SQL/MX 2.x Query Guide (G06.24+, H06.03+)

Parallelism
HP NonStop SQL/MX Query Guide—523728-003
8-5
DAM and ESP Parallelism
For more details about understanding plan fragments, plan fragment boundaries, and
reading the EXPLAIN output, see Explaining Parallel Plans on page 8-12.
DAM and ESP Parallelism
Partitioned parallelism uses different processes, depending on the type of operators
being processed: DAM processes and ESPs.
•
DAM parallelism indicates parallel execution in multiple DAM fragment instances.
The instances might be accessing different tables, such as in a join or union query,
or they might be accessing different partitions of one table under coordination of a
SPLIT_TOP node. DAM parallelism is characterized by no-wait communication
(asynchronous access). This form of parallelism is inexpensive because it uses
existing disk processes; however, it is limited in use. For example, DAM processes
cannot repartition, and they might need to service other requests.
•
ESP parallelism refers to any parallel plan with at least one ESP plan fragment.
ESP parallelism occurs when a plan fragment executes within a special process
called the executor server process (ESP). ESP parallelism is enabled by the
default ATTEMPT_ESP_PARALLELISM.
root [9]
sort [6]
1
Master Executor
sort_partial_groupby_root [7]
esp exchange [5]
4 (range)
split_top [4]
12 (logphys)
ESP Fragment
partition access [3]
hash_partial_groupby_leaf [2]
index_scan [1] - LX3 (m)
DAM Fragment
Root
ESP ESPESP ESP
Process Structure of the Plan
VST081.vsd