Add IT for explain.

Signed-off-by: Yury-Fridlyand <[email protected]>

Author: Yury-Fridlyand

integ-test/src/test/java/org/opensearch/sql/sql/StandalonePaginationIT.java

@@ -22,6 +22,7 @@
 import org.junit.jupiter.api.DisplayNameGenerator;
 import org.opensearch.client.Request;
 import org.opensearch.client.Response;
+import org.opensearch.client.ResponseException;
 import org.opensearch.client.RestHighLevelClient;
 import org.opensearch.common.inject.Injector;
 import org.opensearch.common.inject.ModulesBuilder;
@@ -133,6 +134,7 @@ public void onFailure(Exception e) {
     // act 3: confirm that there's no cursor.
   }
 
+  // Test takes 3+ min due to a big amount of requests issued
   @Test
   @SneakyThrows
   public void test_pagination_blackbox() {
@@ -173,6 +175,25 @@ public void test_pagination_blackbox() {
     }
   }
 
+  @Test
+  @SneakyThrows
+  public void test_explain_not_supported() {
+    var request = new Request("POST", "_plugins/_sql/_explain");
+    // Request should be rejected before index names are resolved
+    request.setJsonEntity("{ \"query\": \"select * from something\", \"fetch_size\": 10 }");
+    var exception = assertThrows(ResponseException.class, () -> client().performRequest(request));
+    var response = new JSONObject(new String(exception.getResponse().getEntity().getContent().readAllBytes()));
+    assertEquals("`explain` feature for paginated requests is not implemented yet.",
+        response.getJSONObject("error").getString("details"));
+
+    // Request should be rejected before cursor parsed
+    request.setJsonEntity("{ \"cursor\" : \"n:0000\" }");
+    exception = assertThrows(ResponseException.class, () -> client().performRequest(request));
+    response = new JSONObject(new String(exception.getResponse().getEntity().getContent().readAllBytes()));
+    assertEquals("`explain` request for cursor requests is not supported. Use `explain` for the initial query request.",
+        response.getJSONObject("error").getString("details"));
+  }
+
   private Settings defaultSettings() {
     return new Settings() {
       private final Map<Key, Integer> defaultSettings = new ImmutableMap.Builder<Key, Integer>()