SQL Server Execution Plans - Red Gate Software

Chapter 8: Advanced Topics
The data passes on to a Parallelism operator, which is the Repartition Streams operator.
This operator is responsible for balancing the streams, trying to make sure that a roughly
equal amount of work is performed by each stream. As you can see in Figure 8.7, the rows
that were retrieved by the streams running in the Clustered Index Scan were retrieved
at different rates by the various streams. This gets rebalanced by the Repartition Streams
operator. It's also possible for this operator to reduce the number of rows, but that's
not the case here. You can see this rebalancing in the operator ToolTip on display in
Figure 8.8.
Look at the top of Figure 8.8, at the Actual Number of Rows property. Note how the
threads have been better balanced out with a roughly even distribution of rows among
them. The idea here is to make each thread do an equal amount of work so that no thread
is waiting too long on the others. You begin to see where the added overhead of parallelism
comes to play. You have to be dealing with quite large volumes of data for this to be
beneficial. The amount of work that it takes to split the streams, balance the streams, and
then bring them all back together can be quite costly. If only a few rows are involved, then
that cost can far outweigh the benefits of putting multiple CPUs to work on the query.
The next several operators after the Repartition Streams operator in Figure 8.5 are
quite common and we covered them in previous chapters. Each one, in this instance,
is operating as a Parallel operator as designated by the yellow icon. Each of these
operators shows how it's processing the data through a set of streams in its output l
isting. The next interesting operator is the final one, right before the Select operator,
the Parallelism Gather Streams operator. This operator is self-explanatory. It pulls the
streams back together in order to present the data gathered by the process as a single
dataset to the query or operator calling it. The output from this operator is now a single
thread of information.
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