Tidb: executor: distsql.selectResult is not necessary to fetch asynchronously.

[SunRunAway]

Performance Questions

• What version of TiDB are you using?
  

ebfb2ae on the master branch

• What's the observed and your expected performance respectively?

distsql.selectResult uses kv.Response.Next() to fetch data from TiKV, since kv.Response is already asynchronously implemented by copIterator, perhaps distsql.selectResult is not necessary to fetch asynchronously again.

If deciding to do this, we need a benchmark result in avoiding performance regression.

[ichn-hu]

I am interested to investigate more on this issue, will start to read related source code!