MySQL is too stupid to create a temporary table for use subquery, so we
have to give it some prompting in the form of a subsubquery.
# File activerecord/lib/arel/visitors/mysql.rb, line 65
def build_subselect(key, o)
subselect = super
# Materialize subquery by adding distinct
# to work with MySQL 5.7.6 which sets optimizer_switch='derived_merge=on'
unless has_limit_or_offset_or_orders?(subselect)
core = subselect.cores.last
core.set_quantifier = Arel::Nodes::Distinct.new
end
Nodes::SelectStatement.new.tap do |stmt|
core = stmt.cores.last
core.froms = Nodes::Grouping.new(subselect).as("__active_record_temp")
core.projections = [Arel.sql(quote_column_name(key.name))]
end
end
