chore: do not shard log queries with empty filter (#13568)

pkg/logql/syntax/ast.go

@@ -318,9 +318,14 @@ func (e *PipelineExpr) Pipeline() (log.Pipeline, error) {
 // HasFilter returns true if the pipeline contains stage that can filter out lines.
 func (e *PipelineExpr) HasFilter() bool {
 	for _, p := range e.MultiStages {
-		switch p.(type) {
+		switch v := p.(type) {
-		case *LineFilterExpr, *LabelFilterExpr:
+		case *LabelFilterExpr:
 			return true
+		case *LineFilterExpr:
+			// ignore empty matchers as they match everything
+			if !((v.Ty == log.LineMatchEqual || v.Ty == log.LineMatchRegexp) && v.Match == "") {
+				return true
+			}
 		default:
 			continue
 		}

pkg/logql/syntax/ast_test.go

@@ -1039,6 +1039,23 @@ func TestParseLargeQuery(t *testing.T) {
 	require.NoError(t, err)
 }
 
+func TestLogSelectorExprHasFilter(t *testing.T) {
+	for query, hasFilter := range map[string]bool{
+		`{foo="bar"} |= ""`:                  false,
+		`{foo="bar"} |= "" |= ""`:            false,
+		`{foo="bar"} |~ ""`:                  false,
+		`{foo="bar"} |= "notempty"`:          true,
+		`{foo="bar"} |= "" |= "notempty"`:    true,
+		`{foo="bar"} != ""`:                  true,
+		`{foo="bar"} | lbl="notempty"`:       true,
+		`{foo="bar"} |= "" | lbl="notempty"`: true,
+	} {
+		expr, err := ParseExpr(query)
+		require.NoError(t, err)
+		require.Equal(t, hasFilter, expr.(LogSelectorExpr).HasFilter())
+	}
+}
+
 func TestGroupingString(t *testing.T) {
 	g := Grouping{
 		Groups:  []string{"a", "b"},