private RexNode useStructIfNeeded(List<? extends RexNode> columns) {
      // Create STRUCT clause
      if (columns.size() == 1) {
        return columns.get(0);
      } else {
        return rexBuilder.makeCall(SqlStdOperatorTable.ROW, columns);
      }
    }
A group of Constraints.

Examples:
 (a=1 && b=1)
 (a=1)

Note: any rexNode is accepted as constraint; but it might be keyed with the empty key;
which means it can't be parsed as a constraint for some reason; but for completeness...
ql/src/java/org/apache/hadoop/hive/ql/optimizer/calcite/rules/HivePointLookupOptimizerRule.java