Correct type checkers for mvappend; fix regexp lookup; correct isempty and isblank (1738/2018)

Signed-off-by: Yuanchun Shen <yuanchu@amazon.com>

Author: yuancu

core/src/main/java/org/opensearch/sql/expression/function/CollectionUDF/MVAppendFunctionImpl.java

@@ -51,7 +51,7 @@ public SqlReturnTypeInference getReturnTypeInference() {
 
   @Override
   public UDFOperandMetadata getOperandMetadata() {
-    return UDFOperandMetadata.wrap(OperandTypes.ARRAY);
+    return UDFOperandMetadata.wrap(OperandTypes.VARIADIC);
   }
 
   private static RelDataType determineElementType(

core/src/main/java/org/opensearch/sql/expression/function/PPLBuiltinOperators.java

@@ -30,6 +30,7 @@
 import org.apache.calcite.sql.SqlKind;
 import org.apache.calcite.sql.SqlNode;
 import org.apache.calcite.sql.SqlOperator;
+import org.apache.calcite.sql.fun.SqlLibraryOperators;
 import org.apache.calcite.sql.fun.SqlStdOperatorTable;
 import org.apache.calcite.sql.type.OperandTypes;
 import org.apache.calcite.sql.type.ReturnTypes;
@@ -504,6 +505,11 @@ public SqlNode rewriteCall(SqlValidator validator, SqlCall call) {
           return call;
         }
       };
+  // SPARK dialect is not included in lookup table to resolve overrides issues (e.g. reverse
+  // function won't work if spark is included because there are multiple overrides of reverse, and
+  // it will choose none of them in the end.) Therefore, SPARK functions used are explicitly
+  // declared here for lookup.
+  public static final SqlFunction REGEXP = SqlLibraryOperators.REGEXP;
 
   /**
    * Returns the PPL specific operator table, creating it if necessary.

core/src/main/java/org/opensearch/sql/expression/function/PPLFuncImpTable.java

@@ -832,7 +832,7 @@ void populate() {
       registerOperator(COALESCE, PPLBuiltinOperators.ENHANCED_COALESCE);
 
       // Register library operator
-      registerOperator(REGEXP, SqlLibraryOperators.REGEXP);
+      registerOperator(REGEXP, PPLBuiltinOperators.REGEXP);
       registerOperator(REGEXP_MATCH, SqlLibraryOperators.REGEXP_CONTAINS);
       registerOperator(CONCAT, SqlLibraryOperators.CONCAT_FUNCTION);
       registerOperator(CONCAT_WS, SqlLibraryOperators.CONCAT_WS);
@@ -1001,7 +1001,7 @@ void populate() {
                 return builder.makeCall(
                     SqlStdOperatorTable.CASE, isEmptyDelimiter, splitChars, normalSplit);
               },
-          PPLTypeChecker.family(SqlTypeFamily.CHARACTER, SqlTypeFamily.CHARACTER));
+          OperandTypes.family(SqlTypeFamily.CHARACTER, SqlTypeFamily.CHARACTER));
 
       // Register MVINDEX to use Calcite's ITEM/ARRAY_SLICE with index normalization
       register(
@@ -1192,7 +1192,7 @@ void populate() {
                   builder.makeCall(
                       SqlStdOperatorTable.OR,
                       builder.makeCall(SqlStdOperatorTable.IS_NULL, arg),
-                      builder.makeCall(SqlStdOperatorTable.IS_EMPTY, arg)),
+                      builder.makeCall(SqlStdOperatorTable.EQUALS, arg, builder.makeLiteral(""))),
           OperandTypes.family(SqlTypeFamily.ANY));
       register(
           IS_BLANK,
@@ -1202,12 +1202,13 @@ void populate() {
                       SqlStdOperatorTable.OR,
                       builder.makeCall(SqlStdOperatorTable.IS_NULL, arg),
                       builder.makeCall(
-                          SqlStdOperatorTable.IS_EMPTY,
+                          SqlStdOperatorTable.EQUALS,
                           builder.makeCall(
                               SqlStdOperatorTable.TRIM,
                               builder.makeFlag(Flag.BOTH),
                               builder.makeLiteral(" "),
-                              arg))),
+                              arg),
+                          builder.makeLiteral(""))),
           OperandTypes.family(SqlTypeFamily.ANY));
       register(
           ILIKE,