docs.intersystems.com
[Home]  
InterSystems: The power behind what matters   
Search:    

%PARALLEL
This optional keyword is specified in the FROM clause of a query. It suggests that Caché perform parallel processing of the query, using multiple processors (if applicable). This can significantly improve performance of some queries that uses one or more COUNT, SUM, AVG, MAX, or MIN aggregate functions, and/or a GROUP BY clause, as well as many other types of queries. These are commonly queries that process a large quantity of data and return a small result set. For example, SELECT AVG(SaleAmt) FROM %PARALLEL User.AllSales GROUP BY Region would likely use parallel processing.
A query that specifies both individual fields and an aggregate function and does not include a GROUP BY clause cannot perform parallel processing. For example, SELECT Name,AVG(Age) FROM %PARALLEL Sample.Person does not perform parallel processing, but SELECT Name,AVG(Age) FROM %PARALLEL Sample.Person GROUP BY Home_State does perform parallel processing.
%PARALLEL is intended for SELECT queries and their subqueries. An INSERT command subquery cannot use %PARALLEL.
Specifying %PARALLEL may degrade performance for some queries. Running a query with %PARALLEL on a system with multiple concurrent users may result in degraded overall performance.
Note:
A query that specifies %PARALLEL must be run in a database that is read/write, not readonly. Otherwise, a <PROTECT> error may occur.
Regardless of the presence of the %PARALLEL keyword in the FROM clause, some queries may use linear processing, not parallel processing: some queries do not support parallel processing; some queries, when optimized, may be found to not benefit from parallel processing. You can determine if and how Caché has partitioned a query for parallel processing using Show Plan.
For further details, refer to Parallel Query Processing in the “Optimizing Query Performance” chapter of the Caché SQL Optimization Guide.


Send us comments on this page
Copyright © 1997-2019 InterSystems Corporation, Cambridge, MA