fix(sql): fix SAMPLE BY queries with ORDER BY timestamp DESC (#3662)

core/src/main/java/io/questdb/griffin/SqlOptimiser.java

@@ -2759,7 +2759,7 @@ public class SqlOptimiser implements Mutable {
         for (int i = 0, k = jm.size(); i < k; i++) {
             QueryModel qm = jm.getQuick(i).getNestedModel();
             if (qm != null) {
-                if (model.getGroupBy().size() == 0) {//order by should not copy through group by
+                if (model.getGroupBy().size() == 0 && model.getSampleBy() == null) { // order by should not copy through group by or sample by
                     qm.setOrderByAdviceMnemonic(orderByMnemonic);
                     qm.copyOrderByAdvice(orderByAdvice);
                     qm.copyOrderByDirectionAdvice(orderByDirectionAdvice);

core/src/test/java/io/questdb/test/griffin/engine/groupby/SampleByTest.java

@@ -9830,6 +9830,32 @@ public class SampleByTest extends AbstractCairoTest {
         );
     }
 
+    @Test
+    public void testSampleByWithOrderByDescTimestamp() throws Exception {
+        assertQuery(
+                "created_at\tfirst\n" +
+                        "1970-01-01T08:00:00.000000Z\t42.17768841969397\n" +
+                        "1970-01-01T06:00:00.000000Z\t34.91070363730514\n" +
+                        "1970-01-01T04:00:00.000000Z\t79.05675319675964\n" +
+                        "1970-01-01T02:00:00.000000Z\t8.43832076262595\n" +
+                        "1970-01-01T00:00:00.000000Z\t80.43224099968394\n",
+                "select created_at, first(price)" +
+                        " from trades" +
+                        " sample by 2h" +
+                        " order by created_at desc",
+                "create table trades as " +
+                        "(" +
+                        "select" +
+                        " rnd_double(0) * 100 price," +
+                        " timestamp_sequence(0, 3600000000) created_at" + // 1 hour step
+                        " from long_sequence(10)" + // 10 rows
+                        ") timestamp(created_at) partition by day",
+                "created_at###DESC",
+                true,
+                false
+        );
+    }
+
     @Test
     public void testSamplePeriodInvalidWithNoUnits() throws Exception {
         testSampleByPeriodFails(