refactor(query engine): parallel inputs reading in aggregator

[forshev]

Description

Parallel inputs reading in aggregator executor

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.34%. Comparing base (38083e8) to head (d0b5630).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #527      +/-   ##
==========================================
+ Coverage   71.30%   71.34%   +0.04%     
==========================================
  Files         250      250              
  Lines       21783    21790       +7     
==========================================
+ Hits        15532    15546      +14     
+ Misses       5095     5092       -3     
+ Partials     1156     1152       -4     

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[dkharms]

We need to introduce some schema for these records -- now these constants are hard-coded all over the codebase.

Maybe we can do something like:

type Col[T any] struct {
      Idx   int
      Type  DataType
}

func (r *Record) Get[T any](c Col[T]) T {
      return r.Vals[c.Idx].Decoded().(T)
}

I am fine with leaving TODO for that and creating an issue.

[forshev]

that's correct. we will have to introduce some column and schema objects

[dkharms]

Do I understand correctly that we will start at most #shards goroutines? Should we limit how many concurrent goroutines we can start?