queryserving, observability: instrument vttablet query cache plan hit…

…s/misses (#14947)

Signed-off-by: Max Englander <max@planetscale.com>
Signed-off-by: Florent Poinsard <florent.poinsard@outlook.fr>
Co-authored-by: Florent Poinsard <florent.poinsard@outlook.fr>

changelog/20.0/20.0.0/summary.md

@@ -11,11 +11,11 @@
   - **[Flag changes](#flag-changes)**
     - [`pprof-http` default change](#pprof-http-default)
 - **[Minor Changes](#minor-changes)**
-
+  - **[New Stats](#new-stats)**
+    - [VTTablet Query Cache Hits and Misses](#vttablet-query-cache-hits-and-misses)
 
 ## <a id="major-changes"/>Major Changes
 
-
 ### <a id="query-compatibility"/>Query Compatibility
 
 #### <a id="vindex-hints"/> Vindex Hints
@@ -64,3 +64,11 @@ To continue enabling these endpoints, explicitly set `--pprof-http` when startin
 
 ## <a id="minor-changes"/>Minor Changes
 
+### <a id="new-stats"/>New Stats
+
+#### <a id="vttablet-query-cache-hits-and-misses"/>VTTablet Query Cache Hits and Misses
+
+VTTablet exposes two new counter stats:
+
+ * `QueryCacheHits`: Query engine query cache hits
+ * `QueryCacheMisses`: Query engine query cache misses

go/vt/vttablet/tabletserver/query_engine.go

@@ -188,6 +188,7 @@ type QueryEngine struct {
 	// stats
 	// Note: queryErrorCountsWithCode is similar to queryErrorCounts except it contains error code as an additional dimension
 	queryCounts, queryCountsWithTabletType, queryTimes, queryErrorCounts, queryErrorCountsWithCode, queryRowsAffected, queryRowsReturned *stats.CountersWithMultiLabels
+	queryCacheHits, queryCacheMisses                                                                                                     *stats.CounterFunc
 
 	// stats flags
 	enablePerWorkloadTableMetrics bool
@@ -280,6 +281,12 @@ func NewQueryEngine(env tabletenv.Env, se *schema.Engine) *QueryEngine {
 	env.Exporter().NewCounterFunc("QueryCacheEvictions", "Query engine query cache evictions", func() int64 {
 		return qe.plans.Metrics.Evicted()
 	})
+	qe.queryCacheHits = env.Exporter().NewCounterFunc("QueryCacheHits", "Query engine query cache hits", func() int64 {
+		return qe.plans.Metrics.Hits()
+	})
+	qe.queryCacheMisses = env.Exporter().NewCounterFunc("QueryCacheMisses", "Query engine query cache misses", func() int64 {
+		return qe.plans.Metrics.Misses()
+	})
 
 	labels := []string{"Table", "Plan"}
 	if config.EnablePerWorkloadTableMetrics {

go/vt/vttablet/tabletserver/query_engine_test.go

@@ -186,11 +186,27 @@ func TestQueryPlanCache(t *testing.T) {
 	ctx := context.Background()
 	logStats := tabletenv.NewLogStats(ctx, "GetPlanStats")
 
+	initialHits := qe.queryCacheHits.Get()
+	initialMisses := qe.queryCacheMisses.Get()
+
 	firstPlan, err := qe.GetPlan(ctx, logStats, firstQuery, false)
 	require.NoError(t, err)
 	require.NotNil(t, firstPlan, "plan should not be nil")
 
 	assertPlanCacheSize(t, qe, 1)
+
+	require.Equal(t, int64(0), qe.queryCacheHits.Get()-initialHits)
+	require.Equal(t, int64(1), qe.queryCacheMisses.Get()-initialMisses)
+
+	secondPlan, err := qe.GetPlan(ctx, logStats, firstQuery, false)
+	require.NoError(t, err)
+	require.NotNil(t, secondPlan, "plan should not be nil")
+
+	assertPlanCacheSize(t, qe, 1)
+
+	require.Equal(t, int64(1), qe.queryCacheHits.Get()-initialHits)
+	require.Equal(t, int64(1), qe.queryCacheMisses.Get()-initialMisses)
+
 	qe.ClearQueryPlanCache()
 }