presto-resource-group-managers/src/main/java/io/prestosql/plugin/resourcegroups/db/DbResourceGroupConfigurationManager.java/DbResourceGroupConfigurationManager/getRootGroups
class DbResourceGroupConfigurationManager:    @Override
    protected List<ResourceGroupSpec> getRootGroups()
    {
        if (lastRefresh.get() == 0) {
            throw new PrestoException(CONFIGURATION_UNAVAILABLE, "Root groups cannot be fetched from database");
        }
        if (this.selectors.get().isEmpty()) {
            throw new PrestoException(CONFIGURATION_INVALID, "No root groups are configured");
        }

        return rootGroups.get();
    }