metrics,executor: add metrics for reading from table cache (pingcap#3…

…2171)

close pingcap#29958

executor/adapter.go

@@ -966,6 +966,10 @@ func (a *ExecStmt) FinishExecuteStmt(txnTS uint64, err error, hasMoreResults boo
 	sessVars.DurationParse = 0
 	// Clean the stale read flag when statement execution finish
 	sessVars.StmtCtx.IsStaleness = false
+
+	if sessVars.StmtCtx.ReadFromTableCache {
+		metrics.ReadFromTableCacheCounter.Inc()
+	}
 }
 
 // CloseRecordSet will finish the execution of current statement and do some record work

metrics/metrics.go

@@ -162,6 +162,7 @@ func RegisterMetrics() {
 	prometheus.MustRegister(TopSQLReportDataHistogram)
 	prometheus.MustRegister(PDApiExecutionHistogram)
 	prometheus.MustRegister(CPUProfileCounter)
+	prometheus.MustRegister(ReadFromTableCacheCounter)
 
 	tikvmetrics.InitMetrics(TiDB, TiKVClient)
 	tikvmetrics.RegisterMetrics()

metrics/server.go

@@ -129,6 +129,15 @@ var (
 			Help:      "Counter of query using plan cache.",
 		}, []string{LblType})
 
+	ReadFromTableCacheCounter = prometheus.NewCounter(
+		prometheus.CounterOpts{
+			Namespace: "tidb",
+			Subsystem: "server",
+			Name:      "read_from_tablecache_total",
+			Help:      "Counter of query read from table cache.",
+		},
+	)
+
 	HandShakeErrorCounter = prometheus.NewCounter(
 		prometheus.CounterOpts{
 			Namespace: "tidb",

table/tables/cache_test.go

@@ -20,11 +20,13 @@ import (
 	"time"
 
 	"github.com/pingcap/tidb/infoschema"
+	"github.com/pingcap/tidb/metrics"
 	"github.com/pingcap/tidb/parser/auth"
 	"github.com/pingcap/tidb/parser/model"
 	"github.com/pingcap/tidb/table/tables"
 	"github.com/pingcap/tidb/testkit"
 	"github.com/pingcap/tidb/util/stmtsummary"
+	dto "github.com/prometheus/client_model/go"
 	"github.com/stretchr/testify/require"
 )
 
@@ -530,3 +532,61 @@ func TestTableCacheLeaseVariable(t *testing.T) {
 	require.True(t, duration > time.Second)
 	require.True(t, duration < 3*time.Second)
 }
+
+func TestMetrics(t *testing.T) {
+	store, clean := testkit.CreateMockStore(t)
+	defer clean()
+
+	tk := testkit.NewTestKit(t, store)
+	tk.MustExec("use test")
+	tk.MustExec("drop table if exists test_metrics;")
+	tk.MustExec(`create table test_metrics(c0 int, c1 varchar(20), c2 varchar(20), unique key uk(c0));`)
+	tk.MustExec(`create table nt (c0 int, c1 varchar(20), c2 varchar(20), unique key uk(c0));`)
+	tk.MustExec(`insert into test_metrics(c0, c1, c2) values (1, null, 'green');`)
+	tk.MustExec(`alter table test_metrics cache;`)
+
+	tk.MustQuery("select * from test_metrics").Check(testkit.Rows("1 <nil> green"))
+	cached := false
+	for i := 0; i < 20; i++ {
+		if tk.HasPlan("select * from test_metrics", "UnionScan") {
+			cached = true
+			break
+		}
+		time.Sleep(50 * time.Millisecond)
+	}
+	require.True(t, cached)
+
+	counter := metrics.ReadFromTableCacheCounter
+	pb := &dto.Metric{}
+
+	queries := []string{
+		// Table scan
+		"select * from test_metrics",
+		// Index scan
+		"select c0 from test_metrics use index(uk) where c0 > 1",
+		// Index Lookup
+		"select c1 from test_metrics use index(uk) where c0 = 1",
+		// Point Get
+		"select c0 from test_metrics use index(uk) where c0 = 1",
+		// // Aggregation
+		"select count(*) from test_metrics",
+		// Join
+		"select * from test_metrics as a join test_metrics as b on a.c0 = b.c0 where a.c1 != 'xxx'",
+	}
+	counter.Write(pb)
+	i := pb.GetCounter().GetValue()
+
+	for _, query := range queries {
+		tk.MustQuery(query)
+		i++
+		counter.Write(pb)
+		hit := pb.GetCounter().GetValue()
+		require.Equal(t, i, hit)
+	}
+
+	// A counter-example that doesn't increase metrics.ReadFromTableCacheCounter.
+	tk.MustQuery("select * from nt")
+	counter.Write(pb)
+	hit := pb.GetCounter().GetValue()
+	require.Equal(t, i, hit)
+}