Cherry-picking a patch from Bug 12828477 from mysql-5.5

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.

mysql-test/r/information_schema.result

@@ -1849,5 +1849,119 @@ unlock tables;
 drop table t1;
 drop view v1;
 #
+# Test for bug #12828477 - "MDL SUBSYSTEM CREATES BIG OVERHEAD FOR
+#                           CERTAIN QUERIES TO INFORMATION_SCHEMA".
+#
+# Check that metadata locks which are acquired during the process
+# of opening tables/.FRMs/.TRG files while filling I_S table are
+# not kept to the end of statement. Keeping the locks has caused
+# performance problems in cases when big number of tables (.FRMs
+# or .TRG files) were scanned as cost of new lock acquisition has
+# increased linearly.
+drop database if exists mysqltest;
+create database mysqltest;
+use mysqltest;
+create table t0 (i int);
+create table t1 (j int);
+create table t2 (k int);
+#
+# Test that we don't keep locks in case when we to fill
+# I_S table we perform full-blown table open.
+#
+# Acquire lock on 't2' so upcoming RENAME is
+# blocked.
+lock tables t2 read;
+#
+# Switching to connection 'con12828477_1'. 
+#
+# The below RENAME should wait on 't2' while
+# keeping X lock on 't1'.
+rename table t1 to t3, t2 to t1, t3 to t2;
+#
+# Switching to connection 'con12828477_2'. 
+#
+# Wait while the above RENAME is blocked.
+# Issue query to I_S which will open 't0' and get
+# blocked on 't1' because of RENAME.
+select table_name, auto_increment from information_schema.tables where table_schema='mysqltest';
+#
+# Switching to connection 'con12828477_3'. 
+#
+# Wait while the above SELECT is blocked.
+#
+# Check that it holds no lock on 't0' so it can be renamed.
+rename table t0 to t4;
+#
+# Switching to connection 'default'.
+#
+#
+# Unblock the first RENAME.
+unlock tables;
+#
+# Switching to connection 'con12828477_1'. 
+#
+# Reap the first RENAME
+#
+# Switching to connection 'con12828477_2'. 
+#
+# Reap SELECT to I_S.
+table_name	auto_increment
+t0	NULL
+t1	NULL
+t2	NULL
+#
+# Switching to connection 'default'.
+#
+#
+# Now test that we don't keep locks in case when we to fill
+# I_S table we read .FRM or .TRG file only (this was the case
+# for which problem existed).
+#
+rename table t4 to t0;
+# Acquire lock on 't2' so upcoming RENAME is
+# blocked.
+lock tables t2 read;
+#
+# Switching to connection 'con12828477_1'. 
+#
+# The below RENAME should wait on 't2' while
+# keeping X lock on 't1'.
+rename table t1 to t3, t2 to t1, t3 to t2;
+#
+# Switching to connection 'con12828477_2'. 
+#
+# Wait while the above RENAME is blocked.
+# Issue query to I_S which will open 't0' and get
+# blocked on 't1' because of RENAME.
+select event_object_table, trigger_name from information_schema.triggers where event_object_schema='mysqltest';
+#
+# Switching to connection 'con12828477_3'. 
+#
+# Wait while the above SELECT is blocked.
+#
+# Check that it holds no lock on 't0' so it can be renamed.
+rename table t0 to t4;
+#
+# Switching to connection 'default'.
+#
+#
+# Unblock the first RENAME.
+unlock tables;
+#
+# Switching to connection 'con12828477_1'. 
+#
+# Reap the first RENAME
+#
+# Switching to connection 'con12828477_2'. 
+#
+# Reap SELECT to I_S.
+event_object_table	trigger_name
+#
+# Switching to connection 'default'.
+#
+#
+# Clean-up.
+drop database mysqltest;
+#
 # End of 5.5 tests
 #

mysql-test/t/information_schema.test

@@ -1543,8 +1543,6 @@ DROP TABLE t1, information_schema.tables;
 LOCK TABLES t1 READ, information_schema.tables READ;
 DROP TABLE t1;
 
-# Wait till all disconnects are completed
---source include/wait_until_count_sessions.inc
 
 #
 # Bug #43834    Assertion in Natural_join_column::db_name() on an I_S query
@@ -1608,6 +1606,186 @@ drop table t1;
 drop view v1;
 
