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Partitioning is usually introduced for large tables processed on big multi-CPU servers. Therefore, it is important to pay attention to parallelism of queries against partitioned tables. When investigating parallelism, we should rst determine whether a parallel query plan is generated at all. Subsequently we can evaluate execution ef ciency of the query from the point of view of parallelism. In this section I will explain how to recognize what parts of a query plan are parallel, what special considerations we should have in the context of partitioning and parallelism, and how to investigate execution ef ciency for parallel query plans. I will also give the details of one substantial change in parallel plan processing for partitioned tables between SQL Server 2005 and SQL Server 2008. Take a look back at the query plan in Figure 11-10 for our BETWEEN query in SQL Server 2005. It has four parallel operators as well as two operators without parallelism (Constant Scan and SELECT). The parallel operators are indicated by a round yellow icon with two arrows at the base of the operator icon. A parallel query plan has at least one Parallelism operator. Figure 11-10 shows two kinds of Parallelism operators: Gather Streams
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Querying Partitioned Tables
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on the left and Distribute Streams after the Constant Scan operator. The Distribute Streams operator creates multiple streams from a single input data stream. The Gather Streams operator merges several input streams into single output. There is one more type of Parallelism operator that does not occur in our example. It is called Redistribute Streams, and it has multiple input and multiple output data streams. Parallel execution of SQL Server query plan is performed by distributing single stream of rows into several streams, each processed by a separate thread. The distribution can be initiated either by a parallel scan or parallel seek operator, or by the afore mentioned Distribute Streams Parallelism operator. The effectiveness of parallel processing is then determined by how equally the work is distributed into the parallel streams. In some cases, the Gather and Redistribute Streams operators must preserve order, and this may introduce stalling because they can produce a new row only when they have received at least one row or end-of-stream indication from all input streams. SQL Server considers query parallelism only if there is more than one processor (multiple CPUs, cores, hyperthreading, or any combination of these) available to SQL Server. Query Optimizer then decides for each individual query whether to generate a parallel plan. For low-cost queries, the overhead of parallelism may be bigger than the gain. Therefore, parallelism is not considered for queries with estimated cost less than 5. (You may see parallel query plans with an estimated cost lower than 5 if the parallelism is what caused the plan s cost to drop below 5.) The degree of parallelism of a query with a parallel plan in SQL Server is the maximum number of active threads executing the query. The number of worker threads required for parallel query is usually much higher because every Parallelism operator creates a thread boundary each input stream and output stream is assigned a separate worker thread. The degree of parallelism restricts the number of active worker threads at any point of single query execution. The same parallel query plan may be executed with different degrees of parallelism. SQL Server decides at the time of query startup what degree of parallelism to use. This is affected by the available resources at that moment. Therefore, the same query with a single parallel query plan may be executed with different degrees of parallelism at different times. You can use either the Degree of Parallelism Event in the SQL Server s Performance Event pro ler category or the Actual Execution Plan captured in SSMS or pro ler to monitor the actual degree of parallelism for a particular instance of query execution. You can use sp_con gure with the max degree or parallelism option to lower the maximum considered by SQL Server for the whole instance. An individual query can include the clause OPTION (MAXDOP <value>) to change the maximum for its own execution. The query in Listing 11-4 demonstrates a major discrepancy in parallel query execution for partitioned tables between SQL Server 2005 and SQL Server 2008. As a by-product of the explanation, we will learn how to investigate the ef ciency of parallelism.
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