Enhance dynamic source clause to support only metadata filters

Signed-off-by: Vamsi Manohar <reddyvam@amazon.com>

Author: vamsimanohar

ppl/src/main/antlr/OpenSearchPPLParser.g4

@@ -526,6 +526,7 @@ tableSourceClause
 
 dynamicSourceClause
    : LT_SQR_PRTHS sourceReferences (COMMA sourceFilterArgs)? RT_SQR_PRTHS
+   | LT_SQR_PRTHS sourceFilterArgs RT_SQR_PRTHS
    ;
 
 sourceReferences

ppl/src/test/java/org/opensearch/sql/ppl/antlr/PPLSyntaxParserTest.java

@@ -5,11 +5,7 @@
 
 package org.opensearch.sql.ppl.antlr;
 
-import static org.junit.Assert.assertEquals;
-import static org.junit.Assert.assertNotEquals;
-import static org.junit.Assert.assertNotNull;
-import static org.junit.Assert.assertThrows;
-import static org.junit.Assert.assertTrue;
+import static org.junit.Assert.*;
 
 import java.util.List;
 import org.antlr.v4.runtime.CommonTokenStream;
@@ -199,11 +195,49 @@ public void testDynamicSourceWithSingleSource() {
   }
 
   @Test
-  public void testDynamicSourceRequiresAtLeastOneSource() {
-    // The grammar requires at least one source reference before optional filter args
-    // This test documents that behavior
-    exceptionRule.expect(RuntimeException.class);
-    new PPLSyntaxParser().parse("source=[fieldIndex=\"httpStatus\", region=\"us-west-2\"]");
+  public void testDynamicSourceWithoutSource() {
+    String query = "source=[fieldIndex=\"httpStatus\", region=\"us-west-2\"]";
+    OpenSearchPPLLexer lexer = new OpenSearchPPLLexer(new CaseInsensitiveCharStream(query));
+    OpenSearchPPLParser parser = new OpenSearchPPLParser(new CommonTokenStream(lexer));
+
+    OpenSearchPPLParser.RootContext root = parser.root();
+    OpenSearchPPLParser.SearchFromContext searchFrom =
+        (OpenSearchPPLParser.SearchFromContext)
+            root.pplStatement().queryStatement().pplCommands().searchCommand();
+    OpenSearchPPLParser.DynamicSourceClauseContext dynamicSource =
+        searchFrom.fromClause().dynamicSourceClause();
+    assertNull(dynamicSource.sourceReferences());
+    assertEquals(
+        "Should have 2 filter arg", 2, dynamicSource.sourceFilterArgs().sourceFilterArg().size());
+    assertEquals(
+        "Filter arg text",
+        "fieldIndex=\"httpStatus\",region=\"us-west-2\"",
+        dynamicSource.sourceFilterArgs().getText());
+  }
+
+  @Test
+  public void testDynamicSourceWithAllSources() {
+    String query = "source=[`*`, fieldIndex=\"httpStatus\", region=\"us-west-2\"]";
+    OpenSearchPPLLexer lexer = new OpenSearchPPLLexer(new CaseInsensitiveCharStream(query));
+    OpenSearchPPLParser parser = new OpenSearchPPLParser(new CommonTokenStream(lexer));
+
+    OpenSearchPPLParser.RootContext root = parser.root();
+    OpenSearchPPLParser.SearchFromContext searchFrom =
+        (OpenSearchPPLParser.SearchFromContext)
+            root.pplStatement().queryStatement().pplCommands().searchCommand();
+    OpenSearchPPLParser.DynamicSourceClauseContext dynamicSource =
+        searchFrom.fromClause().dynamicSourceClause();
+    assertEquals(
+        "Should have 1 source reference",
+        1,
+        dynamicSource.sourceReferences().sourceReference().size());
+    assertEquals("Source reference text", "`*`", dynamicSource.sourceReferences().getText());
+    assertEquals(
+        "Should have 2 filter arg", 2, dynamicSource.sourceFilterArgs().sourceFilterArg().size());
+    assertEquals(
+        "Filter arg text",
+        "fieldIndex=\"httpStatus\",region=\"us-west-2\"",
+        dynamicSource.sourceFilterArgs().getText());
   }
 
   @Test