![]() ![]() Given two sets of parallel execution servers SS1 and SS2 for the query plan illustrated in Figure 8-1, the execution proceeds as follows: each server set (SS1 and SS2) has four execution processes because of the PARALLEL hint in the query that specifies the DOP.Ĭhild set SS1 first scans the table customers and sends rows to SS2, which builds a hash table on the rows. Predicate Information (identified by operation id):Ħ - access("SALES"."CUST_ID"="CUSTOMERS"."CUST_ID")įigure 8-1 illustrates the data flow or query plan for the query in Example 8-1. | Id | Operation | Name | Rows | Bytes | TQ |IN-OUT| PQ Distrib | ![]() The number of parallel execution servers in one set is the degree of parallelism (DOP).Įxample 8-2 Explain Plan Output for a Query on Customers and Sales ![]() The coordinator must decide whether an operation can be performed in parallel and, if so, how many parallel execution servers to enlist. PARALLEL DEFINITION FULLFor example, the parallel execution method might be to perform a parallel full table scan by block range or a parallel index range scan by partition. For example, a parallel query with a SUM() operation requires adding the individual subtotals calculated by each parallel server.įinally, the query coordinator returns any results to the user.Īfter the optimizer determines the execution plan of a statement, the parallel execution coordinator determines the parallel execution method for each operation in the plan. When the parallel servers are finished executing the statement, the query coordinator performs any portion of the work that cannot be executed in parallel. The parallel execution servers performs each operation in parallel if possible. The SQL statement is executed as a sequence of operations (a full table scan to perform a join on a nonindexed column, an ORDER BY clause, and so on). The query coordinator obtains the necessary number of parallel servers. The user session or shadow process takes on the role of a coordinator, often called the query coordinator. If parallel execution is chosen, then the following steps occur: Each SQL statement undergoes an optimization and parallelization process when it is parsed. ![]()
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