presto-raptor-legacy/src/main/java/io/prestosql/plugin/raptor/legacy/RaptorMetadata.java/RaptorMetadata/listTableColumns
class RaptorMetadata:    @Override
    public Map<SchemaTableName, List<ColumnMetadata>> listTableColumns(ConnectorSession session, SchemaTablePrefix prefix)
    {
        requireNonNull(prefix, "prefix is null");

        ImmutableListMultimap.Builder<SchemaTableName, ColumnMetadata> columns = ImmutableListMultimap.builder();
        for (TableColumn tableColumn : dao.listTableColumns(prefix.getSchema().orElse(null), prefix.getTable().orElse(null))) {
            ColumnMetadata columnMetadata = new ColumnMetadata(tableColumn.getColumnName(), tableColumn.getDataType());
            columns.put(tableColumn.getTable(), columnMetadata);
        }
        return Multimaps.asMap(columns.build());
    }