Add support for MAXFILESIZE option in UNLOAD

[atrigent]

The UNLOAD command now has a very useful MAXFILESIZE option to control the maximum size of the produced files. The UnloadFromSelect construct in the dialect should have support for that.

http://docs.aws.amazon.com/redshift/latest/dg/r_UNLOAD.html

[jleclanche]

I wanted to implement this but then, what would the API look like? The syntax is max-size [MB | GB]. So we have two values to choose.

Choices:

• UnloadFromSelect(max_file_size_mb=123), UnloadFromSelect(max_file_size_gb=6.2) (mutually-exclusive)
• UnloadFromSelect(max_file_size=(123, "MB")), UnloadFromSelect(max_file_size=(6.2, "GB")) (tuple int or float with string)
• UnloadFromSelect(max_file_size=(123, Size.MB)), UnloadFromSelect(max_file_size=(6.2, Size.GB)) (tuple int or float with enum)
• UnloadFromSelect(max_file_size=123), UnloadFromSelect(max_file_size=6.2 * 1024) (only accept megabytes, auto-calculate when to show GB instead [if we even need to? Does the syntax allow for "6348 MB"?])

[graingert]

what sort of MB are these? MiB or MB?

[jleclanche]

Docs don't specify. Probably MiB, but would have to test.

[graingert]

once someone has found out, I think this should accept bytes and convert to "MB"

[jleclanche]

I don't think a bytes-level API is ideal, makes it a little impractical to use :/

[graingert]

@jleclanche bytes is perfect, then people can multiply using our own flags, or their own:

class Size(enum.IntEnum):
    MiB = 2**20
    MB = 10**6
    GiB = 2**30
    GB =  10**9

UnloadFromSelect(max_file_size=123 * Size.MB) // can use our flags
UnloadFromSelect(max_file_size=123_000_000) // can use fancy new _ numbers
UnloadFromSelect(max_file_size=123 * 1024 * 1024) // can use their own multiplies