Parallel Processing
The %PARALLEL keyword supports parallelism and distributed processing on a multiprocessor system. It causes InterSystems IRIS to perform parallel processing on the UNION queries, assigning each query to a separate process on the same machine. In some cases that process will send the query to a different machine to be processed. These processes communicate via pipes, with InterSystems IRIS creating one or more temporary files to hold subquery results. The main process combines the resulting rows and returns the final results. For further details, refer to the Show Plan for a UNION query, comparing the Show Plan with and without the %PARALLEL keyword. To determine the number of processors on the current system use the %SYSTEM.Util.NumberOfCPUs()Opens in a new tab method.
In general, the more effort expended to produce each row, the more beneficial %PARALLEL becomes.
Specifying the %PARALLEL keyword disables automatic UNION-to-OR optimizations.
The following examples show the use of the %PARALLEL keyword:
SELECT Name FROM Sample.Employee WHERE Name %STARTSWITH 'A'
UNION %PARALLEL
SELECT Name FROM Sample.Person WHERE Name %STARTSWITH 'A'
ORDER BY Name
SELECT Name FROM Sample.Employee WHERE Name %STARTSWITH 'A'
UNION ALL %PARALLEL
SELECT Name FROM Sample.Person WHERE Name %STARTSWITH 'A'
ORDER BY Name
%PARALLEL is intended for SELECT queries and their subqueries. An INSERT command subquery cannot use %PARALLEL.
Adding the %PARALLEL keyword may not be appropriate for all UNION queries, and may result in an error. The following SQL constructs generally do not support UNION %PARALLEL execution: an outer join, a correlated field, an IN predicate condition containing a subquery, or a collection predicate. UNION %PARALLEL is supported for a FOR SOME predicate, but not for a FOR SOME %ELEMENT collection predicate. To determine if a UNION query can successfully use %PARALLEL, test each leg of the UNION separately. Separately test each leg query by adding a FROM %PARALLEL keyword. If one of the FROM %PARALLEL queries generates a query plan that does not show parallelization, then the UNION query will not support %PARALLEL.
