12. A computer-implemented process, comprising: using a computer to perform process actions for: receiving a crowd-sourced query formatted as a multi-layer structure, the crowd-sourced query comprising two or more branches, each branch associated with a corresponding set of worker qualifications; applying one or more optimizations to the crowd-sourced query to construct a reformulated query, said optimizations serving to reduce a complexity of the crowd-sourced query; reducing one or more of expected completion time and expected cost associated with the reformulated query by matching the reformulated query to an optimized execution process; presenting the reformulated query and matching optimized execution process as an optimized version of the crowd-sourced query for execution via a computer-based crowd-sourcing backend; during execution of the optimized version of the crowd-sourced query, improving query efficiency by automatically changing the matching optimized execution process to a different optimized execution processes and dynamically changing the reformulated query to correspond to the automatically changed execution process in response to collected runtime statistics relating to execution of the optimized version of the crowd-sourced query; and further during execution, presenting the changed reformulated query and changed matching optimized execution process for continued execution via the computer-based crowd-sourcing backend.
15. The computer-implemented process of claim 12 wherein a common subexpression elimination optimization comprises: identifying common subexpressions in the input query; and merging the identified common subexpressions so that expressions in the input query reference merged subexpressions in place of the common subexpressions.