[Enhancement] optimize the performace for topn with large offset (backport #55886) #56369
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Why I'm doing:
SSB100G dop=4 1BE
baseline:1m42s patched:28s339ms
The reasons why baseline performance is too low are:
1.The merge operation is too frequent and needs to be done every 256 chunks. But the total input data is too large. This results in too many merge operations.
This PR adds max_buffer_size, which depends on offset + limit /chunk_size, to reduce the frequency of merge operations. But it may result in using more memory.
This PR additionally optimizes memory for merge chunks. It can reduce the peak memory of merge.
What I'm doing:
Fixes #issue
What type of PR is this:
Does this PR entail a change in behavior?
If yes, please specify the type of change:
Checklist: