Update documentation for _routing

Signed-off-by: acarbonetto <[email protected]>

docs/user/dql/basics.rst

@@ -158,6 +158,9 @@ Result set:
 One can also provide meta-field name(s) to retrieve reserved-fields (beginning with underscore) from OpenSearch documents.  They may also be used
 in the query `WHERE` or `ORDER BY` clauses. Meta-fields are not output from wildcard calls (`SELECT *`) and must be explicitly included to be returned.
 
+Note: `_routing` is used differently in the `SELECT` and `WHERE` clauses.  In `WHERE`, it contains the routing hash id. In `SELECT`,
+it returns the shard used for the query (unless shards aren't active, in which case it returns the routing hash id).
+
 SQL query::
 
 	POST /_plugins/_sql

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

@@ -169,8 +169,10 @@ public void testMetafieldIdentifierRoutingFilterTest() throws IOException {
     var datarows = result.getJSONArray("datarows");
     assertEquals(1, datarows.length());
 
-    // note that _routing in the SELECT clause returns the shard
+    assertEquals("test4", datarows.getJSONArray(0).getString(0));
+    // note that _routing in the SELECT clause returns the shard, not the routing hash id
     assertTrue(datarows.getJSONArray(0).getString(2).contains("[" + index + "]"));
+
   }
 
   @Test