a2l007 opened a new issue #6057: Broker sends sequential requests to the historical for union queries URL: https://github.com/apache/incubator-druid/issues/6057 Analyzing groupBy queries based on Union datasources, we have seen that for a given query if a historical has segments for the multiple datasources part of this query, the broker generates multiple requests to this historical, but in a serial manner. For example, running the following union query: ``` { "queryType": "groupBy", "dataSource": { "type": "union", "dataSources": [ { "type": "table", "name": "tableA" }, { "type": "table", "name": "tableB" }, { "type": "table", "name": "tableC" } ] }, "intervals": { "type": "LegacySegmentSpec", "intervals": [ "2018-07-23T00:00:00.000Z\/2018-07-24T00:00:00.000Z" ] }, "virtualColumns": [ ], "filter": null, "granularity": "DAY", "dimensions": [ { "type": "default", "dimension": "acc_id" } ], "aggregations": [ { "fieldName": "clicks", "name": "clicks", "type": "longSum" } ], "postAggregations": [ ], "having": null, "limitSpec": { "type": "NoopLimitSpec" }, "descending": false } ``` generates logs such as: ``` 2018-07-24T14:50:06,500 INFO [qtp1256384385-231[groupBy_q_test_1]] com.metamx.http.client.pool.ChannelResourceFactory - Generating: https://tier .historical.foo.com:4443 2018-07-24T14:50:40,366 INFO [qtp1256384385-231[groupBy_q_test_1]] com.metamx.http.client.pool.ChannelResourceFactory - Generating: https://tier .historical.foo.com:4443 ``` This historical contains segments for datasources `tableA` and `tableB` and therefore it sends two request to the historical but as it can be seen from the timestamps, there is a delay between both the requests. [BrokerServerView](https://github.com/apache/incubator-druid/blob/master/server/src/main/java/io/druid/client/BrokerServerView.java#L291) confirms this behaviour that at an instant for a given query, there can be only a single request to a historical. This behaviour clearly causes query execution delays for union queries. Before I work on investigating if this can be parallelized, I wanted to check with the community for any comments on whether this is already a known issue with union queries or if it is actually a bug.
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