ColumnType.Length() Support

[hundredwatt]

The README states "This driver supports the ColumnType interface introduced in Go 1.8, with the exception of ColumnType.Length(), which is currently not supported."

I found the ColumnTypeLength function commented out here:

mysql/rows.go

Lines 66 to 68 in 578c4c8

	// func (rows *mysqlRows) ColumnTypeLength(i int) (length int64, ok bool) {
	// return int64(rows.rs.columns[i].length), true
	// }

Based on the discussion in #667, it appears there were no limiting factors preventing this from being implemented: #667 (comment).

@julienschmidt can you confirm? Is this something that no one has gotten to yet? Or did you find some sort of limitation trying to implement this?

[julienschmidt]

As written in the discussion at #667, the length MySQL reports is not the same as a string in Go with the same content would have. It seems to be the internal space requirement for storing it within MySQL, which is rather useless IMO.
If you think you can implement in a way, which produces reliable and useful lengths: go for it!

[kirs]

In my use case with this driver, I'm looking at columns that are received with resultset, and when the column/field is TINYINT(1) I want to treat it in a certain way that's different from when the field is TINYINT(2) or TINYINT(3).

Unless the client exposes ColumnType.Length(), there's no way for me to look that up.

I get that Length() can be misleading value for string types, but I'm wondering if we could make it work so that at least the length of other types could be looked up.

[kirs]

Could we expose a RawLength() where the func name would signal that it's not part of the Length() interface?

[methane]

MySQL is deprecating column width for integers, except tinyint(1).
https://dev.mysql.com/doc/relnotes/mysql/8.0/en/news-8-0-19.html#:~:text=Display%20width%20specification%20for%20integer%20data%20types%20was%20deprecated%20in%20MySQL%208.0.17%2C%20and%20now%20statements%20that%20include%20data%20type%20definitions%20in%20their%20output%20no%20longer%20show%20the%20display%20width%20for%20integer%20types%2C%20with%20these%20exceptions%3A

I reopen #1453 for it.