Implement ColumnType interface

[julienschmidt]

See golang/go@2a85578 and golang/go#16652

[hasryan]

I have an immediate need for this functionality in a small project I am working on, so I hacked together a partial, very rough implementation for the single interface I need: hasryan@f2e81dd

When I was working on that I noticed that empty results are represented by the emptyRows{} struct which simply returns nil from its Columns() method. I think column details are still available from MySQL even if the result set is empty, so it would be nice if when these new interfaces are implemented it includes the ability to get column details for empty results as well.

My use case is one where I am executing ad-hoc queries against MySQL and replicating the results into another database (SQLite in my case). I need the type information from MySQL so that I can create appropriately-typed columns in the SQLite table I generate. An empty resultset is valid, and I would like to be able to create an empty table with correctly-typed columns.

[torkelo]

This i also something I would find very useful (working on MySQL data source support in Grafana where I need to visualize ad-hoc SQL queries in different forms) and knowing column types is going to be very important.

@hasryan any plans to open a PR for your implementation?

[hasryan]

@torkelo sorry for not responding sooner. I do not have any immediate plans to open a PR because I suspect my referenced implementation is not up to to the quality standards of this project. It's something I put together that worked for me in my specific use case, but I have not had enough time to review the codebase to know if it is consistent with existing patterns. Also, I only implemented one of the five new interfaces.