sqlstats: fix insights benchmark bugs

- Transactions slice length was incorrectly initialized.
- Transactions needed to set a UUID in order to get observed
  by the `observeTransaction` function and actually trigger
  the code that's being benchmarked.
- Indexing into the `transactions` slice in the benchmark was
  incorrectly using `idx` instead of `i` and grabbing empty
  objects.

Resolves: #151300
Epic: None
Release note: None

Author: dhartunian

cpu.out

@@ -20,6 +20,7 @@ import (
 	"github.com/cockroachdb/cockroach/pkg/sql/sqlstats/insights"
 	"github.com/cockroachdb/cockroach/pkg/util/stop"
 	"github.com/cockroachdb/cockroach/pkg/util/uint128"
+	"github.com/cockroachdb/cockroach/pkg/util/uuid"
 )
 
 // Here we benchmark the entire insights stack, so that we can include in our
@@ -51,7 +52,7 @@ func BenchmarkInsights(b *testing.B) {
 			var sessions sync.WaitGroup
 			sessions.Add(numSessions)
 			statements := make([]sqlstats.RecordedStmtStats, b.N)
-			transactions := make([]sqlstats.RecordedTxnStats, b.N)
+			transactions := make([]sqlstats.RecordedTxnStats, numSessions)
 			for i := 0; i < numSessions; i++ {
 				sessionID := clusterunique.ID{Uint128: uint128.FromInts(rand.Uint64(), uint64(i))}
 				for j := 0; j < numTransactionsPerSession; j++ {
@@ -66,7 +67,8 @@ func BenchmarkInsights(b *testing.B) {
 					}
 				}
 				transactions[i] = sqlstats.RecordedTxnStats{
-					SessionID: sessionID,
+					SessionID:     sessionID,
+					TransactionID: uuid.MakeV4(),
 				}
 			}
 
@@ -76,7 +78,7 @@ func BenchmarkInsights(b *testing.B) {
 					defer sessions.Done()
 					for j := 0; j < numTransactionsPerSession; j++ {
 						idx := numTransactionsPerSession*i + j
-						provider.ObserveTransaction(ctx, &transactions[idx],
+						provider.ObserveTransaction(ctx, &transactions[i],
 							[]*sqlstats.RecordedStmtStats{&statements[idx]})
 					}
 				}(i)