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POSQL Selecting rows at varying intervals
 

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  1. POSQL Selecting rows at varying intervals
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    <p>I've got a situation where I have a huge table, containing a huge number of rows, which looks like (for example):</p> <pre><code>id Timestamp Value 14574499 2011-09-28 08:33:32.020 99713.3000 14574521 2011-09-28 08:33:42.203 99713.3000 14574540 2011-09-28 08:33:47.017 99713.3000 14574559 2011-09-28 08:38:53.177 99720.3100 14574578 2011-09-28 08:38:58.713 99720.3100 14574597 2011-09-28 08:39:03.590 99720.3100 14574616 2011-09-28 08:39:08.950 99720.3100 14574635 2011-09-28 08:39:13.793 99720.3100 14574654 2011-09-28 08:39:19.063 99720.3100 14574673 2011-09-28 08:39:23.780 99720.3100 14574692 2011-09-28 08:39:29.167 99758.6400 14574711 2011-09-28 08:39:33.967 99758.6400 14574730 2011-09-28 08:39:40.803 99758.6400 14574749 2011-09-28 08:39:49.297 99758.6400 </code></pre> <p>Ok, so the rules are: The timestamps can be any n number of seconds apart, 5s, 30s, 60s etc, it varies depending on how old the record is (archiving takes place).</p> <p>I want to be able to query this table to select each nth row based on the timestamp.</p> <p>So for example:</p> <p>Select * from mytable where intervalBetweenTheRows = 30s</p> <p>(for the purposes of this question, based on the presumption the interval requested is always to a higher precision than available in the database)</p> <p>So, every nth row based on the time between each row</p> <p>Any ideas?!</p> <p>Karl</p> <p>For those of you who are interested, recursive CTE was actually quite slow, I thought of a slightly different method:</p> <pre><code>SELECT TOP 500 MIN(pvh.[TimeStamp]) as [TimeStamp], AVG(pvh.[Value]) as [Value] FROM PortfolioValueHistory pvh WHERE pvh.PortfolioID = @PortfolioID AND pvh.[TimeStamp] &gt;= @StartDate AND pvh.[TimeStamp] &lt;= @EndDate GROUP BY FLOOR(DateDiff(Second, '01/01/2011 00:00:00', pvh.[TimeStamp]) / @ResolutionInSeconds) ORDER BY [TimeStamp] ASC </code></pre> <p>I take the timestamp minus an arbitrary date to give a base int to work with, then floor and divide this by my desired resolution, I then group by this, taking the min timestamp (the first of that 'region' of stamps) and the average value for that 'period'.</p> <p>This is used to plot a graph of historical data, so the average value does me fine. </p> <p>This was the fastest execution based on the table size that I could come up with</p> <p>Thanks for your help all.</p>
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    1. COPlease explain further. You want the 30s to be a **minimum** interval between returned rows? Or an exact interval?
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    2. COWhen you say "every nth row" you make it sound like you want rows 2, 4, 6, 8, etc. But really you just want to discard all records that happen within N seconds of the last returned record? Or perhaps, the first record after 30s, and the first records after 60s, and the first record after 90s, etc? (Perhaps you could give an example result set that you're looking for?) Also, can I make these assumptions? No two rows have the same Timestamp? And/Or that a records with a higher `id` CAN'T happen BEFORE a record with a lower `id`?
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    3. COThe "NO CURSORS" comment is a given. We don't do no cursin cursors no more.
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