Changes based on upstream SQL PR comments

Signed-off-by: Margarit Hakobyan <margarith@bitquilltech.com>

Author: margarit-h

ppl/src/main/java/org/opensearch/sql/ppl/parser/AstExpressionBuilder.java

@@ -9,6 +9,7 @@
 import static org.opensearch.sql.ast.dsl.AstDSL.qualifiedName;
 import static org.opensearch.sql.expression.function.BuiltinFunctionName.IS_NOT_NULL;
 import static org.opensearch.sql.expression.function.BuiltinFunctionName.IS_NULL;
+import static org.opensearch.sql.expression.function.BuiltinFunctionName.POSITION;
 import static org.opensearch.sql.ppl.antlr.parser.OpenSearchPPLParser.BinaryArithmeticContext;
 import static org.opensearch.sql.ppl.antlr.parser.OpenSearchPPLParser.BooleanFunctionCallContext;
 import static org.opensearch.sql.ppl.antlr.parser.OpenSearchPPLParser.BooleanLiteralContext;
@@ -69,7 +70,6 @@
 import org.opensearch.sql.ast.expression.Literal;
 import org.opensearch.sql.ast.expression.Not;
 import org.opensearch.sql.ast.expression.Or;
-import org.opensearch.sql.ast.expression.PositionFunction;
 import org.opensearch.sql.ast.expression.QualifiedName;
 import org.opensearch.sql.ast.expression.RelevanceFieldList;
 import org.opensearch.sql.ast.expression.Span;
@@ -294,8 +294,10 @@ public UnresolvedExpression visitTableSource(TableSourceContext ctx) {
   @Override
   public UnresolvedExpression visitPositionFunction(
           OpenSearchPPLParser.PositionFunctionContext ctx) {
-    return new PositionFunction(visitFunctionArg(ctx.functionArg(0)),
-            visitFunctionArg(ctx.functionArg(1)));
+    return new Function(
+            POSITION.getName().getFunctionName(),
+            Arrays.asList(visitFunctionArg(ctx.functionArg(0)),
+                    visitFunctionArg(ctx.functionArg(1))));
   }
 
   /**

ppl/src/test/java/org/opensearch/sql/ppl/parser/AstExpressionBuilderTest.java

@@ -35,7 +35,6 @@
 import static org.opensearch.sql.ast.dsl.AstDSL.not;
 import static org.opensearch.sql.ast.dsl.AstDSL.nullLiteral;
 import static org.opensearch.sql.ast.dsl.AstDSL.or;
-import static org.opensearch.sql.ast.dsl.AstDSL.position;
 import static org.opensearch.sql.ast.dsl.AstDSL.projectWithArg;
 import static org.opensearch.sql.ast.dsl.AstDSL.qualifiedName;
 import static org.opensearch.sql.ast.dsl.AstDSL.relation;
@@ -184,12 +183,13 @@ public void testEvalFunctionExprNoArgs() {
 
   @Test
   public void testPositionFunctionExpr() {
-    assertEqual("source=t | eval f=position(fieldA IN fieldB)",
+    assertEqual("source=t | eval f=position('substr' IN 'str')",
         eval(
             relation("t"),
             let(
                 field("f"),
-                position(field("fieldA"), field("fieldB"))
+                function("position",
+                        stringLiteral("substr"), stringLiteral("str"))
             )
         ));
   }