Method HeadSnapshot

internal/storage/postgres/data_reader.go:553–587

HeadSnapshot retrieves the latest snapshot token associated with the tenant.

(ctx context.Context, tenantID string)

// HeadSnapshot retrieves the latest snapshot token associated with the tenant.
func (r *DataReader) HeadSnapshot(ctx context.Context, tenantID string) (token.SnapToken, error) {
	// Start a new trace span and end it when the function exits.
	ctx, span := internal.Tracer.Start(ctx, "data-reader.head-snapshot")
	defer span.End()
	// Log snapshot operation
	slog.DebugContext(ctx, "getting head snapshot for tenant_id", slog.String("tenant_id", tenantID))
	// Declare transaction ID and snapshot variables
	var xid db.XID8
	var snapshotValue string

	// Build the query to find the highest transaction ID and snapshot associated with the tenant.
	builder := r.database.Builder.Select("id", "snapshot").From(TransactionsTable).Where(squirrel.Eq{"tenant_id": tenantID}).OrderBy("id DESC").Limit(1)
	query, args, err := builder.ToSql()
	if err != nil {
		return nil, utils.HandleError(ctx, span, err, base.ErrorCode_ERROR_CODE_SQL_BUILDER)
	}

	// TODO: To optimize this query, create the following index concurrently to avoid table locks:
	// CREATE INDEX CONCURRENTLY idx_transactions_tenant_id_id ON transactions(tenant_id, id DESC);

	// Execute the query and retrieve the highest transaction ID and snapshot.
	err = r.database.ReadPool.QueryRow(ctx, query, args...).Scan(&xid, &snapshotValue)
	if err != nil {
		// If no rows are found, return a snapshot token with a value of 0.
		if errors.Is(err, pgx.ErrNoRows) {
			return snapshot.NewToken(db.XID8{Uint: 0}, ""), nil
		}
		return nil, utils.HandleError(ctx, span, err, base.ErrorCode_ERROR_CODE_SCAN)
	}

	slog.DebugContext(ctx, "successfully retrieved latest snapshot token")
	// Return snapshot token
	// Return the latest snapshot token associated with the tenant.
	return snapshot.NewToken(xid, snapshotValue), nil
}