NonStop SQL/MP Reference Manual

NonStop SQL/MP Reference Manual—142115
E-20
Considerations—EXPLAIN
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 CPUs 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
Query plan n
Specifies the number of the query and whether the plan involves
parallel execution.
Requested
Specifies whether the sort or hash operation was requested by
the optimizer or explicitly requested by the user.
SBB for [Insert]
[Update]
Specifies whether virtual sequential block buffering (VSBB) is
used for insert or update operations.
SBB for reads
Specifies whether real or virtual sequential block buffering
(SBB) is used for read operations.
Seq Blockspli
t
Indicates that the user requested blocks to be split as for
sequential inserts by specifying the CONTROL TABLE
SEQUENTIAL BLOCKSPLIT ON option.