Comments on: Trivial MySQL Variable of the Day

By: tom

tom — Sat, 16 Apr 2011 19:40:42 +0000

http://bugs.mysql.com/bug.php?id=40344

By: MC Brown

MC Brown — Thu, 18 Jun 2009 07:42:18 +0000

Documentation have been alerted to the presence of this item and it will be fixed soon

By: Mark Leith

Mark Leith — Thu, 18 Jun 2009 07:38:59 +0000

Well, I also really wouldn’t call it “online alter table” – it’s not “online”, there are still metadata locks taken, etc. – it just happens a lot quicker because of the lack of the table copying.

I’ll point the docs team over here and get it documented better.

By: Sheeri Cabral

Sheeri Cabral — Wed, 17 Jun 2009 21:53:31 +0000

Mark — thanx, so I was half right — my guess on the functionality was right, it makes it so the new online changes are not online, but my guess that it wasn’t implemented yet was wrong.

Still, documentation would be nice.

By: Mark Leith

Mark Leith — Wed, 17 Jun 2009 20:44:00 +0000

Hi Sheeri,

It’s used to disable the optimizations that were added in 5.1 for “Faster Alter Table” – in ./sql/sql_table.cc down in the depths of mysql_alter_table():

if (thd->variables.old_alter_table
|| (table->s->db_type() != create_info->db_type)
#ifdef WITH_PARTITION_STORAGE_ENGINE
|| partition_changed
#endif
)
need_copy_table= ALTER_TABLE_DATA_CHANGED;
else
{
enum_alter_table_change_level need_copy_table_res;
/* Check how much the tables differ. */
if (compare_tables(table, alter_info,
create_info, order_num,
&need_copy_table_res,
&key_info_buffer,
&index_drop_buffer, &index_drop_count,
&index_add_buffer, &index_add_count,
&candidate_key_count))
goto err;

if (need_copy_table == ALTER_TABLE_METADATA_ONLY)
need_copy_table= need_copy_table_res;
}

So – if you have change an ENUM value, or rename a column, or change a column default etc. it will still follow the old 5.0 way of making a temporary table, copy over, and switch etc.

I can’t imagine why anybody would want to use this, other than testing / debugging. :)