`rows per task` of IndexLookUp is hardcoded
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sig/planner
type/enhancement
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Description
## Enhancement
As we can see, the `batchSize` is a const(default is 20000) and the `taskPerBatch` is 32.
This indicates that each task is expected to scan 625 rows.
It's a hardcoded value to lead to a suboptimal execution plan. (we may think that the number of tasks is too large and decide not to choose index scan. but one task may scan 10000 rows).
We need to consider the data distribution and calculate it more correctly.
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