that minute (for each key).
>
> I don't think your examples will give me the separate hstore key-value pairs, extracted as the last in the interval for each key & reassembled as an hstore list in the result. The sensor id is the hstore key, as described above, not a separate attribute. That said, the keys can be extracted from the hstore much like a column, but I'm not sure that is the best approach.
>
> Treating each set of hstores in an interval as an array & extracting the last elements may be viable. But I['m not sure how...
>
Hi Brent,
With the table structure like that, you'll need to project all the
sensor data onto each timestamp and then collapse it. Something like
this:
SELECT date_trunc('minute',timer), key, first(value) over (partition
by key order by timer desc) FROM (SELECT timer, (each(readings)).*
FROM table WHERE date_trunc('minute',timer) = '2021-04-15
08:10:00-04')x;
If your queries will always target a specific timestamp truncated to
the minute, you'll find an index on date_trunc('minute',timer) will be
useful, I think. If your intervals are more complex than date_trunc()
can handle then you'll have to get more creative, but that's the basic
structure -- normalize the primary key, project the sensor data to
make it table-ish, then use regular SQL to pull out what you want.
--
Mike Rylander
| Research and Development Manager
| Equinox Open Library Initiative
| phone: 1-877-OPEN-ILS (673-6457)
| email: miker@xxxxxxxxxxxxxx
| web: https://equinoxOLI.org
>
>
> On Thursday, April 15, 2021, 6:33:08 AM GMT+12, Michael Lewis <mlewis@xxxxxxxxxxx> wrote:
>
>
>
>
>
> If you share example schema and desired output (like a dummy table or even pseudo code SQL), then I'm sure many people could help you. Right now, the description of your desired result seems a bit unclear, at least to me.
>
>
> If you wanted to run this hourly for the last 1 hour, it sounds a bit like want this-
>
> select sensor_id, date_trunc( 'minute', timestamptz_field_name_here ), last( value_from_hstore ) over ( partition by sensor_id, date_trunc( 'minute', timestamptz_field_name_here ) ) as last_value_recorded
> from data_table_here
> where timestamptz_field_name_here BETWEEN NOW() - interval '1 hour' and NOW()
> group by sensor_id, date_trunc( 'minute', timestamptz_field_name_here )
>
>
> You could also use the left join "where is null" pattern to check that a prior record in the minute period of time does not exist for that same key. Something like this-
>
> select d1.sensor_id, date_trunc( 'minute', d1.timestamptz_field_name_here ), d1.timestamptz_field_name_here as last_value_recorded
> from data_table_here as d1
> left join data_table_here as prior_d1 ON prior_d1.sensor_id = d1.sensor_id AND prior_d1.timestamptz_field_name_here < d1.timestamptz_field_name_here and prior_d1.timestamptz_field_name_here >= date_trunc( 'minute', d1.timestamptz_field_name_here )
> where d1.timestamptz_field_name_here BETWEEN NOW() - interval '1 hour' and NOW()
>
>
> >
>
>
