chore(persistence): prefers using SEQUENCE to retrieve table max. id

See 0e995d4 and #543.

PostgreSQL and CockroachDB support `last_value` column from `SEQUENCE`
virtual tables.

Author: HorlogeSkynet

persistence/postgres.go

@@ -155,14 +155,14 @@ func (db *postgresDatabase) getExactCount() (uint, error) {
 	ctx, cancel := context.WithTimeout(context.Background(), time.Second/2)
 	defer cancel()
 
-	rows, err := db.conn.QueryContext(ctx, "SELECT id::BIGINT AS exact_count FROM torrents ORDER BY id DESC LIMIT 1;")
+	rows, err := db.conn.QueryContext(ctx, "SELECT last_value::BIGINT AS exact_count FROM seq_torrents_id;")
 	if err != nil {
 		return 0, err
 	}
 	defer rows.Close()
 
 	if !rows.Next() {
-		return 0, errors.New("no rows returned from `SELECT id::BIGINT AS exact_count FROM torrents ORDER BY id DESC LIMIT 1;`")
+		return 0, errors.New("no rows returned from `SELECT last_value::BIGINT AS exact_count FROM seq_torrents_id;`")
 	}
 
 	// Returns int64: https://godoc.org/github.com/lib/pq#hdr-Data_Types

persistence/postgres_test.go

@@ -116,7 +116,7 @@ func TestPostgresDatabase_GetExactCount(t *testing.T) {
 
 	// Test case 1: Valid row returned
 	rows := sqlmock.NewRows([]string{"exact_count"}).AddRow(int64(10))
-	mock.ExpectQuery("SELECT id::BIGINT AS exact_count FROM torrents ORDER BY id DESC LIMIT 1;").
+	mock.ExpectQuery("SELECT last_value::BIGINT AS exact_count FROM seq_torrents_id;").
 		WillReturnRows(rows)
 
 	count, err := db.getExactCount()
@@ -129,7 +129,7 @@ func TestPostgresDatabase_GetExactCount(t *testing.T) {
 
 	// Test case 2: No rows returned
 	rows = sqlmock.NewRows([]string{"exact_count"})
-	mock.ExpectQuery("SELECT id::BIGINT AS exact_count FROM torrents ORDER BY id DESC LIMIT 1;").
+	mock.ExpectQuery("SELECT last_value::BIGINT AS exact_count FROM seq_torrents_id;").
 		WillReturnRows(rows)
 
 	count, err = db.getExactCount()
@@ -142,7 +142,7 @@ func TestPostgresDatabase_GetExactCount(t *testing.T) {
 
 	// Test case 3: Null value returned
 	rows = sqlmock.NewRows([]string{"exact_count"}).AddRow(nil)
-	mock.ExpectQuery("SELECT id::BIGINT AS exact_count FROM torrents ORDER BY id DESC LIMIT 1;").
+	mock.ExpectQuery("SELECT last_value::BIGINT AS exact_count FROM seq_torrents_id;").
 		WillReturnRows(rows)
 
 	count, err = db.getExactCount()
@@ -226,7 +226,7 @@ func TestPostgresDatabase_GetNumberOfTorrents(t *testing.T) {
 
 	// Test case 1: getExactCount succeeds
 	rows := sqlmock.NewRows([]string{"exact_count"}).AddRow(int64(10))
-	mock.ExpectQuery("SELECT id::BIGINT AS exact_count FROM torrents ORDER BY id DESC LIMIT 1;").
+	mock.ExpectQuery("SELECT last_value::BIGINT AS exact_count FROM seq_torrents_id;").
 		WillReturnRows(rows)
 
 	count, err := db.GetNumberOfTorrents()
@@ -238,7 +238,7 @@ func TestPostgresDatabase_GetNumberOfTorrents(t *testing.T) {
 	}
 
 	// Test case 2: getExactCount fails, getFuzzyCount succeeds
-	mock.ExpectQuery("SELECT id::BIGINT AS exact_count FROM torrents ORDER BY id DESC LIMIT 1;").
+	mock.ExpectQuery("SELECT last_value::BIGINT AS exact_count FROM seq_torrents_id;").
 		WillReturnError(fmt.Errorf("exact count failed"))
 	rows = sqlmock.NewRows([]string{"estimate_count"}).AddRow(int64(20))
 	mock.ExpectQuery("SELECT reltuples::BIGINT AS estimate_count FROM pg_class WHERE relname='torrents';").
@@ -253,7 +253,7 @@ func TestPostgresDatabase_GetNumberOfTorrents(t *testing.T) {
 	}
 
 	// Test case 3: both getExactCount and getFuzzyCount fail
-	mock.ExpectQuery("SELECT id::BIGINT AS exact_count FROM torrents ORDER BY id DESC LIMIT 1;").
+	mock.ExpectQuery("SELECT last_value::BIGINT AS exact_count FROM seq_torrents_id;").
 		WillReturnError(fmt.Errorf("exact count failed"))
 	mock.ExpectQuery("SELECT reltuples::BIGINT AS estimate_count FROM pg_class WHERE relname='torrents';").
 		WillReturnError(fmt.Errorf("fuzzy count failed"))