Extract check for VALUES syntax

The change to upserts (rails#51274) is causing
an issue with Vitess, since it doesn't support the `row_alias` syntax added
in MySQL 8.0.19.

There is an ongoing work (vitessio/vitess#15510)
to add that support and it is likely to be included into Vitess v20,
but in the meantime it would be nice to have an ability to control that
behavior in the existing apps.

activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb

@@ -643,33 +643,33 @@ def build_insert_sql(insert) # :nodoc:
 
         # MySQL 8.0.19 replaces `VALUES(<expression>)` clauses with row and column alias names, see https://dev.mysql.com/worklog/task/?id=6312 .
         # then MySQL 8.0.20 deprecates the `VALUES(<expression>)` see https://dev.mysql.com/worklog/task/?id=13325 .
-        if !mariadb? && database_version >= "8.0.19"
-          values_alias = quote_table_name("#{insert.model.table_name}_values")
-          sql = +"INSERT #{insert.into} #{insert.values_list} AS #{values_alias}"
+        if supports_values_syntax?
+          sql = +"INSERT #{insert.into} #{insert.values_list}"
 
           if insert.skip_duplicates?
-            sql << " ON DUPLICATE KEY UPDATE #{no_op_column}=#{values_alias}.#{no_op_column}"
+            sql << " ON DUPLICATE KEY UPDATE #{no_op_column}=#{no_op_column}"
           elsif insert.update_duplicates?
+            sql << " ON DUPLICATE KEY UPDATE "
             if insert.raw_update_sql?
-              sql = +"INSERT #{insert.into} #{insert.values_list} ON DUPLICATE KEY UPDATE #{insert.raw_update_sql}"
+              sql << insert.raw_update_sql
             else
-              sql << " ON DUPLICATE KEY UPDATE "
-              sql << insert.touch_model_timestamps_unless { |column| "#{insert.model.quoted_table_name}.#{column}<=>#{values_alias}.#{column}" }
-              sql << insert.updatable_columns.map { |column| "#{column}=#{values_alias}.#{column}" }.join(",")
+              sql << insert.touch_model_timestamps_unless { |column| "#{column}<=>VALUES(#{column})" }
+              sql << insert.updatable_columns.map { |column| "#{column}=VALUES(#{column})" }.join(",")
             end
           end
         else
-          sql = +"INSERT #{insert.into} #{insert.values_list}"
+          values_alias = quote_table_name("#{insert.model.table_name}_values")
+          sql = +"INSERT #{insert.into} #{insert.values_list} AS #{values_alias}"
 
           if insert.skip_duplicates?
-            sql << " ON DUPLICATE KEY UPDATE #{no_op_column}=#{no_op_column}"
+            sql << " ON DUPLICATE KEY UPDATE #{no_op_column}=#{values_alias}.#{no_op_column}"
           elsif insert.update_duplicates?
-            sql << " ON DUPLICATE KEY UPDATE "
             if insert.raw_update_sql?
-              sql << insert.raw_update_sql
+              sql = +"INSERT #{insert.into} #{insert.values_list} ON DUPLICATE KEY UPDATE #{insert.raw_update_sql}"
             else
-              sql << insert.touch_model_timestamps_unless { |column| "#{column}<=>VALUES(#{column})" }
-              sql << insert.updatable_columns.map { |column| "#{column}=VALUES(#{column})" }.join(",")
+              sql << " ON DUPLICATE KEY UPDATE "
+              sql << insert.touch_model_timestamps_unless { |column| "#{insert.model.quoted_table_name}.#{column}<=>#{values_alias}.#{column}" }
+              sql << insert.updatable_columns.map { |column| "#{column}=#{values_alias}.#{column}" }.join(",")
             end
           end
         end
@@ -894,6 +894,10 @@ def remove_index_for_alter(table_name, column_name = nil, **options)
           "DROP INDEX #{quote_column_name(index_name)}"
         end
 
+        def supports_insert_values_syntax?
+          !mariadb? && database_version >= "8.0.19"
+        end
+
         def supports_rename_index?
           if mariadb?
             database_version >= "10.5.2"