Identifying parallel scans in SET EXPLAIN output

When PDQ is turned on, the SET EXPLAIN output shows whether the optimizer chose parallel scans. If the optimizer chose parallel scans, the output lists Parallel. (If PDQ is turned off, the output lists Serial.)

If PDQ is turned on, the optimizer also indicates the maximum number of threads that are required to answer the query. The # of Secondary Threads field in the SET EXPLAIN output indicates the number of threads that are required in addition to your user session thread. The total number of threads necessary is the number of secondary threads plus 1.

The following example shows SET EXPLAIN output for a table with fragmentation and PDQ priority set to LOW:

SELECT * FROM t1 WHERE c1 > 20

Estimated Cost: 2
Estimated # of Rows Returned: 2

1) informix.t1: SEQUENTIAL SCAN (Parallel, fragments: 2)

 Filters: informix.t1.c1 > 20

# of Secondary Threads = 1

The following example of partial SET EXPLAIN output shows a query with a hash join between two fragmented tables and PDQ priority set to ON. The query is marked with

DYNAMIC HASH
JOIN

, and the table on which the hash is built is marked with Build Outer.

QUERY:
------
SELECT h1.c1, h2.c1 FROM h1, h2 WHERE h1.c1 = h2.c1

Estimated Cost: 2
Estimated # of Rows Returned: 5

1) informix.h1: SEQUENTIAL SCAN (Parallel, fragments: ALL)

2) informix.h2: SEQUENTIAL SCAN (Parallel, fragments: ALL)


DYNAMIC HASH JOIN (Build Outer)
 Dynamic Hash Filters: informix.h1.c1 = informix.h2.c1

# of Secondary Threads = 6

The following example of partial SET EXPLAIN output shows a table with fragmentation, PDQ priority set to LOW, and an index that was selected as the access plan:

SELECT * FROM t1 WHERE c1 < 13

Estimated Cost: 2
Estimated # of Rows Returned: 1

1) informix.t1: INDEX PATH

 (1) Index Keys: c1 (Parallel, fragments: ALL)
 Upper Index Filter: informix.t1.c1 < 13


# of Secondary Threads = 3