[Enhancement] Optimize the algorithm of selecting host for a bucket scan task when a backend not alive

[xinghuayu007]

Proposed changes

When query uses bucket join, fuction Coordinator#BucketShuffleJoinController#getExecHostPortForFragmentIDAndBucketSeq is responseful for making sure each host have average bucket to scan. That means if there are 10 buckets to scan and 5 hosts, the strategy will distributed 2 buckets to each host. The algorithm is like this:

a. use data structure buckendIdToBucketCountMap to represents how many buckets distributed to the backend；
b. traverse every backend, find a backend which owns minimum buckets. We call it mini_backend;
c. distribute the bucket to the mini_backend;
d. update buckendIdToBucketCountMap for mini_backend;

When all bakends are all alive, the algorithm is available. But when mini_backend is not alive, it will chose a replica host as final host randomly and buckendIdToBucketCountMap is not updated. This will cause the bucket scan task not load balance.

This patch optimize the algorithm. When the mini_backend is not alive, update buckendIdToBucketCountMap.

Related Issue: #5132

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[caiconghui]

Suggested change

	buckendIdToBucketCountMap.put(backendIdRef.getRef(), buckendIdToBucketCountMap.get(backendIdRef.getRef())+1);
	buckendIdToBucketCountMap.put(backendIdRef.getRef(), buckendIdToBucketCountMap.get(backendIdRef.getRef()) + 1);

[caiconghui]

LGTM

[morningman]

@xinghuayu007 there are conflict, please resolve them first.

[xinghuayu007]

@xinghuayu007 there are conflict, please resolve them first.

Thank you for your code review. The conflict has been resolved.

[morningman]

LGTM

[kangkaisen]

+1