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POSQL query executing slowly (for some parameter values)
 

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  1. POSQL query executing slowly (for some parameter values)
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    <p>I have a SQL Server 2005 database with several tables. One of the tables is used to store timestamps and message counters for several devices, and has the following columns:</p> <pre><code>CREATE TABLE [dbo].[Timestamps] ( [Id] [uniqueidentifier] NOT NULL, [MessageCounter] [bigint] NULL, [TimeReceived] [bigint] NULL, [DeviceTime] [bigint] NULL, [DeviceId] [int] NULL ) </code></pre> <p><code>Id</code> is the unique primary key (Guid.Comb), and I have indexes on both <code>DeviceId</code> and <code>MessageCounter</code> columns.</p> <p>What I want to do is find the last inserted row (the row with the largest <code>MessageCounter</code>) for a certain device.</p> <p>The thing that is strange is that a query for <strong>device no. 4</strong> (and all other devices except no.1) returns almost instantaneously:</p> <pre><code>select top 1 * from "Timestamps" where DeviceId = 4 order by MessageCounter desc </code></pre> <p>but the same query for <strong>device no. 1</strong> takes forever to complete:</p> <pre><code>select top 1 * from "Timestamps" where DeviceId = 1 /* this is the only line changed */ order by MessageCounter desc </code></pre> <p>The strangest thing is that device 1 has <strong>much less rows</strong> than device 4:</p> <pre><code>select count(*) from "Timestamps" where DeviceId = 4 (returns 1,839,210) select count(*) from "Timestamps" where DeviceId = 1 (returns 323,276). </code></pre> <p>Does anyone have a clue what I could be doing wrong?</p> <p><strong>[Edit]</strong></p> <p>From the execution plans for both queries, it is clearly visible that Device 1 (lower diagram) creates a much larger number of rows in Index scan:</p> <p><a href="http://img295.imageshack.us/img295/5784/execplans.png" rel="nofollow noreferrer">Execution plans for device 4 (upper) and device 1 (lower) http://img295.imageshack.us/img295/5784/execplans.png</a></p> <p>The difference is when I hover the Index Scan nodes on execution plan diagrams:</p> <pre><code>Device 4 Actual Number of Rows: 1 Device 1 Actual Number of Rows: approx. 6,500,000 </code></pre> <p>6,500,000 rows is a very strange number, since my <code>select count(*)</code> query returns around 300,000 rows for device 1!</p>
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    1. USGroo
    1. USGroo
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      1. VTUpMod
    1. COMaybe refresh the index on DeviceID?
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      1. USFrustratedWithFormsDesigner
    2. COcan you add the execution plan on both queries? than we know, what it changes. and afterwards we can guess, why it is that way... :-)
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      1. UScRichter
    3. COOk, it looks like the statistics are out of sync - I assume the databases are set to auto update statsistics. Follow OMG Ponies' advice
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      1. USPeter Schofield
 

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