paymentsdb: benchmark FetchInFlightPayments on Postgres

diff --git a/payments/db/fetch_inflight_benchmark_postgres_test.go b/payments/db/fetch_inflight_benchmark_postgres_test.go
new file mode 100644
index 0000000..9e6f9e9
--- /dev/null
+++ b/payments/db/fetch_inflight_benchmark_postgres_test.go
@@ -0,0 +1,60 @@
+//go:build test_db_postgres && !test_db_sqlite
+
+package paymentsdb
+
+import (
+	"database/sql"
+	"testing"
+
+	"github.com/lightningnetwork/lnd/sqldb"
+)
+
+// postgresFetchInFlightBenchBackend creates Postgres-backed benchmark stores.
+type postgresFetchInFlightBenchBackend struct {
+	fixture *sqldb.TestPgFixture
+}
+
+// newFetchInFlightBenchBackend creates the benchmark backend selected by the
+// active build tags.
+func newFetchInFlightBenchBackend(b *testing.B) fetchInFlightBenchBackend {
+	b.Helper()
+
+	fixture := sqldb.NewTestPgFixture(
+		b, sqldb.DefaultPostgresFixtureLifetime,
+	)
+	b.Cleanup(func() {
+		fixture.TearDown(b)
+	})
+
+	return postgresFetchInFlightBenchBackend{fixture: fixture}
+}
+
+// newStore creates a migrated Postgres SQLStore and seeds it with one benchmark
+// scenario.
+func (backend postgresFetchInFlightBenchBackend) newStore(b *testing.B,
+	scenario fetchInFlightBenchScenario,
+	totalPayments int) (*SQLStore, fetchInFlightBenchSeedStats) {
+
+	b.Helper()
+
+	pgDB := sqldb.NewTestPostgresDB(b, backend.fixture)
+	baseDB := pgDB.BaseDB
+
+	withTx := func(tx *sql.Tx) SQLQueries {
+		return baseDB.WithTx(tx)
+	}
+	queries := sqldb.NewTransactionExecutor(baseDB, withTx)
+
+	store, err := NewSQLStore(&SQLStoreConfig{
+		QueryCfg: sqldb.DefaultPostgresConfig(),
+	}, queries)
+	if err != nil {
+		b.Fatalf("new SQL store: %v", err)
+	}
+
+	stats := seedFetchInFlightBenchDB(
+		b, baseDB.DB, withTx, scenario, totalPayments,
+	)
+
+	return store, stats
+}