Support default expression and expression indexes for MySQL

[kamipo]

MySQL 8.0.13 is released a few days ago.

https://mysqlserverteam.com/the-mysql-8-0-13-maintenance-release-is-generally-available/

Now we can supports default expression and expression indexes for mysql2 adapter.

[chrismaximin]

Why would we want to dup quote_column_name(name) here?

[kamipo]

This is because the quoted columns would be mutated by add_index_length and add_index_sort_order:

rails/activerecord/lib/active_record/connection_adapters/mysql/schema_statements.rb

Lines 109 to 112 in a3dcba4

	def add_index_length(quoted_columns, **options)
	lengths = options_for_index_columns(options[:length])
	quoted_columns.each do |name, column|
	column << "(#{lengths[name]})" if lengths[name].present?

rails/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb

Lines 1195 to 1198 in a3dcba4

	def add_index_sort_order(quoted_columns, **options)
	orders = options_for_index_columns(options[:order])
	quoted_columns.each do |name, column|
	column << " #{orders[name].upcase}" if orders[name].present?

[chrismaximin]

Ah I see, I thought quote_column_name would return something that was already duped somehow... because it is the case of

rails/activerecord/lib/active_record/connection_adapters/abstract/quoting.rb

Lines 67 to 69 in 6149080

	def quote_column_name(column_name)
	column_name.to_s
	end

... but I didn't realise the method may always return the same object like ...

rails/activerecord/lib/active_record/connection_adapters/mysql/quoting.rb

Lines 7 to 9 in 6149080

	def quote_column_name(name)
	@quoted_column_names[name] ||= "`#{super.gsub('`', '``')}`"
	end

... got it :)

I'm just realising that there's no way (AFAIK) to know whether a method returns an object that is referenced elsewhere or not, which is kind of sad, because we may be duplicating strings for no reason (if a method returns a new object or an existing object), maybe we should do something about that one day.

[chrismaximin]

Nevermind, I misread this whole thing 👍