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

On Sat, Oct 1, 2011 at 2:30 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...
>
How many rows per vessel?

Either way it seems to me we are talking about selecting the current
record, and then selecting the max of another record within a
timeframe.  I would probably use a plpgsql function that would do this
in two stages, ideally being able to do an index scan twice....

Best Wishes,
Chris Travers