join: simplify logic passed to Deferred::work
#392
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Previously, the join code had to wrap the `result` closure into another closure to invoke `Deferred::work`, to deal with the fact that matches from the `todo1` list had their value and diff fields swapped. Instead of having a closure to unswap the fields, we can simply pass them to the `Deferred` constructor in the correct order instead. That is, the `Deferred` constructor should always receive the cursor/storage from input 1 first and from input 2 second. If we make this change, then a) no swapping closure is necessary, and b) the input values match the types (`C1`, `C2`) of the `Deferred` struct.
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Previously, the join code had to wrap the
resultclosure into another closure to invokeDeferred::work, to deal with the fact that matches from thetodo1list had their value and diff fields swapped.Instead of having a closure to unswap the fields, we can simply pass them to the
Deferredconstructor in the correct order instead. That is, theDeferredconstructor should always receive the cursor/storage from input 1 first and from input 2 second. If we make this change, then a) no swapping closure is necessary, and b) the input values match the types (C1,C2) of theDeferredstruct.This PR is a backport of MaterializeInc/materialize#19667, to avoid that the implementations diverge unnecessarily. In
mz_join_core, the readability improvement from this change is more pronounced than in DD.