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Which issue does this PR close?
Related to XXX
Running benchmarks to see if this helps
Rationale for this change
I spent time profiling the sql_planner benchmarks as part of #17419
and the amount of time spent planning unions was surprising.
What changes are included in this PR?
Are these changes tested?
I also felt bad we haven't really improved planning speed in a while!
I couldn't help myself and implemented a simple optimization to flatten unions
as soon as possible in the planner rather than waiting maybe for the optimizers to do it.
The basic idea is quite simple: if the input is a union, just add its inputs to the list of inputs
Are there any user-facing changes?
not sure