Druid not using Both Historical nodes

Hi

I have setup 3 node cluster.

Node 1: Coordinator ,overlord,middle manager,router

Node 2: Broker,Historical

Node 3: Broker,Historical

Node is 16 cores,110 GB ram and Disk of 350GB each

I created the druid segment using Hive .I could see it created in deep storage ,also segment gets downloaded to both Historical nodes.

I have 9 million records,total data size is 572M.

Now I able to do select * uptil 250K records

select * from table limit 250000

But when I give more than 250K ,let says just select * from table for enitre 9 million.I get get an error ,saying GC overhead limit exceeded.

If increase druid.historical.jvm.heap.memory ,it works for higher count and not entire 9 million.

This is the same behaviour via hive and the rest api calls

From broker logs I see only one broker being hit i.e node 2.

I have set hive.druid.broker.address.default to point to node.

To my understanding the router will distribute the work load to all available broker and historical nodes and retrieve records.

Question 1:

Is My Understanding correct ? and if yes what configuration should I set to all a distributed run.

Question 2:

Does query performance depends on sized of the segment or the number of Segments/Records created.

Question 3:

For 9 million plus records would be the ideal cluster set up.Like How many nodes will required,how many cores and RAM etc?

Question 4:

Is tableau a recommended for visualization using druid

Thanks

Vinayak