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IBM Informix OnLine Database Server Administrator’s Guide

Psort is a sorting package that improves performance by taking advantage of
multiprocessors to start and synchronize multiple sort processes. This
parallel-process sorting package is transparent to end users. (If you are
working on a uniprocessor machine and you set any of the parallel-sort
parameters, OnLine ignores the parameters and proceeds with nonparallel
sorting.)

Psort becomes an option for OnLine under either of three conditions:

■

The database server process is ordering query results. (An

ORDER BY

clause appears as part of a

SELECT

 statement.)

■

The database server process is eliminating duplicates. (The

UNIQUE

or

DISTINCT

 keyword appears in the

SELECT

 statement.)

■

The database server process is executing a sort-merge join, a new
multitable join method that is used with the older “nested-loop” join
method.

To enable Psort, set the

PSORT_NPROCS

environment variable, which defines

the upper limit for the number of processes used to sort a query. To disable
Psort, unset the

PSORT_NPROCS

 environment variable. (Refer to the next

topic for guidelines for setting the value of

PSORT_NPROCS

.)

If Psort is enabled, OnLine performs parallel sorting only when performance
is likely to improve. OnLine does not employ parallel sorting for a small
number of input rows or if a table index supports the order requested in the
query.

If OnLine engages Psort, multiple sorted runs are made in memory, written
to disk, and then merged from disk into a single result stream. OnLine calcu-
lates the number of processes to use in the sort based on the size of the query
and the number of processors on the system. You can tune Psort by limiting
the maximum number of processes available to OnLine and by directing
OnLine to use directories on different disks for the intermediate writes.