presto-resource-group-managers/src/main/java/io/prestosql/plugin/resourcegroups/db/DbResourceGroupConfigurationManager.java/DbResourceGroupConfigurationManager/getSelectors
class DbResourceGroupConfigurationManager:    @VisibleForTesting
    public List<ResourceGroupSelector> getSelectors()
    {
        if (lastRefresh.get() == 0) {
            throw new PrestoException(CONFIGURATION_UNAVAILABLE, "Selectors cannot be fetched from database");
        }
        if (selectors.get().isEmpty()) {
            throw new PrestoException(CONFIGURATION_INVALID, "No selectors are configured");
        }
        return selectors.get();
    }