C++: Fix HashCons library performance #1951
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As the linearity of the disjuncts is different, this enables us to pick better join orders for each disjunct separately.
The user knows that an expression functionally determines its hashCons value, and that an expression functionally determines its number of children, but this is not provable from the definitions, and so not usable by the optimiser. By storing the result of those known-functional calls in a variable, rather than repeating the call, we enable better join orders.
pavgust
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Empirically, computing hashcons values for certain snapshots (especially where many array aggregate literals were used) was very slow. This PR proposes a bunch of changes (intended to be behaviour-preserving) which allow the optimiser to do a better job.