Implement master_aggregate_table_shards(source table, destination table, aggregation_query)

[ozgune]

Our documentation refers to the user-defined function master_aggregate_table_shards to help users create distributed materialized views. We need to implement this function or remove the reference to it from our documentation.

https://www.citusdata.com/documentation/citusdb-documentation/examples/id_querying_aggregated_data.html

The mechanism through which we implement this function could potentially be similar to #10

[ozgune]

@onderkalaci -- Now that we implemented INSERT INTO (SELECT ...) with #508, what do you think about closing this issue?

[onderkalaci]

https://www.citusdata.com/documentation/citusdb-documentation/examples/id_querying_aggregated_data.html

The link is dead, so I cannot see what the UDF is intended for. If it only intends to make co-located roll-ups, INSERT INTO .. SELECT already does that with a wide SQL coverage. But, if it intends to run arbitrary multi-shard roll-ups, it might make sense to keep the issue open.

[ozgune]

Good point. The intent with this user-defined function was to roll-up data from a source distributed table to a target colocated table. I'm therefore closing this issue.