Allow equal expression as a function argument (opensearch-project#4001)

* Remove named function arg from functions other than table functions

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

* Test eval if function with equal as condition

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

---------

Signed-off-by: Yuanchun Shen <yuanchu@amazon.com>
(cherry picked from commit 9f2b19a)

Author: yuancu

integ-test/src/test/java/org/opensearch/sql/calcite/remote/CalcitePPLConditionBuiltinFunctionIT.java

@@ -194,6 +194,19 @@ public void testIfWithLike() throws IOException {
         actual, rows(0.0, "John"), rows(0.0, "Jane"), rows(0.0, "Jake"), rows(1.0, "Hello"));
   }
 
+  @Test
+  public void testIfWithEquals() throws IOException {
+    JSONObject actual =
+        executeQuery(
+            String.format(
+                "source=%s | eval jake = if(name='Jake', 1, 0) | fields name, jake",
+                TEST_INDEX_STATE_COUNTRY));
+
+    verifySchema(actual, schema("name", "string"), schema("jake", "int"));
+
+    verifyDataRows(actual, rows("Jake", 1), rows("Hello", 0), rows("John", 0), rows("Jane", 0));
+  }
+
   @Test
   public void testIsPresent() throws IOException {
     JSONObject actual =

ppl/src/main/antlr/OpenSearchPPLParser.g4

@@ -522,7 +522,7 @@ tableSource
    ;
 
 tableFunction
-   : qualifiedName LT_PRTHS functionArgs RT_PRTHS
+   : qualifiedName LT_PRTHS namedFunctionArgs RT_PRTHS
    ;
 
 // fields
@@ -597,10 +597,17 @@ functionArgs
    : (functionArg (COMMA functionArg)*)?
    ;
 
+namedFunctionArgs
+   : (namedFunctionArg (COMMA namedFunctionArg)*)?
+   ;
+
 functionArg
-   : (ident EQUAL)? functionArgExpression
+   : functionArgExpression
    ;
 
+namedFunctionArg
+   : (ident EQUAL)? functionArgExpression
+   ;
 
 functionArgExpression
    : lambda

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

@@ -550,8 +550,8 @@ public UnresolvedPlan visitTableSourceClause(TableSourceClauseContext ctx) {
   @Override
   public UnresolvedPlan visitTableFunction(TableFunctionContext ctx) {
     ImmutableList.Builder<UnresolvedExpression> builder = ImmutableList.builder();
-    ctx.functionArgs()
-        .functionArg()
+    ctx.namedFunctionArgs()
+        .namedFunctionArg()
         .forEach(
             arg -> {
               String argName = (arg.ident() != null) ? arg.ident().getText() : null;