Fix PPL eval command string concatenation with + operator

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

@@ -689,7 +689,17 @@ void populate() {
       registerOperator(AND, SqlStdOperatorTable.AND);
       registerOperator(OR, SqlStdOperatorTable.OR);
       registerOperator(NOT, SqlStdOperatorTable.NOT);
-      registerOperator(ADD, SqlStdOperatorTable.PLUS);
+
+      // Register ADD (+ symbol) for numeric addition
+      register(
+          ADD,
+          (RexBuilder builder, RexNode... args) -> builder.makeCall(SqlStdOperatorTable.PLUS, args),
+          new PPLTypeChecker.PPLFamilyTypeChecker(SqlTypeFamily.NUMERIC, SqlTypeFamily.NUMERIC));
+
+      // Register ADD (+ symbol) for string concatenation
+      registerOperator(ADD, SqlStdOperatorTable.CONCAT);
+
+      // Register ADDFUNCTION for numeric addition only
       registerOperator(ADDFUNCTION, SqlStdOperatorTable.PLUS);
       registerOperator(SUBTRACT, SqlStdOperatorTable.MINUS);
       registerOperator(SUBTRACTFUNCTION, SqlStdOperatorTable.MINUS);

ppl/src/test/java/org/opensearch/sql/ppl/calcite/CalcitePPLEvalTest.java

@@ -354,4 +354,102 @@ public void testDependedLateralEval() {
             + "GROUP BY `DEPTNO`";
     verifyPPLToSparkSQL(root, expectedSparkSql);
   }
+
+  @Test
+  public void testEvalStringConcatenationWithPlus() {
+    String ppl =
+        "source=EMP | eval full_name = ENAME + ' ' + JOB | fields EMPNO, ENAME, JOB, full_name |"
+            + " head 3";
+    RelNode root = getRelNode(ppl);
+    String expectedLogical =
+        "LogicalSort(fetch=[3])\n"
+            + "  LogicalProject(EMPNO=[$0], ENAME=[$1], JOB=[$2], full_name=[||(||($1, ' '),"
+            + " $2)])\n"
+            + "    LogicalTableScan(table=[[scott, EMP]])\n";
+    verifyLogical(root, expectedLogical);
+
+    String expectedSparkSql =
+        "SELECT `EMPNO`, `ENAME`, `JOB`, `ENAME` || ' ' || `JOB` `full_name`\n"
+            + "FROM `scott`.`EMP`\n"
+            + "LIMIT 3";
+    verifyPPLToSparkSQL(root, expectedSparkSql);
+  }
+
+  @Test
+  public void testEvalStringConcatenationWithNullableField() {
+    // Test with COMM field which naturally contains nulls in the EMP table
+    // When COMM is null, concatenation should result in null per SQL standard
+    String ppl =
+        "source=EMP | eval "
+            + "comm_desc = 'Commission: ' + CAST(COMM AS STRING) "
+            + "| fields EMPNO, ENAME, COMM, comm_desc | head 5";
+    RelNode root = getRelNode(ppl);
+
+    String expectedLogical =
+        "LogicalSort(fetch=[5])\n"
+            + "  LogicalProject(EMPNO=[$0], ENAME=[$1], COMM=[$6], "
+            + "comm_desc=[||('Commission: ':VARCHAR, NUMBER_TO_STRING($6))])\n"
+            + "    LogicalTableScan(table=[[scott, EMP]])\n";
+    verifyLogical(root, expectedLogical);
+
+    String expectedSparkSql =
+        "SELECT `EMPNO`, `ENAME`, `COMM`, "
+            + "'Commission: ' || `NUMBER_TO_STRING`(`COMM`) `comm_desc`\n"
+            + "FROM `scott`.`EMP`\n"
+            + "LIMIT 5";
+    verifyPPLToSparkSQL(root, expectedSparkSql);
+  }
+
+  @Test
+  public void testEvalStringConcatenationChained() {
+    // Test chained concatenation
+    String ppl =
+        "source=EMP | eval "
+            + "description = ENAME + ' - ' + JOB + ' (Dept: ' + CAST(DEPTNO AS STRING) + ')' "
+            + "| where EMPNO IN (7369, 7499) | fields EMPNO, description";
+    RelNode root = getRelNode(ppl);
+
+    String expectedLogical =
+        "LogicalProject(EMPNO=[$0], description=[$8])\n"
+            + "  LogicalFilter(condition=[SEARCH($0, Sarg[7369, 7499])])\n"
+            + "    LogicalProject(EMPNO=[$0], ENAME=[$1], JOB=[$2], MGR=[$3], HIREDATE=[$4],"
+            + " SAL=[$5], COMM=[$6], DEPTNO=[$7], description=[||(||(||(||(||($1, ' - ':VARCHAR),"
+            + " $2), ' (Dept: ':VARCHAR), SAFE_CAST($7)), ')')])\n"
+            + "      LogicalTableScan(table=[[scott, EMP]])\n";
+    verifyLogical(root, expectedLogical);
+
+    String expectedSparkSql =
+        "SELECT `EMPNO`, `description`\n"
+            + "FROM (SELECT `EMPNO`, `ENAME`, `JOB`, `MGR`, `HIREDATE`, `SAL`, `COMM`, `DEPTNO`,"
+            + " `ENAME` || ' - ' || `JOB` || ' (Dept: ' || SAFE_CAST(`DEPTNO` AS STRING) || ')'"
+            + " `description`\n"
+            + "FROM `scott`.`EMP`) `t`\n"
+            + "WHERE `EMPNO` IN (7369, 7499)";
+    verifyPPLToSparkSQL(root, expectedSparkSql);
+  }
+
+  @Test
+  public void testEvalStringConcatenationMultipleExpressions() {
+    // Test multiple concatenation expressions in a single eval command
+    String ppl =
+        "source=EMP | eval "
+            + "name_job = ENAME + ' - ' + JOB, "
+            + "name_dept = ENAME + ' (Dept ' + CAST(DEPTNO AS STRING) + ')' "
+            + "| fields EMPNO, name_job, name_dept | head 2";
+    RelNode root = getRelNode(ppl);
+
+    String expectedLogical =
+        "LogicalSort(fetch=[2])\n"
+            + "  LogicalProject(EMPNO=[$0], name_job=[||(||($1, ' - ':VARCHAR), $2)], "
+            + "name_dept=[||(||(||($1, ' (Dept ':VARCHAR), SAFE_CAST($7)), ')')])\n"
+            + "    LogicalTableScan(table=[[scott, EMP]])\n";
+    verifyLogical(root, expectedLogical);
+
+    String expectedSparkSql =
+        "SELECT `EMPNO`, `ENAME` || ' - ' || `JOB` `name_job`, "
+            + "`ENAME` || ' (Dept ' || SAFE_CAST(`DEPTNO` AS STRING) || ')' `name_dept`\n"
+            + "FROM `scott`.`EMP`\n"
+            + "LIMIT 2";
+    verifyPPLToSparkSQL(root, expectedSparkSql);
+  }
 }