part: added least func

[thorfour]

Move least of a part into a function. Lifted out all the calls in table where we fetched the least to use this helper function.

Also changed add part calls in addparttogranule to use the internal form that takes the least row so it doesn't need to be called again.

This addresses #86

[brancz]

So if I'm understanding this right, it all boils down to parquet readers not being safe for concurrent reading (because the values buffer is concurrently reused)?

[thorfour]

So if I'm understanding this right, it all boils down to parquet readers not being safe for concurrent reading (because the values buffer is concurrently reused)?

That's my understanding

[brancz]

In that case, this seems like we're maybe fixing one specific case, but not the general problem and other cases where this data race can happen. I feel like we should then instead create a new *parquet.File instead of reusing one instance. If this ends up being too expensive we should use a sync.Pool to reuse them safely.

[thorfour]

In that case, this seems like we're maybe fixing one specific case, but not the general problem and other cases where this data race can happen. I feel like we should then instead create a new *parquet.File instead of reusing one instance. If this ends up being too expensive we should use a sync.Pool to reuse them safely.

That may be a solution. I want to write another read based unit test to see if it exacerbates this problem. That said, I think this change is still good because it lifts and shifts code that makes the code overall more readable.

[brancz]

That makes sense. Happy with that!