to mysql-5.5.16-release.

Original revision:
# revision-id: dmitry.lenev@oracle.com-20110811155849-feyt3h7tj48padiu
# parent: tatjana.nuernberg@oracle.com-20110811120945-c6x9a5d2du8s9oj2
# committer: Dmitry Lenev <Dmitry.Lenev@oracle.com>
# branch nick: mysql-5.5-12828477
# timestamp: Thu 2011-08-11 19:58:49 +0400
# message:
#   Fix for bug #12828477 - "MDL SUBSYSTEM CREATES BIG OVERHEAD 
#   FOR CERTAIN QUERIES TO INFORMATION_SCHEMA".
#   
#   The problem was that metadata locking subsystem introduced
#   too much overhead for queries to I_S which were processed by
#   opening only .FRM or .TRG files and had to scanned a lot of
#   tables (e.g. SELECT COUNT(*) FROM I_S.TRIGGERS was affected). 
#   The same effect was not observed for similar queries which 
#   performed full-blown table open in order to fill I_S table.
#   
#   The problem stemmed from the fact that in case when I_S 
#   implementation opened only .FRM or .TRG file for each table 
#   processed it didn't release metadata lock it has acquired on 
#   the table after finishing its processing. As result, list
#   of acquired metadata locks were growing until the end of 
#   statement. Since acquisition of each new lock required 
#   search in the list of already acquired locks performance
#   degraded.
#   
#   The same effect is not observed when I_S implementation
#   performs full-blown table open for each table being
#   processed, as in the latter cases metadata lock on the
#   table is released right after table processing.
#   
#   This fix addressed the problem by ensuring that I_S 
#   implementation releases metadata lock after processing
#   the table in both cases of full-blown table open and in 
#   case when only .FRM or .TRG file is read.