vb net 2d barcode generator Inside Microsoft SQL Server 2008: T-SQL Programming in .NET

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Inside Microsoft SQL Server 2008: T-SQL Programming
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This generates the following output:
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app ---------app1 app2 mx ----------4 3
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Notice that instead of using a BETWEEN predicate to determine whether a session was active at a certain point in time (ts), I used ts >= starttime AND ts < endtime . If a session ends at the ts point in time, I don t want to consider it as active . The execution plan for this query is shown in Figure 8-3 . Note that I captured this plan after populating the Sales table with a more realistic volume of data . With very little data you might get a different plan .
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FiguRe 8-3 Execution plan for Maximum Concurrent Sessions, set-based solution
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First, the index I created on (app, starttime, endtime) is scanned . The number of rows returned will be the same as the number of rows in the table . For each app, starttime (call it ts) returned, a Nested Loops operator initiates activity that calculates the count of active sessions (by a seek within the index, followed by a partial scan to count active sessions) . The number of pages read in each iteration of the Nested Loops operator is the number of levels in the index plus at least the number of pages consumed by the rows representing active sessions . To make my point, I ll focus on the number of rows scanned at the leaf level because this number varies based on active sessions . Of course, to do adequate performance estimations, you should take page counts (logical reads) as well as many other factors into consideration . If you have n rows in the table, and there are c concurrent sessions on average at any given point in time when a session starts, you re looking at the following at least: n c rows scanned in total at the leaf level, beyond the pages scanned by the seek operations that got you to the leaf . You now need to figure out how this solution scales when the data volume increases . Typically, such reports are required periodically for example, once a month, for the most recent month . With the recommended index in place, the performance shouldn t change as long as the traffic doesn t increase for a month s worth of activity . But suppose that you anticipate traffic increase by a factor of f . If traffic increases by a factor of f, both total rows and number of active sessions at a given time grow by that factor; so in total, the number of rows scanned at the leaf level becomes at least (n f)(c f) = n c f 2 . You see, as the traffic grows, the cost doesn t increase linearly; rather, it increases in a quadratic manner .
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8 Cursors
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Next, let s talk about a cursor-based solution . The power of a cursor-based solution is that it can scan data in order . Relying on the fact that each session represents two events one that increases the count of active sessions, and one that decreases the count I ll declare a cursor for the following query:
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SELECT app, starttime AS ts, 1 AS event_type FROM dbo.Sessions UNION ALL SELECT app, endtime, -1 FROM dbo.Sessions ORDER BY app, ts, event_type;
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This query returns the following for each session start or end event: the application (app), the timestamp (ts); an event type (event_type) of +1 for a session start event or 1 for a session end event . The events are sorted by app, ts, and event_type . The reason for sorting by app, ts is obvious . The reason for adding event_type to the sort is to guarantee that if a session ends at the same time another session starts, you will take the end event into consideration first (because sessions are considered to have ended at their end time) . Other than that, the cursor code is straightforward simply scan the data in order and keep adding up the +1s and 1s for each application . With every new row scanned, check whether the cumulative value to that point is greater than the current maximum for that application, which you store in a variable . If it is, store it as the new maximum . When you re done with an application, insert a row containing the application ID and maximum into a table variable . That s about it . The following code has the complete cursor solution:
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DECLARE @app AS VARCHAR(10), @prevapp AS VARCHAR (10), @ts AS datetime, @event_type AS INT, @concurrent AS INT, @mx AS INT; DECLARE @Result AS TABLE(app VARCHAR(10), mx INT); DECLARE C CURSOR FAST_FORWARD FOR SELECT app, starttime AS ts, 1 AS event_type FROM dbo.Sessions UNION ALL SELECT app, endtime, -1 FROM dbo.Sessions ORDER BY app, ts, event_type; OPEN C; FETCH NEXT FROM C INTO @app, @ts, @event_type; SELECT @prevapp = @app, @concurrent = 0, @mx = 0; WHILE @@fetch_status = 0 BEGIN IF @app <> @prevapp BEGIN INSERT INTO @Result VALUES(@prevapp, @mx); SELECT @prevapp = @app, @concurrent = 0, @mx = 0; END SET @concurrent = @concurrent + @event_type; IF @concurrent > @mx SET @mx = @concurrent;
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