Merge pull request #21318 from yahonda/pr21108

Support MySQL 5.7.8 which enables show_compatibility_56=off

activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb

@@ -727,8 +727,12 @@ def type_to_sql(type, limit = nil, precision = nil, scale = nil)
 
       # SHOW VARIABLES LIKE 'name'
       def show_variable(name)
-        variables = select_all("SHOW VARIABLES LIKE '#{name}'", 'SCHEMA')
-        variables.first['Value'] unless variables.empty?
+        begin
+          variables = select_all("select @@#{name} as 'Value'", 'SCHEMA')
+          variables.first['Value'] unless variables.empty?
+        rescue ActiveRecord::StatementInvalid => _e
+          nil
+        end
       end
 
       # Returns a table's primary key and belonging sequence.

activerecord/lib/active_record/connection_adapters/mysql_adapter.rb

@@ -223,7 +223,7 @@ def client_encoding
         return @client_encoding if @client_encoding
 
         result = exec_query(
-          "SHOW VARIABLES WHERE Variable_name = 'character_set_client'",
+          "select @@character_set_client",
           'SCHEMA')
         @client_encoding = ENCODINGS[result.rows.last.last]
       end