On Sat, Oct 1, 2011 at 5:30 PM, Jeff Adams
<Jeff.Adams@noaa.gov> wrote:
Greetings,
I have a large table (~19 million records). Records contains a field
identifying a vessel and a field containing an time (epoch). Using the
current rows vessel and time values, I need to be able to find the next
lowest time value for the vessel and use it to compute how much time has
elapsed between the records. I have tried a scalar subquery in the SELECT,
which works, but it runs quite slowly. Is there an alternative approach that
might perform better for this type of query. Any information would be
greatly appreciated. Thanks...
Jeff
Would a self-join with a MAX() help, like this? (Where "v" is your vessel_id and "e" is your time value?)
create table stuff
( v int
, e timestamp
);
insert into stuff (v, e) values (1, '1/1/2011');
insert into stuff (v, e) values (1, '1/2/2011');
insert into stuff (v, e) values (1, '1/3/2011');
insert into stuff (v, e) values (2, '2/1/2011');
insert into stuff (v, e) values (2, '2/2/2011');
select a.v, a.e, max(b.e), a.e - max(b.e)
from stuff a
join stuff b on a.v = b.v
where a.e > b.e
group by a.v, a.e
I don't have a multi-million row table handy, but I'd be interested to hear your results.
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