A method for joining tables in multiple heterogeneous distributed databases implemented by at least two data sources accessible to a federal database server over a network includes: transmitting from the federated database server a sub-command to a first of the data sources responsive to the federated database server receiving a data query; retrieving, with the federated database server, block data from the first data source related to the data query using block fetching according to the sub-command; transmitting, with the federated database server, at least a portion of the block data to a second of the data sources together with an instruction for the second data source to perform a join operation on the portion of the block data and a data table stored by the second data source related to the query; and retrieving a result of the join operation with the federated database server.