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Does anyone know what happens after the query plan is generated in most database? I'm assuming individual step, like index scan, hashjoin are coded already and the plan steps are iterated and respective methods are called? So the execution steps are already compiled but the step traversal is kind of interpreted. With Peloton LLVM engine everything is merged together in a single sequence of machine code?

How much advantage does this give you? Is there really so many steps in the execution plan (the visible steps are usually < 50) but what about the internal actual compiled steps? Unless this is allowing merging and further simplification steps identifying redundant operation that gets trimmed of not sure where 100x performance improvement comes from.

Though I remember seeing the scala based in-memory query engine that was sort of doing simplification of the actual steps and doing very well in benchmark, maybe this is similar.




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