Add support for returning result rows as structs. (:as => :struct)

[joe07734]

This patch adds a new query option to return result rows as structs.

At Bandcamp we've been using this patch (and the other) in our fork of the mysql2 gem since the beginning. We think dot notation is cleaner and more object-oriented-looking, and wanted to use it in our code that processes query results. I believe it can also make the code faster if it's doing a lot of column accesses.

This is a default for all our queries:

opts = Mysql2::Client.default_query_options
opts[:as] = :struct

And query results can be used like this:

band = query(<<-SQL
    SELECT band.name, bio.bio
    FROM bands AS band
    LEFT JOIN bios AS bio ON bio.band_id = band.id
    WHERE band.id = #{band_id}
    SQL
).first

puts "#{band.name} - #{band.bio}"

I've been meaning to make these two pull requests since forever. We've had years of experience with them and think they're useful. If you have any questions about how Bandcamp's Linux/MySQL/Ruby stack works, just ask.

[joe07734]

Fixed for those failed tests, now going to update the unit tests for statements, like in the other PR.

[sodabrew]

Nit: make this the else case in the block below. (or the default case, and convert to a switch statement)

[joe07734]

Done.

[sodabrew]

This is still using an intermediary array in the row fetch loop above. Is that better/more efficient to create the struct once than to incrementally add members to the struct in the same way elements are pushed into the array or added into the hash? I think the latter would have more clarity / all three options being handled as cases in the same loop.

[joe07734]

I was trying to avoid modifying the the main loop for clarity! It also allowed me to factor the "cast_row_as_struct" code with a simple parameter list. The statement support added a lot of duplicate code and I really wanted to not contribute to that. Also, the struct.new native call has a nice array form.

[joe07734]

Happy to rethink this if you think it's critical.

[sodabrew]

These don't get GC marked, why does the struct variant need to be GC marked? (w->rowStruct above) (Or conversely, should these also get GC marked?)

[joe07734]

I admit I'm hazy on when something needs to be gc-marked and when it doesn't. The "prototype" struct needs to be marked so that it'll stick around while control hops between the gem and ruby, while enumerating the results. Each row result struct, tho', is an instance of this prototype struct and is made right here, just like the hash and array, and returned to the client asap and will be gc marked there. That's my understanding anyway.

[sodabrew]

This is also awesome. Few comments. Thank you for contributing upstream!