Support Descending Indexes for MySQL

MySQL 8.0.1 and higher supports descending indexes: `DESC` in an index
definition is no longer ignored.

See https://dev.mysql.com/doc/refman/8.0/en/descending-indexes.html.

4 files changed, 14 insertions, 4 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index ffe588b3c5..f74425b281 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,10 @@
+*   Support Descending Indexes for MySQL.
+
+    MySQL 8.0.1 and higher supports descending indexes: `DESC` in an index definition is no longer ignored.
+    See https://dev.mysql.com/doc/refman/8.0/en/descending-indexes.html.
+
+    *Ryuta Kamizono*
+
 *   Fix inconsistency with changed attributes when overriding AR attribute reader.
 
     *bogdanvlviv*
diff --git a/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb b/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
index 0866f58015..71fa7b929c 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
@@ -90,10 +90,8 @@ module ActiveRecord
         true
       end
 
-      # Technically MySQL allows to create indexes with the sort order syntax
-      # but at the moment (5.5) it doesn't yet implement them
       def supports_index_sort_order?
-        true
+        !mariadb? && version >= "8.0.1"
       end
 
       def supports_transaction_isolation?
diff --git a/activerecord/lib/active_record/connection_adapters/mysql/schema_statements.rb b/activerecord/lib/active_record/connection_adapters/mysql/schema_statements.rb
index 1e58513387..571edffec7 100644
--- a/activerecord/lib/active_record/connection_adapters/mysql/schema_statements.rb
+++ b/activerecord/lib/active_record/connection_adapters/mysql/schema_statements.rb
@@ -38,6 +38,7 @@ module ActiveRecord
 
               indexes.last.columns << row[:Column_name]
               indexes.last.lengths.merge!(row[:Column_name] => row[:Sub_part].to_i) if row[:Sub_part]
+              indexes.last.orders.merge!(row[:Column_name] => :desc) if row[:Collation] == "D"
             end
           end
 
diff --git a/activerecord/test/cases/schema_dumper_test.rb b/activerecord/test/cases/schema_dumper_test.rb
index fccba4738f..cb8d449ba9 100644
--- a/activerecord/test/cases/schema_dumper_test.rb
+++ b/activerecord/test/cases/schema_dumper_test.rb
@@ -182,7 +182,11 @@ class SchemaDumperTest < ActiveRecord::TestCase
     if current_adapter?(:PostgreSQLAdapter)
       assert_equal 't.index ["firm_id", "type", "rating"], name: "company_index", order: { rating: :desc }', index_definition
     elsif current_adapter?(:Mysql2Adapter)
-      assert_equal 't.index ["firm_id", "type", "rating"], name: "company_index", length: { type: 10 }', index_definition
+      if ActiveRecord::Base.connection.supports_index_sort_order?
+        assert_equal 't.index ["firm_id", "type", "rating"], name: "company_index", length: { type: 10 }, order: { rating: :desc }', index_definition
+      else
+        assert_equal 't.index ["firm_id", "type", "rating"], name: "company_index", length: { type: 10 }', index_definition
+      end
     else
       assert_equal 't.index ["firm_id", "type", "rating"], name: "company_index"', index_definition
     end