sqldb: add benchmark for sqlite connection number

diff --git a/sqldb/sqlite_bench_test.go b/sqldb/sqlite_bench_test.go
new file mode 100644
index 0000000..bc5cea3
--- /dev/null
+++ b/sqldb/sqlite_bench_test.go
@@ -0,0 +1,204 @@
+//go:build !js && !(windows && (arm || 386)) && !(linux && (ppc64 || mips || mipsle || mips64))
+
+package sqldb
+
+import (
+	"context"
+	"database/sql"
+	"fmt"
+	"path/filepath"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/lightningnetwork/lnd/sqldb/sqlc"
+	"github.com/stretchr/testify/require"
+)
+
+// BenchmarkSqliteMaxConns benchmarks sequential reads against a SQLite
+// database with varying MaxConnections settings.
+//
+// Run with:
+//
+//	go test -bench=BenchmarkSqliteMaxConns -benchmem -run=^$ ./sqldb/
+func BenchmarkSqliteMaxConns(b *testing.B) {
+	const numInvoices = 500
+
+	// connCounts contains the MaxConnections values we want to compare.
+	// 0 means "use the library default" (currently 2 for SQLite).
+	connCounts := []int{1, 2, 4, 8, 16, 0}
+
+	// Build a fresh SQLite database that will be shared across all
+	// sub-benchmarks. We insert a fixed set of invoices once and then
+	// execute read-only queries from multiple goroutines.
+	dbFileName := filepath.Join(b.TempDir(), "bench.db")
+
+	// Open the store once with migrations so the schema is in place.
+	setupStore, err := NewSqliteStore(&SqliteConfig{
+		SkipMigrations: false,
+	}, dbFileName)
+	require.NoError(b, err)
+
+	require.NoError(b, setupStore.ApplyAllMigrations(
+		context.Background(), GetMigrations(),
+	))
+
+	ctx := context.Background()
+
+	// Insert test invoices. We use a predictable hash per invoice so we
+	// can look them up deterministically during the benchmark.
+	hashes := make([][]byte, numInvoices)
+	for i := range numInvoices {
+		hash := make([]byte, 32)
+		hash[0] = byte(i)
+		hash[1] = byte(i >> 8)
+		hashes[i] = hash
+
+		_, err := setupStore.InsertInvoice(
+			ctx, sqlc.InsertInvoiceParams{
+				Hash:               hash,
+				PaymentAddr:        hash,
+				PaymentRequestHash: hash,
+				Expiry:             3600,
+				CreatedAt:          time.Now(),
+			},
+		)
+		require.NoError(b, err)
+	}
+
+	require.NoError(b, setupStore.DB.Close())
+
+	for _, maxConns := range connCounts {
+		name := fmt.Sprintf("MaxConns=%d", maxConns)
+		if maxConns == 0 {
+			name = fmt.Sprintf("MaxConns=default(%d)",
+				DefaultSqliteMaxConns)
+		}
+
+		b.Run(name, func(b *testing.B) {
+			store, err := NewSqliteStore(
+				&SqliteConfig{
+					SkipMigrations: true,
+					MaxConnections: maxConns,
+				}, dbFileName,
+			)
+			require.NoError(b, err)
+
+			b.Cleanup(func() {
+				require.NoError(b, store.DB.Close())
+			})
+
+			var i int
+			for b.Loop() {
+				hash := hashes[i%numInvoices]
+				i++
+
+				_, err := store.GetInvoice(
+					ctx,
+					sqlc.GetInvoiceParams{
+						Hash: hash,
+					},
+				)
+				if err != nil {
+					require.ErrorIs(b, err, sql.ErrNoRows)
+				}
+			}
+		})
+	}
+}
+
+// BenchmarkSqliteMaxConnsConcurrentReads measures aggregate read throughput
+// for a fixed level of goroutine concurrency to complement the sequential
+// benchmark above. Each iteration launches a fixed number of goroutines that
+// all issue reads simultaneously, directly stressing the connection pool.
+func BenchmarkSqliteMaxConnsConcurrentReads(b *testing.B) {
+	const (
+		numInvoices = 500
+		goroutines  = 16
+	)
+
+	connCounts := []int{1, 2, 4, 8, 16, 0}
+
+	dbFileName := filepath.Join(b.TempDir(), "bench_conc.db")
+
+	setupStore, err := NewSqliteStore(&SqliteConfig{
+		SkipMigrations: false,
+	}, dbFileName)
+	require.NoError(b, err)
+
+	require.NoError(b, setupStore.ApplyAllMigrations(
+		context.Background(), GetMigrations(),
+	))
+
+	ctx := context.Background()
+
+	hashes := make([][]byte, numInvoices)
+	for i := range numInvoices {
+		hash := make([]byte, 32)
+		hash[0] = byte(i)
+		hash[1] = byte(i >> 8)
+		hashes[i] = hash
+
+		_, err := setupStore.InsertInvoice(
+			ctx, sqlc.InsertInvoiceParams{
+				Hash:               hash,
+				PaymentAddr:        hash,
+				PaymentRequestHash: hash,
+				Expiry:             3600,
+				CreatedAt:          time.Now(),
+			},
+		)
+		require.NoError(b, err)
+	}
+
+	require.NoError(b, setupStore.DB.Close())
+
+	for _, maxConns := range connCounts {
+		name := fmt.Sprintf("MaxConns=%d", maxConns)
+		if maxConns == 0 {
+			name = fmt.Sprintf("MaxConns=default(%d)",
+				DefaultSqliteMaxConns)
+		}
+
+		b.Run(name, func(b *testing.B) {
+			store, err := NewSqliteStore(
+				&SqliteConfig{
+					SkipMigrations: true,
+					MaxConnections: maxConns,
+				}, dbFileName,
+			)
+			require.NoError(b, err)
+
+			b.Cleanup(func() {
+				require.NoError(b, store.DB.Close())
+			})
+
+			for b.Loop() {
+				var wg sync.WaitGroup
+				wg.Add(goroutines)
+
+				for g := range goroutines {
+					go func() {
+						defer wg.Done()
+
+						hash := hashes[g%numInvoices]
+						_, err := store.GetInvoice(
+							ctx,
+							sqlc.GetInvoiceParams{
+								Hash: hash,
+							},
+						)
+						if err != nil &&
+							err != sql.ErrNoRows {
+
+							b.Errorf("GetInvoice:"+
+								" %v", err)
+						}
+					}()
+				}
+
+				wg.Wait()
+			}
+		})
+	}
+}