Adjust bare-distinct logic.

If an index is provided to a bare distinct which happens
to be the index handling the query, then the query needs
to behave as if no index argument was provided.

For example:

When querying against index `i`,
```
Distinct(index="i", field="ints")`
```
should behave exactly like
```
Distinct(field="ints")
```

executor.go

@@ -438,7 +438,7 @@ func (e *executor) execute(ctx context.Context, index string, q *pql.Query, shar
 		// still need to handle them. Since everything else was
 		// already precomputed by handlePreCallChildren, though,
 		// we don't need this logic in executeCall.
-		if newIndex := call.CallIndex(); newIndex != "" {
+		if newIndex := call.CallIndex(); newIndex != "" && newIndex != index {
 			v, err = e.executeCall(ctx, newIndex, call, nil, opt)
 		} else {
 			v, err = e.executeCall(ctx, index, call, shards, opt)

executor_test.go

@@ -5082,3 +5082,32 @@ func TestExecutor_Execute_CountDistinct(t *testing.T) {
 		}
 	})
 }
+
+// Ensure that a top-level, bare distinct on multiple nodes
+// is handled correctly.
+func TestExecutor_BareDistinct(t *testing.T) {
+	t.Helper()
+	c := test.MustRunCluster(t, 2)
+	defer c.Close()
+
+	c.CreateField(t, "i", pilosa.IndexOptions{}, "ints",
+		pilosa.OptFieldTypeInt(0, math.MaxInt64),
+	)
+
+	// Populate integer data.
+	c.Query(t, "i", fmt.Sprintf(`
+			Set(0, ints=1)
+			Set(%d, ints=2)
+		`, ShardWidth))
+
+	for _, pql := range []string{
+		`Distinct(field="ints")`,
+		`Distinct(index="i", field="ints")`,
+	} {
+		exp := []uint64{1, 2}
+		res := c.Query(t, "i", pql).Results[0].(pilosa.SignedRow)
+		if got := res.Pos.Columns(); !reflect.DeepEqual(exp, got) {
+			t.Fatalf("expected: %v, but got: %v", exp, got)
+		}
+	}
+}