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A recent change, IMPALA-5016, added an expr rewrite rule to simplfy coalesce(). This rule eliminates the coalesce() when its first parameter (that isn't constant null) is a SlotRef pointing to a SlotDescriptor that is non-nullable (for example because it is from a non-nullable Kudu column or because it is from an HDFS partition column with no null partitions), under the assumption that the SlotRef could never have a null value. This assumption is violated when the SlotRef is the output of an outer join, leading to incorrect results being returned. The problem is that the nullability of a SlotDescriptor (which determines whether there is a null indicator bit in the tuple for that slot) is a slightly different property than the nullability of a SlotRef pointing to that SlotDescriptor (since the SlotRef can still be NULL if the entire tuple is NULL). This patch removes the portion of the rewrite rule that considers the nullability of the SlotDescriptor. This means that we're missing out on some optimizations opportunities and we should revisit this in a way that works with outer joins (IMPALA-5753) Testing: - Updated FE tests. - Added regression tests to exprs.test Change-Id: I1ca6df949f9d416ab207016236dbcb5886295337 Reviewed-on: http://gerrit.cloudera.org:8080/7567 Reviewed-by: Matthew Jacobs <mj@cloudera.com> Reviewed-by: Thomas Tauber-Marshall <tmarshall@cloudera.com> Tested-by: Impala Public Jenkins