1. A computer-implemented method comprising: receiving, by a first distributed system, a query that requests a join of first rows of a first table stored by a first distributed system comprising multiple segment nodes with second rows of an external table, the external table representing data stored in multiple data fragments on multiple respective data nodes in a second distributed system, wherein each of the data nodes of the second distributed system are distinct from the segment nodes of the first distributed system, wherein the first distributed system operates under control of a first master node and the second distributed system operates under control of a second master node that is distinct from the first master node; in response to receiving the query, initiating communication by a plurality of extension services between nodes of the first distributed system and nodes of the second distributed system, including a first extension service communicating with the master nodes of the first and second distributed system and a plurality of second extension services communicating between data nodes of the second distributed system and segment nodes of the first distributed system; requesting, by the master node of the first distributed system from the first extension service, location information for a plurality of the data fragments in the second distributed system represented by the external table; allocating each data fragment of the plurality of data fragments to a respective one of the plurality of second extension services and each of the second extension services to one of a plurality of segment nodes of the first distributed system, including providing each segment node with location information for respective data fragments of the plurality of fragments allocated to the segment node; receiving, by each of the plurality of segment nodes of the first distributed system, a stream of data from a respective data node of the second distributed system through a respective second extension service of the plurality of extension services that obtains fragments from the respective data node of the second distributed system according to the location information obtained by the master node of the first distributed; computing, by each of the plurality of segment nodes of the first distributed system, joined rows between the first rows of the first table and the stream of data corresponding to second rows of the external table; and computing a result for the received query using the joined rows.