presto-mongodb/src/main/java/io/prestosql/plugin/mongodb/MongoMetadata.java/MongoMetadata/getColumnHandles
class MongoMetadata:    @Override
    public Map<String, ColumnHandle> getColumnHandles(ConnectorSession session, ConnectorTableHandle tableHandle)
    {
        MongoTableHandle table = (MongoTableHandle) tableHandle;
        List<MongoColumnHandle> columns = mongoSession.getTable(table.getSchemaTableName()).getColumns();

        ImmutableMap.Builder<String, ColumnHandle> columnHandles = ImmutableMap.builder();
        for (MongoColumnHandle columnHandle : columns) {
            columnHandles.put(columnHandle.getName(), columnHandle);
        }
        return columnHandles.build();
    }