Quick Beginnings for DB2 Extended Enterprise Edition for UNIX
DB2 Extended Enterprise Edition uses a cost-based query
optimizer, which compares different methods for doing a unit of work,
and selects the most efficient one. The optimizer provides the
- Transparent parallelism
- Both new and existing applications that use data-manipulating SQL
statements do not have to be changed when they are migrated to DB2 Extended
Enterprise Edition. You only have to rebind them so the optimizer can
generate the best plans for existing SQL queries.
- Comprehensive use of data partitioning information
- The optimizer uses information about how base tables, and the intermediate
tables that result from queries, are partitioned across database
partitions. This information is used to determine the best execution
- Full-fledged, cost-based optimization
- The optimizer considers different execution plans and chooses the one with
the lowest cost. While comparing different strategies, it accounts for
the inherent parallelism of different operations, and the costs introduced by
- Inter-partition parallelism of all relational operations.
- All operations, including index and table scans, aggregation, set
operations, joins, inserts, deletes, and updates can employ
inter-partition parallelism. (Inter-partition parallelism
means that the operator is executed in parallel by each database partition
server; intra-partition parallelism means that different
operators in the same query can be executed in parallel by the same database
partition server.) When generating plans, the optimizer considers
different parallel joining methods, including collocated,
directed, and broadcast joins.
As a simple example, assume that you want to fetch specific columns from
rows that meet some condition. The coordinator node sends a request to
the other database partition servers to select the columns from rows that meet
that condition. The other database partition servers then send this
data to the coordinator node, which does the final processing.
If a row is being added to a table, the database partition server checks
the partitioning map, which specifies the database partition server
where the row is stored. The row is only sent to that database
partition server, with the result that only the interested database partition
servers take part in the insert. This keeps communications overhead as
low as possible.
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