add Test coverage

Signed-off-by: Kai Huang <ahkcs@amazon.com>

Author: ahkcs

docs/user/interfaces/endpoint.rst

@@ -275,6 +275,8 @@ Description
 
 PPL also supports the ``fetch_size`` parameter, but with different semantics from SQL. In PPL, ``fetch_size`` limits the number of rows returned in a single, complete response. **PPL does not support cursor-based pagination** — no cursor is returned and there is no way to fetch additional pages. The value of ``fetch_size`` should be greater than ``0``. In absence of ``fetch_size`` or a value of ``0``, it will use the system default behavior (no limit). The effective upper bound is governed by the ``plugins.query.size_limit`` cluster setting (defaults to ``index.max_result_window``, which is 10000).
 
+``fetch_size`` can be specified either as a URL parameter or in the JSON request body. If both are provided, the JSON body value takes precedence.
+
 +--------------------+-------------------------------------+------------------------------------+
 | Aspect             | SQL ``fetch_size``                  | PPL ``fetch_size``                 |
 +====================+=====================================+====================================+
@@ -285,7 +287,7 @@ PPL also supports the ``fetch_size`` parameter, but with different semantics fro
 | Can fetch more?    | Yes (with cursor)                   | No (single response)               |
 +--------------------+-------------------------------------+------------------------------------+
 
-Example
+Example 1: JSON body
 -------
 
 PPL query::
@@ -295,6 +297,15 @@ PPL query::
 	  "query" : "source = accounts | fields firstname, lastname | where age > 20"
 	}'
 
+Example 2: URL parameter
+-------
+
+PPL query::
+
+	>> curl -H 'Content-Type: application/json' -X POST localhost:9200/_plugins/_ppl?fetch_size=5 -d '{
+	  "query" : "source = accounts | fields firstname, lastname | where age > 20"
+	}'
+
 Result set::
 
     {

integ-test/src/test/java/org/opensearch/sql/ppl/FetchSizeIT.java

@@ -123,6 +123,34 @@ public void testFetchSizeLargerThanDataset() throws IOException {
     assertEquals(7, dataRows.length());
   }
 
+  @Test
+  public void testFetchSizeAtSystemLimit() throws IOException {
+    // fetch_size at the default system limit (10000) should work without error
+    JSONObject result =
+        executeQueryWithFetchSize(String.format("source=%s", TEST_INDEX_BANK), 10000);
+    JSONArray dataRows = result.getJSONArray("datarows");
+    // Bank index has 7 documents, so we get all of them
+    assertEquals(7, dataRows.length());
+  }
+
+  @Test
+  public void testFetchSizeExceedingSystemLimitIsCapped() throws IOException {
+    // fetch_size > system limit (10000) is accepted but capped by LogicalSystemLimit
+    JSONObject result =
+        executeQueryWithFetchSize(String.format("source=%s", TEST_INDEX_BANK), 10001);
+    JSONArray dataRows = result.getJSONArray("datarows");
+    // Bank index has 7 documents, result is capped by system limit but dataset is smaller
+    assertEquals(7, dataRows.length());
+  }
+
+  @Test
+  public void testNegativeFetchSizeReturnsAllResults() throws IOException {
+    // Negative fetch_size is treated as "no limit" (same as 0)
+    JSONObject result = executeQueryWithFetchSize(String.format("source=%s", TEST_INDEX_BANK), -1);
+    JSONArray dataRows = result.getJSONArray("datarows");
+    assertEquals(7, dataRows.length());
+  }
+
   @Test
   public void testFetchSizeOne() throws IOException {
     JSONObject result =

plugin/src/main/java/org/opensearch/sql/plugin/request/PPLQueryRequestFactory.java

@@ -88,9 +88,14 @@ private static PPLQueryRequest parsePPLRequestFromPayload(RestRequest restReques
       // Support fetch_size as a URL parameter if not already in the JSON body
       if (!jsonContent.has(QUERY_PARAMS_FETCH_SIZE)
           && restRequest.params().containsKey(QUERY_PARAMS_FETCH_SIZE)) {
-        jsonContent.put(
-            QUERY_PARAMS_FETCH_SIZE,
-            Integer.parseInt(restRequest.params().get(QUERY_PARAMS_FETCH_SIZE)));
+        try {
+          jsonContent.put(
+              QUERY_PARAMS_FETCH_SIZE,
+              Integer.parseInt(restRequest.params().get(QUERY_PARAMS_FETCH_SIZE)));
+        } catch (NumberFormatException e) {
+          throw new IllegalArgumentException(
+              "Invalid fetch_size parameter: must be a valid integer", e);
+        }
       }
       PPLQueryRequest pplRequest =
           new PPLQueryRequest(

ppl/src/test/java/org/opensearch/sql/ppl/domain/PPLQueryRequestTest.java

@@ -77,6 +77,15 @@ public void testGetFetchSizeReturnsZeroWhenJsonContentIsNull() {
     assertEquals(0, request.getFetchSize());
   }
 
+  @Test
+  public void testGetFetchSizeHandlesExplicitNull() {
+    JSONObject json = new JSONObject();
+    json.put("query", "source=t");
+    json.put("fetch_size", JSONObject.NULL);
+    PPLQueryRequest request = new PPLQueryRequest("source=t", json, "/_plugins/_ppl");
+    assertEquals(0, request.getFetchSize());
+  }
+
   @Test
   public void testGetFetchSizeWithLargeValue() {
     JSONObject json = new JSONObject("{\"query\": \"source=t\", \"fetch_size\": 15000}");