72cfc17aef
This patch causes unknown-type Consts to be coerced to the resolved output type of the set operation at parse time. Formerly such Consts were left alone until late in the planning stage. The disadvantage of that approach is that it disables some optimizations, because the planner sees the set-op leaf query as having different output column types than the overall set-op. We saw an example of that in a recent performance gripe from Claudio Freire. Fixing such a Const requires scribbling on the leaf query in transformSetOperationTree, but that should be all right since if the leaf query's semantics depended on that output column, it would already have resolved the unknown to something else. Most of the bulk of this patch is a simple adjustment of transformSetOperationTree's API so that upper levels can get at the TargetEntry containing a Const to be replaced: it now returns a list of TargetEntries, instead of just the bare expressions.
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