test(ruler): add integration test for select merging query reduction

Author: alanprot

pkg/ruler/select_merger_integration_test.go

@@ -0,0 +1,73 @@
+package ruler
+
+import (
+	"context"
+	"sync/atomic"
+	"testing"
+	"time"
+
+	"github.com/prometheus/prometheus/model/labels"
+	"github.com/prometheus/prometheus/promql"
+	"github.com/prometheus/prometheus/rules"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestSelectMerger_Integration(t *testing.T) {
+	// 3 rules with overlapping selectors on http_requests_total.
+	rls := []rules.Rule{
+		mustParseRule(t, `rate(http_requests_total{job="api",deploy_color="blue"}[5m])`),
+		mustParseRule(t, `rate(http_requests_total{job="api",deploy_color="green"}[5m])`),
+		mustParseRule(t, `rate(http_requests_total{job="api",deploy_color=~".*"}[5m])`),
+	}
+
+	// Step 1: Plan merged selects.
+	plan := planMergedSelects(rls, 2)
+	require.Len(t, plan, 1, "expected 1 merged entry for http_requests_total")
+	assert.Equal(t, "http_requests_total", plan[0].metricName)
+
+	// Mock data: series for blue, green, and red.
+	allSeries := promql.Vector{
+		{Metric: labels.FromStrings("__name__", "http_requests_total", "job", "api", "deploy_color", "blue"), T: 1000, F: 10},
+		{Metric: labels.FromStrings("__name__", "http_requests_total", "job", "api", "deploy_color", "green"), T: 1000, F: 20},
+		{Metric: labels.FromStrings("__name__", "http_requests_total", "job", "api", "deploy_color", "red"), T: 1000, F: 30},
+	}
+
+	// Mock QueryFunc that counts calls and returns allSeries.
+	var callCount atomic.Int32
+	mockQueryFunc := func(ctx context.Context, qs string, t time.Time) (promql.Vector, error) {
+		callCount.Add(1)
+		return allSeries, nil
+	}
+
+	// Step 2: Execute prefetch — should call QueryFunc exactly once.
+	ts := time.Now()
+	cache := executePrefetch(context.Background(), plan, mockQueryFunc, ts)
+	assert.Equal(t, int32(1), callCount.Load(), "executePrefetch should call QueryFunc exactly once")
+
+	// Step 3: Create cached query func wrapping the mock.
+	cached := cachedQueryFunc(mockQueryFunc, cache)
+
+	// Step 4: Query with each rule's expression — should all be served from cache.
+	callsBefore := callCount.Load()
+
+	// Blue rule: should return only blue series.
+	vecBlue, err := cached(context.Background(), `http_requests_total{job="api",deploy_color="blue"}`, ts)
+	require.NoError(t, err)
+	assert.Len(t, vecBlue, 1)
+	assert.Equal(t, "blue", vecBlue[0].Metric.Get("deploy_color"))
+
+	// Green rule: should return only green series.
+	vecGreen, err := cached(context.Background(), `http_requests_total{job="api",deploy_color="green"}`, ts)
+	require.NoError(t, err)
+	assert.Len(t, vecGreen, 1)
+	assert.Equal(t, "green", vecGreen[0].Metric.Get("deploy_color"))
+
+	// Regex .* rule: should return all series.
+	vecAll, err := cached(context.Background(), `http_requests_total{job="api",deploy_color=~".*"}`, ts)
+	require.NoError(t, err)
+	assert.Len(t, vecAll, 3)
+
+	// Step 5: Verify no additional calls to the underlying QueryFunc.
+	assert.Equal(t, callsBefore, callCount.Load(), "cachedQueryFunc should not call underlying QueryFunc")
+}