Re: SQL Help - Finding Next Lowest Value of Current Row Value

[Jeff Adams <Jeff.Adams@xxxxxxxx>]

Thanks for the suggestions Chris (and Chris). After a bit more investigation I stumbled upon the Window functions. The approach below turned out to be much more efficient that a function or self join approach. The SQL that I used is provided below (event_id and mmsi uniquely identify a vessel transit for which I wished to compute how much time had elapsed between successive records):

 

SELECT

  a.event_id,

  a.mmsi,

  (a.epoch - lag(epoch) OVER (PARTITION BY event_id, mmsi ORDER BY epoch ASC))/60 AS elapsed

FROM

   dmas_ais a

 

Jeff

 

From: ccurvey@xxxxxxxxx [mailto:ccurvey@xxxxxxxxx] On Behalf Of Chris Curvey
Sent: Saturday, October 01, 2011 10:55 PM
To: Jeff Adams
Cc: pgsql-general@xxxxxxxxxxxxxx
Subject: Re: SQL Help - Finding Next Lowest Value of Current Row Value

 

On Sat, Oct 1, 2011 at 5:30 PM, Jeff Adams <Jeff.Adams@xxxxxxxx> 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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