Adding filter to vector scanner

[harsh-ps-2003]

Fixes #338

Changes
Adding a filtering function to the vector scanner which would take in a step vector and produce a new step vector with filtered samples to optimize the query execution process.

Verification
Added a test case sum(kube_pod_info == 1)

Tests are passing locally using make test

[harsh-ps-2003]

@fpetkovski the make tests are passing locally :

❯ make test
>> running unit tests (without cache)
?       github.com/thanos-io/promql-engine/api  [no test files]
ok      github.com/thanos-io/promql-engine/engine       589.694s
?       github.com/thanos-io/promql-engine/execution    [no test files]
?       github.com/thanos-io/promql-engine/execution/aggregate  [no test files]
?       github.com/thanos-io/promql-engine/execution/binary     [no test files]
?       github.com/thanos-io/promql-engine/execution/exchange   [no test files]
?       github.com/thanos-io/promql-engine/execution/function   [no test files]
?       github.com/thanos-io/promql-engine/execution/model      [no test files]
?       github.com/thanos-io/promql-engine/execution/noop       [no test files]
?       github.com/thanos-io/promql-engine/execution/remote     [no test files]
?       github.com/thanos-io/promql-engine/execution/parse      [no test files]
?       github.com/thanos-io/promql-engine/execution/step_invariant     [no test files]
?       github.com/thanos-io/promql-engine/execution/scan       [no test files]
?       github.com/thanos-io/promql-engine/execution/unary      [no test files]
?       github.com/thanos-io/promql-engine/execution/warnings   [no test files]
?       github.com/thanos-io/promql-engine/extlabels    [no test files]
ok      github.com/thanos-io/promql-engine/execution/storage    (cached)
?       github.com/thanos-io/promql-engine/query        [no test files]
?       github.com/thanos-io/promql-engine/worker       [no test files]
ok      github.com/thanos-io/promql-engine/logicalplan  (cached)
ok      github.com/thanos-io/promql-engine/ringbuffer   (cached)

but make lint is failing!
Please point out if I have messed up something!

[MichaHoffmann]

I dont think this is ever populated anywhere right?

[harsh-ps-2003]

There might be some gaps in my understanding (sorry) but I thought, the filterFunc field within the vectorScanner struct is being populated within the Next method of the vectorSelector struct. The Next method creates a new vectorScanner and assigns the filterFunc to it before returning the result. After the filterFunc is assigned within the Next method of the vectorSelector struct, the actual filtering process occurs when iterating through the vectors and applying the filterFunc to each StepVector. When the Next method is called, it returns a slice of model.StepVector instances. Each model.StepVector in the returned slice is processed by the filterFunc that was assigned to the corresponding vectorScanner during its creation. The filterFunc is applied to each model.StepVector to perform the desired filtration based on the defined logic within the filterFunc. Hence I dont understand how is it not getting populated!

[MichaHoffmann]

I mean it needs to be instantiated somewhere, right now it is always nil as it is never assigned any filter function. You would need to do that. Thats also what the linter is complaining about!

[MichaHoffmann]

I think we need to add a "FilterFunc" to logicalplan.VectorSelector and then pass it down from execute.go into the constructor of the scan.VectorScan operator. The logical filter should be populated by an optimizer that also removes the binary expression (which is the ultimate benefit ~ no binary operator on top of the vector scan anymore). I think that would be the way to implement the feature.

Edit: that is probably a lot of information! If you have any questions or i can help somehow, let me know!

[harsh-ps-2003]

@MichaHoffmann thanks for the guidance! I am kinda noob to PromQL engine, I am trying to solve this just for getting some exposure!