Adrian Klaver wrote:
On Wednesday 24 November 2010 1:08:27 am Mark Morgan Lloyd wrote:
What is best practice when extracting both current and average from a
table? Demonstration table here contains data from a cheap weather station.
I can obviously get the current reading like this:
select temp_out, dewpoint
from weather
where datetime between (now() - '10 minutes'::interval) and now()
order by datetime desc
limit 1;
and I can get averages like this:
select avg(temp_out) as avg_temp_out, avg(dewpoint) as avg_dewpoint
from weather
where datetime between (now() - '45 minutes'::interval) and now();
In both cases there are a dozen or so columns in total. How are these
best merged to yield a single row? Some form of join, or window functions?
I am not seeing a dozen columns, maybe rows?
I'd only put in a couple of columns as an example, but I was also making
the point that it would be nice to avoid having to type in an excessive
number of column names.
I quick and dirty solution(testing needed):
select
temp_out,dewpoint,atbl.avg_temp_out,atbl.avg_dewpoint
from
(select avg(temp_out) as avg_temp_out, avg(dewpoint) as avg_dewpoint
from weather
where datetime between (now() - '45 minutes'::interval) and now()) as atbl,
weather
order by datetime desc limit 1;
Has to be a bit more complex than that to make sure that the current
reading really is current:
select
ctbl.temp_out,ctbl.dewpoint,
atbl.avg_temp_out,atbl.avg_dewpoint
from (
select avg(temp_out) as avg_temp_out, avg(dewpoint) as avg_dewpoint
from weather
where datetime between (now() - '45 minutes'::interval) and now()
) as atbl, (
select * from weather
where datetime between (now() - '8 minutes'::interval) and now()
order by datetime desc limit 1
) as ctbl;
explain prices that as Nested Loop (cost=8.30..16.62 rows=1 width=84).
I think it's more elegant than the SQL I'm currently using
select * from (
select * from weather
where datetime between (now() - '10 minutes'::interval) and now()
order by datetime desc
limit 1
) as foo left outer join (
select datetime, avg(temp_out) as avg_temp_out, avg(dewpoint) as
avg_dewpoint
from weather
where datetime between (now() - '45 minutes'::interval) and now()
group by datetime
) as bar using (datetime);
but I note that explain prices that as Nested Loop Left Join
(cost=0.02..16.63 rows=1 width=215).
Does that mean that the query using the nested join will, on average, be
more efficient?
--
Mark Morgan Lloyd
markMLl .AT. telemetry.co .DOT. uk
[Opinions above are the author's, not those of his employers or colleagues]
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