SQL/MP Reference Manual
HP NonStop SQL/MP Reference Manual—523352-013
E-18
Considerations—EXPLAIN
Operation cost Indicates the relative cost of performing the operation. Lower
numbers indicate more efficient and less costly operation.
Parallel Execution
Plan
Describes the parallel execution plan for the operation, including:
The number of executor server processes (ESPs) that perform
each operation in parallel
The name of the system and the processors in which the ESPs
are started
The names of the volumes on which partitions of the base table
reside
The names of the volumes on which rows are redistributed to
promote parallel access
The number and purpose of each sort started by each ESP in
parallel.
If the query requires a sort, the plan shows the number of sorts
performed in parallel.
For a join query, describes the plan as one of these three types:
Matching partitions (nested join only)
Parallel access
Repartitioned
Plan Forced Indicates that the join method, the join sequence, or both were forced
by the user with a CONTROL TABLE directive.
Plan step Indicates the number of the plan step.
Specifies whether the plan step involves a union or join operation. If
the plan step involves a join operation, specifies the type of join
(inner or left) and the join strategy: nested, (sort) merge, key-
sequenced merge, or hash.
Pred. selectivity Specifies the percentage of the table or index that is evaluated to
test the search conditions of the predicates.
Purpose Specifies why the sort or hash join is required:
To order rows before the join phase
To order rows for an ORDER BY
To form groups of rows for a GROUP BY
To compute an aggregate DISTINCT
To discard duplicates for a DISTINCT
Plan step Indicates the number of the plan step.
Specifies whether the plan step involves a union or join operation. If
the plan step involves a join operation, specifies the type of join
(inner or left) and the join strategy: nested, (sort) merge, key-
sequenced merge, or hash.
Table E-1. EXPLAIN Plan Header Lines (page3of5)
Item Action