pkg/clickhouse: answer HasProfileData with one row, not a scan

pkg/clickhouse/querier.go

@@ -264,11 +264,34 @@ func (q *Querier) ProfileTypes(
 
 // HasProfileData checks if there is any profile data in the store.
 func (q *Querier) HasProfileData(ctx context.Context) (bool, error) {
-	types, err := q.ProfileTypes(ctx, time.UnixMilli(0), time.UnixMilli(0))
+	ctx, span := q.tracer.Start(ctx, "ClickHouse/HasProfileData")
+	defer span.End()
+
+	// This asks whether the table holds anything at all, so it wants one row,
+	// not every distinct profile type. It used to call
+	// ProfileTypes(UnixMilli(0), UnixMilli(0)), whose time filter is skipped
+	// when both bounds are zero -- so the question "is there any data?" was
+	// answered with an unbounded SELECT DISTINCT over six columns, a full scan
+	// of a table that grows without bound. Measured on a server with 72h of
+	// retention: 108,591,120 rows and 7.9 GB read to return 9 rows, 1.3s warm
+	// and 61s cold, every time the UI asked.
+	rows, err := q.client.Query(ctx, hasProfileDataQuery(q.client.FullTableName()))
 	if err != nil {
-		return false, err
+		return false, fmt.Errorf("failed to query for profile data: %w", err)
 	}
-	return len(types) > 0, nil
+	defer rows.Close()
+
+	if rows.Next() {
+		return true, nil
+	}
+	return false, rows.Err()
+}
+
+// hasProfileDataQuery is the statement HasProfileData runs. It is a separate
+// function so a test can pin its shape without a server: existence needs one
+// row, and must not become a scan again.
+func hasProfileDataQuery(table string) string {
+	return fmt.Sprintf("SELECT 1 FROM %s LIMIT 1", table)
 }
 
 // QueryRange executes a range query and returns time series data.

pkg/clickhouse/querier_test.go

@@ -0,0 +1,45 @@
+// Copyright 2026 The Parca Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package clickhouse
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+// HasProfileData asks whether the table holds anything at all, so it must read
+// one row rather than compute every distinct profile type.
+//
+// It used to answer by calling ProfileTypes(UnixMilli(0), UnixMilli(0)), and
+// ProfileTypes only applies its time filter when both bounds are non-zero --
+// so the cheapest question in the API ran an unbounded SELECT DISTINCT over six
+// columns, scanning a table that grows without bound. On a server with 72h of
+// retention that read 108,591,120 rows and 7.9 GB to return 9 rows, every time
+// the UI asked.
+//
+// The shape is asserted rather than the result because this query needs a
+// server to run, while the regression is entirely visible in the statement.
+func TestHasProfileDataQueryShape(t *testing.T) {
+	q := hasProfileDataQuery("parca.stacktraces")
+
+	require.Equal(t, "SELECT 1 FROM parca.stacktraces LIMIT 1", q)
+
+	upper := strings.ToUpper(q)
+	require.Contains(t, upper, "LIMIT 1", "existence needs one row")
+	require.NotContains(t, upper, "DISTINCT", "existence must not be answered with a distinct scan")
+	require.NotContains(t, upper, "GROUP BY")
+	require.NotContains(t, upper, "ORDER BY")
+}