Cost-Based Optimization of Integration Flows - Datenbanken ...

2.2 Adaptive Query Processing
Adaptive Query Processing
System
Aspect
Plan-Based
Adaptation
Integration-Flow
Adaptation
Continuous-Query
Adaptation
Tuple
Routing
Inter-
Query
Late
Binding
Inter-
Operator
Intra-
Operator
Inter-
Instance
Intra-
Operator
Time
Aspect
Proactive Reactive Proactive Reactive
synchronous
asynchronous
Figure 2.7: Classification of Adaptive Query Processing
natural blocking operators such as Sort and Group By in order to evaluate statistics and
to re-optimize the remaining query plan during runtime. Third, inter-operator adaptation
can re-optimize a plan at any possible point between operators by inserting artificial
materialization points (Checkpoint operators). Fourth, the fine-grained intra-operator
adaptation allows for re-optimization during the runtime of a single operator by data
partitioning across concurrent subplans (and thus, concurrent instances of an operator).
From a time perspective, we additionally use the notation of reactive (react to suboptimalities)
and proactive (create switchable plans before execution) re-optimization [BBD05a].
Furthermore, we distinguish between synchronous and asynchronous re-optimization. For
synchronous re-optimization (late-binding and inter-operator), execution is blocked because
the current execution process requires the optimization result (in terms of the optimal
remaining subplan), while for asynchronous optimization (intra-operator), execution
is not blocked and plans are exchanged or merged at the next possible point.
Putting it all together, in the context of plan-based adaptation in DBMS, there exist
approaches for inter-query optimization, late-binding, inter-operator and intra-operator reoptimization.
Here, only intra-operator re-optimization can be executed asynchronously,
while all other approaches require synchronous re-optimization because the remaining plan
is optimized. Furthermore, the integration flow adaptation is classified as inter-instance
adaptation, which is similar to inter-query adaptation. In the opposite, continuous query
adaptation refers to intra-operator adaptation because theoretically, an infinite stream
of tuples is processed (and thus, we cannot block execution until all tuples are read by
any operator). Finally, the tuple routing strategies, as an alternative for continuous query
adaptation, represent by themselves the most fine-grained time scheme for re-optimization,
where the optimization decisions in the sense of routing paths over operators can be made
for each individual tuple. In the following, we use this general classification to put existing
techniques into the context of adaptive query processing.
2.2.2 Plan-Based Adaptation
For reactive, inter-operator re-optimization, the traditional optimizer is used to create a
plan. During execution, intermediate results are materialized, and if estimation errors are
detected, the current plan is re-optimized. In ReOpt [KD98], the optimizer is invoked
if statistics differ significantly, regardless of whether or not this will produce another
19