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Re-order union operands descending by their estimated per-host memory, s.t. parent nodes can gauge the peak memory consumption of a MergeNode after opening it during execution (a MergeNode opens its first operand in Open()). Scan nodes are always ordered last because they can dynamically scale down their memory usage, whereas many other nodes cannot (e.g., joins, aggregations). One goal is to decrease the likelihood of a SortNode parent claiming too much memory in its Open(), possibly causing the mem limit to be hit when subsequent union operands are executed. Change-Id: Ia51caaffd55305ea3dbd2146cd55acc7da67f382 Reviewed-on: http://gerrit.ent.cloudera.com:8080/3146 Reviewed-by: Alex Behm <alex.behm@cloudera.com> Tested-by: Alex Behm <alex.behm@cloudera.com> Reviewed-on: http://gerrit.ent.cloudera.com:8080/3213 Tested-by: jenkins