planner: do not remove the first row func if it is a constant (#50287)

close #38756

pkg/executor/test/executor/executor_test.go

@@ -4299,3 +4299,15 @@ func TestProcessInfoOfSubQuery(t *testing.T) {
 	tk2.MustQuery("select 1 from information_schema.processlist where TxnStart != '' and info like 'select%sleep% from t%'").Check(testkit.Rows("1"))
 	wg.Wait()
 }
+
+func TestIssue38756(t *testing.T) {
+	store := testkit.CreateMockStore(t)
+	tk := testkit.NewTestKit(t, store)
+
+	tk.MustExec("use test")
+	tk.MustExec("create table t (c1 int)")
+	tk.MustExec("insert into t values (1), (2), (3)")
+	tk.MustQuery("SELECT SQRT(1) FROM t").Check(testkit.Rows("1", "1", "1"))
+	tk.MustQuery("(SELECT DISTINCT SQRT(1) FROM t)").Check(testkit.Rows("1"))
+	tk.MustQuery("SELECT DISTINCT cast(1 as double) FROM t").Check(testkit.Rows("1"))
+}

pkg/planner/core/task.go

@@ -1893,6 +1893,12 @@ func RemoveUnnecessaryFirstRow(
 					// the firstrow in root task can not be removed.
 					break
 				}
+				// Skip if it's a constant.
+				// For SELECT DISTINCT SQRT(1) FROM t.
+				// We shouldn't remove the firstrow(SQRT(1)).
+				if _, ok := gbyExpr.(*expression.Constant); ok {
+					continue
+				}
 				if gbyExpr.Equal(sctx, aggFunc.Args[0]) {
 					canOptimize = true
 					firstRowFuncMap[aggFunc].Args[0] = finalGbyItems[j]