It is the addition of the fast_index_creation variable: http://www.percona.com/doc/percona-server/5.5/management/innodb_fast_index_creation.html?highlight=fast_index_creation#fast_index_creation
Post: MySQL Users Conference - Innodb
… amount of pages which do not fit in compressed space. Fast Index Creation This is another feature which I was asking Heikki about… tables may take a lot of time. Fast Index creation will allow Innodb to build indexes by sorting rather by row by row insertions… as for MyISAM tables by separate phase of building Indexes ? Will be UNIQUE indexes built by sorting in case of ALTER TABLE…
Post: Percona Server and XtraBackup weekly news, April 4th
… “fast index creation” for ALTER TABLE and OPTIMIZE TABLE, which can make these blocking operations much faster and result in less fragmented indexes. He also improved mysqldump so indexes are created with “fast index creation” when restoring a dump, which can…
Post: Percona Server 5.1.49-rel12.0
… innodb_flush_log_at_trx_commit. (Yasufumi Kinoshita) innodb_fast_index_creation – Allows disabling of fast index creation. (Yasufumi Kinoshita) innodb_use_sys_stats_table – If… – Fixes long stalls while accessing the innodb_buffer_pool_pages_index table on systems with a large number of tables. Bug…
Comment: ALTER TABLE: Creating Index by Sort and Buffer Pool Size
expand_fast_index_creation is a Percona Server feature, introduced in 5.1.56 and 5.5.11. http://www.mysqlperformanceblog.com/2011/11/06/improved-innodb-fast-index-creation/
Post: Percona Server 5.5.11-20.2 Stable Release
… to the latest stable version as April 11th, 2011. InnoDB fast index creation now works with mysqldump, ALTER TABLE and OPTIMIZE TABLE. (Alexey… table could fail due to a bug in innodb_expand_fast_index_creation.patch. (Alexey Kopytov) Bug #764395 – InnoDB crashed with an assertion…
Comment: Thinking about running OPTIMIZE on your Innodb Table ? Stop!
The InnoDB plugin documentation says that altering foreign keys on tables will cause a table copy instead of fast index creation. Is there an alternate way to optimize the creation of these indexes?
Post: XtraDB: The Top 10 enhancements
… statistics are inserted into the slow query log (see docs). Fast Crash Recovery – In the built-in InnoDB, the crash recovery… Features – XtraDB is derived from the InnoDB plugin, which has fast index creation (as opposed to recreating the whole table!) and page compression… allows you to delay the writing of non-unique secondary index pages, which can often lead to a lot of merged…
Post: InnoDB: look after fragmentation
… it (tough it is possible in XtraDB / InnoDB-plugin with FAST INDEX creation) so be careful with queries scan many records by secondary…
Comment: MySQL 5.0, 5.1 and Innodb Plugin CPU Efficiency
…)? Assuming both tests used tables created and indexed using the InnoDB Plugin with its new Fast Index Creation capability (which does not require Barracuda format), there should be no difference in the density of the index vs…

