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Threaded Reads

Autopartitioning Techniques in SAS/ACCESS

SAS/ACCESS products share an autopartitioning scheme based on the MOD function. Some products support additional techniques. For example, if your Oracle tables are physically partitioned in the DBMS, SAS/ACCESS Interface to Oracle automatically partitions in accordance with Oracle physical partitions rather than using MOD. SAS/ACCESS Interface to Teradata uses FastExport, if available, which lets the FastExport Utility direct partitioning.

MOD is a mathematical function that produces the remainder of a division operation. Your DBMS table must contain a column to which SAS can apply the MOD function -- a numeric column constrained to integral values. DBMS integer and small integer columns suit this purpose. Integral decimal (numeric) type columns can work as well. On each thread, SAS appends a WHERE clause to your SQL that uses the MOD function with the numeric column to create a subset of the result set. Combined, these subsets add up to exactly the result set for your original single SQL statement.

For example, assume that your original SQL that SAS produced is SELECT CHR1, CHR2 FROM DBTAB and that table Dbtab contains integer column IntCol. SAS creates two threads and issues:

SELECT CHR1, CHR2 FROM DBTAB WHERE (MOD(INTCOL,2)=0)

and 

SELECT CHR1, CHR2 FROM DBTAB WHERE (MOD(INTCOL,2)=1)

Rows with an even value for IntCol are retrieved by the first thread. Rows with an odd value for IntCol are retrieved by the second thread. Distribution of rows across the two threads is optimal if IntCol has a 50/50 distribution of even and odd values.

SAS modifies the SQL for columns containing negative integers, for nullable columns, and to combine SAS WHERE clauses with the partitioning WHERE clauses. SAS can also run more than two threads. You use the second parameter of the DBSLICEPARM= option to increase the number of threads.

The success of this technique depends on the distribution of the values in the chosen integral column. Without knowledge of the distribution, your SAS/ACCESS product attempts to pick the best possible column. For example, indexed columns are given preference for some DBMSs. However, column selection is more or less a guess, and the SAS guess might cause poor distribution of the result set across the threads. If no suitable numeric column is found, MOD cannot be used at all, and threaded reads will not occur if your SAS/ACCESS product has no other partitioning technique. For these reasons, you should explore autopartitioning particulars for your DBMS and judiciously use DBSLICE= to augment autopartitioning. See the information for your DBMS for specific autopartitioning details.

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