Intro to Query Execution Bitmap Filters

One of the least understood Query Execution operators is the Bitmap.  I’d like to give a fairly brief overview of how Bitmap filters are used, as well as some technical details about their limitations and functionality.  Bitmap filters are often mistaken as Bitmap indexes.  The two are actually very distinct concepts — Bitmap indexes are physical structures that are…

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Statistics Profile Output Formatting

Statistics profile output is an important tool when it comes to troubleshooting query plan issues. When enabled, it returns a textual representation of the query plan with a lot of detail about cost and cardinality estimates as well as actual counts.   When working in SQL Server management Studio (SSMS), it is advised to enable…

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Showplan Trace Events

Besides SSMS, another great tool available to database developers and DBAs to view query plans and troubleshoot query compilation or execution issues is the SQL Server Profiler. In the Profiler, all the showplan events are listed under the Performance Event category. All the SQL Trace events generate showplan information at query-level granularity, i.e. a single…

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What’s this cost?

Oftentimes when people include actual execution plan (Ctrl-M, see previous posts for a good primer on execution plans) while executing a batch in SQL Server Management Studio, and they see this “Query cost (relative to the batch)” thing on top of each query in the batch, they start to ask: What does this mean? But when…

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Viewing and Interpreting XML Showplans

As mentioned in our previous blog posting, SQL Server 2005 supports Showplan generation in XML format. XML-based Showplans provide greater flexibility in viewing query plans and saving them to files as compared to legacy Showplans. In addition to the usability benefits, XML-based Showplans also contain certain plan-specific information which is not available in Legacy Showplans. For example, Showplan XML contains the cached plan…

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