Building blocksBuilding blocks mk Tue, 03/30/2010 - 12:07
Optimizer Building Blocks
Some instructions are independent of the execution context. In particular, expressions over side-effect free functions with constant parameters could be evaluated before the program block is considered further.
A major task for an optimizer is to select instruction (sequences) which can and should be replaced with cheaper ones. The cost model underlying this decision depends on the processing stage and the overall objective. For example, based on a symbolic analysis their may exist better implementations within the interpreter to perform the job (e.g. hashjoin vs mergejoin). Alternative, expensive intermediates may be cached for later use.
Plan enumeration is often implemented as a Memo structure, which designates alternative sub-plans based on a cost metric. Perhaps we can combine these memo structures into a large table for all possible combinations encountered for a user.
The MAL language does not imply a specific optimizer to be used. Its programs are merely a sequence of specifications, which is interpreted by an engine specific to a given task. Activation of the engine is controlled by a scenario, which currently includes two hooks for optimization; a strategic optimizer and a tactical optimizer. Both engines take a MAL program and produce a (new/modified) MAL program for execution by the lower layers.
MAL programs end-up in the symbol table linked to a user session. An optimizer has the freedom to change the code, provided it is known that the plan derived is invariant to changes in the environment. All others lead to alternative plans, which should be collected as a trail of MAL program blocks. These trails can be inspected for a posteriori analysis, at least in terms of some statistics on the properties of the MAL program structures automatically. Alternatively, the trail may be pruned and re-optimized when appropriate from changes in the environment.
The rule applied for all optimizers is to not-return before checking the state of the MAL program, and to assure the dataflow and variable scopes are properly set. It costs some performance, but the difficulties that arise from optimizer interference are very hard to debug. One of the easiest pitfalls is to derive an optimized version of a MAL function while it is already referenced by or when polymorphic typechecking is required afterwards.
Building Your Own Optimizer
Implementation of your own MAL-MAL optimizer can best be started from refinement of one of the examples included in the code base. Beware that only those used in the critical path of SQL execution are thorouhly tested. The others are developed up to the point that the concept and approach can be demonstrated.
The general structure of most optimizers is to actively copy a MAL block into a new program structure. At each step we determine the action taken, e.g. replace the instruction or inject instructions to achieve the desired goal.
A tally on major events should be retained, because it gives valuable insight in the effectiveness of your optimizer. The effects of all optimizers is collected in a system catalog.
Each optimizer ends with a strong defense line,
optimizerCheck() It performs a complete type and data flow analysis before returning. Moreover, if you are in debug mode, it will keep a copy of the plan produced for inspection. Studying the differences between optimizer steps provide valuable information to improve your code.
The functionality of the optimizer should be clearly delineated. The guiding policy is that it is always safe to not apply an optimizer step. This helps to keep the optimizers as independent as possible.
It really helps if you start with a few tiny examples to test your optimizer. They should be added to the Tests directory and administered in Tests/All.
Breaking up the optimizer into different components and grouping them together in arbitrary sequences calls for careful programming.
One of the major hurdles is to test interference of the optimizer. The test set is a good starting point, but does not garantee that all cases have been covered.
In principle, any subset of optimizers should work flawlessly. With a few tens of optimizers this amounts to potential millions of runs. Adherence to a partial order reduces the problem, but still is likely to be too resource consumptive to test continously.