comparing rows

On Mon, 2007-12-10 at 13:31 -0800, hjenkins wrote:

I would like to take a timeseries of data and extract the rows of data
flanking the gaps in it. So I need to compare timestamps from two
adjacent
rows, and determine if the interval is greater than the standard
sampling
interval.

It often helps for us to have a snippet of a table definition to frame
replies. I'll assume that you have a "data" table with a timestamp
column called "ts". I suspect you could use a subquery, like this:

=> select D1.ts as ts1,(select ts from data D2 where D2.ts>D1.ts limit
1) as ts2 from data D1;

I'm uncertain about the performance of this subquery in modern PGs. If
this query works for you, then you can wrap the whole thing in a view or
another subquery in order to compute ts2-ts1, like this:

=> select ts1,ts2,ts2-ts1 as delta from ( <above query> ) X;

This will get you only the timestamps of adjacent rows with large
deltas. The easiest way to get the associated data is to join on the
original data table where ts1=ts or ts2=ts.

-Reece

--
Reece Hart, http://harts.net/reece/, GPG:0x25EC91A0

On Mon, Dec 10, 2007 at 02:00:24PM -0800, Reece Hart wrote:

On Mon, 2007-12-10 at 13:31 -0800, hjenkins wrote:

I would like to take a timeseries of data and extract the rows of data
flanking the gaps in it. So I need to compare timestamps from two
adjacent
rows, and determine if the interval is greater than the standard
sampling
interval.

It often helps for us to have a snippet of a table definition to frame
replies. I'll assume that you have a "data" table with a timestamp
column called "ts". I suspect you could use a subquery, like this:

=> select D1.ts as ts1,(select ts from data D2 where D2.ts>D1.ts limit
1) as ts2 from data D1;

I'd make this a JOIN on some (set of) column(s). Let's call those
columns a, b and c, and let's assume none are NULLable.

SELECT d1.ts AS ts1, d2.ts AS ts2
FROM
data d1
JOIN
data d2
ON (
(d1.a, d2.b, d2.c) = (d2.a, d2.b, d2.c)
AND
d1.ts < d2.ts
)

Cheers,
David.

I'm uncertain about the performance of this subquery in modern PGs. If
this query works for you, then you can wrap the whole thing in a view or
another subquery in order to compute ts2-ts1, like this:

=> select ts1,ts2,ts2-ts1 as delta from ( <above query> ) X;

This will get you only the timestamps of adjacent rows with large
deltas. The easiest way to get the associated data is to join on the
original data table where ts1=ts or ts2=ts.

-Reece

--
Reece Hart, http://harts.net/reece/, GPG:0x25EC91A0

--
David Fetter <david@fetter.org> http://fetter.org/
Phone: +1 415 235 3778 AIM: dfetter666 Yahoo!: dfetter
Skype: davidfetter XMPP: david.fetter@gmail.com

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