 
+--echo #
+--echo # Test for bug #12828477 - "MDL SUBSYSTEM CREATES BIG OVERHEAD FOR
+--echo #                           CERTAIN QUERIES TO INFORMATION_SCHEMA".
+--echo #
+--echo # Check that metadata locks which are acquired during the process
+--echo # of opening tables/.FRMs/.TRG files while filling I_S table are
+--echo # not kept to the end of statement. Keeping the locks has caused
+--echo # performance problems in cases when big number of tables (.FRMs
+--echo # or .TRG files) were scanned as cost of new lock acquisition has
+--echo # increased linearly.
+--disable_warnings
+drop database if exists mysqltest;
+--enable_warnings
+create database mysqltest;
+use mysqltest;
+create table t0 (i int);
+create table t1 (j int);
+create table t2 (k int);
+
+--echo #
+--echo # Test that we don't keep locks in case when we to fill
+--echo # I_S table we perform full-blown table open.
+--echo #
+
+--echo # Acquire lock on 't2' so upcoming RENAME is
+--echo # blocked.
+lock tables t2 read;
+
+--echo #
+--echo # Switching to connection 'con12828477_1'. 
+--echo #
+connect (con12828477_1, localhost, root,,mysqltest);
+--echo # The below RENAME should wait on 't2' while
+--echo # keeping X lock on 't1'.
+--send rename table t1 to t3, t2 to t1, t3 to t2
+
+--echo #
+--echo # Switching to connection 'con12828477_2'. 
+--echo #
+connect (con12828477_2, localhost, root,,mysqltest);
+--echo # Wait while the above RENAME is blocked.
+let $wait_condition=
+  select count(*) = 1 from information_schema.processlist
+  where state = "Waiting for table metadata lock" and
+        info = "rename table t1 to t3, t2 to t1, t3 to t2";
+--source include/wait_condition.inc
+
+--echo # Issue query to I_S which will open 't0' and get
+--echo # blocked on 't1' because of RENAME.
+--send select table_name, auto_increment from information_schema.tables where table_schema='mysqltest'
+
+--echo #
+--echo # Switching to connection 'con12828477_3'. 
+--echo #
+connect (con12828477_3, localhost, root,,mysqltest);
+--echo # Wait while the above SELECT is blocked.
+let $wait_condition=
+  select count(*) = 1 from information_schema.processlist
+  where state = "Waiting for table metadata lock" and
+        info = "select table_name, auto_increment from information_schema.tables where table_schema='mysqltest'";
+--source include/wait_condition.inc
+
+--echo #
+--echo # Check that it holds no lock on 't0' so it can be renamed.
+rename table t0 to t4;
+
+--echo #
+--echo # Switching to connection 'default'.
+--echo #
+connection default;
+--echo #
+--echo # Unblock the first RENAME.
+unlock tables;
+
+--echo #
+--echo # Switching to connection 'con12828477_1'. 
+--echo #
+connection con12828477_1;
+--echo # Reap the first RENAME
+--reap
+
+--echo #
+--echo # Switching to connection 'con12828477_2'. 
+--echo #
+connection con12828477_2;
+--echo # Reap SELECT to I_S.
+--reap
+
+--echo #
+--echo # Switching to connection 'default'.
+--echo #
+connection default;
+
+--echo #
+--echo # Now test that we don't keep locks in case when we to fill
+--echo # I_S table we read .FRM or .TRG file only (this was the case
+--echo # for which problem existed).
+--echo #
+
+rename table t4 to t0;
+--echo # Acquire lock on 't2' so upcoming RENAME is
+--echo # blocked.
+lock tables t2 read;
+
+--echo #
+--echo # Switching to connection 'con12828477_1'. 
+--echo #
+connection con12828477_1;
+--echo # The below RENAME should wait on 't2' while
+--echo # keeping X lock on 't1'.
+--send rename table t1 to t3, t2 to t1, t3 to t2
+
+--echo #
+--echo # Switching to connection 'con12828477_2'. 
+--echo #
+connection con12828477_2;
+--echo # Wait while the above RENAME is blocked.
+let $wait_condition=
+  select count(*) = 1 from information_schema.processlist
+  where state = "Waiting for table metadata lock" and
+        info = "rename table t1 to t3, t2 to t1, t3 to t2";
+--source include/wait_condition.inc
+
+--echo # Issue query to I_S which will open 't0' and get
+--echo # blocked on 't1' because of RENAME.
+--send select event_object_table, trigger_name from information_schema.triggers where event_object_schema='mysqltest'
+
+--echo #
+--echo # Switching to connection 'con12828477_3'. 
+--echo #
+connection con12828477_3;
+--echo # Wait while the above SELECT is blocked.
+let $wait_condition=
+  select count(*) = 1 from information_schema.processlist
+  where state = "Waiting for table metadata lock" and
+        info = "select event_object_table, trigger_name from information_schema.triggers where event_object_schema='mysqltest'";
+--source include/wait_condition.inc
+
+--echo #
+--echo # Check that it holds no lock on 't0' so it can be renamed.
+rename table t0 to t4;
+
+--echo #
+--echo # Switching to connection 'default'.
+--echo #
+connection default;
+--echo #
+--echo # Unblock the first RENAME.
+unlock tables;
+
+--echo #
+--echo # Switching to connection 'con12828477_1'. 
+--echo #
+connection con12828477_1;
+--echo # Reap the first RENAME
+--reap
+
+--echo #
+--echo # Switching to connection 'con12828477_2'. 
+--echo #
+connection con12828477_2;
+--echo # Reap SELECT to I_S.
+--reap
+
+--echo #
+--echo # Switching to connection 'default'.
+--echo #
+connection default;
+disconnect con12828477_1;
+disconnect con12828477_2;
+disconnect con12828477_3;
+
+--echo #
+--echo # Clean-up.
+drop database mysqltest;
+
+
 --echo #
 --echo # End of 5.5 tests
 --echo #
+
+# Wait till all disconnects are completed
+--source include/wait_until_count_sessions.inc

sql/sql_show.cc

@@ -3157,6 +3157,10 @@ fill_schema_table_by_open(THD *thd, bool is_show_fields_or_keys,
   */
   thd->temporary_tables= NULL;
   close_thread_tables(thd);
+  /*
+    Release metadata lock we might have acquired.
+    See comment in fill_schema_table_from_frm() for details.
+  */
   thd->mdl_context.rollback_to_savepoint(open_tables_state_backup->mdl_system_tables_svp);
 
   thd->lex= old_lex;
@@ -3339,6 +3343,9 @@ try_acquire_high_prio_shared_mdl_lock(THD *thd, TABLE_LIST *table,
   @param[in]      db_name                  database name
   @param[in]      table_name               table name
   @param[in]      schema_table_idx         I_S table index
+  @param[in]      open_tables_state_backup Open_tables_state object which is used
+                                           to save/restore original state of metadata
+                                           locks.
   @param[in]      can_deadlock             Indicates that deadlocks are possible
                                            due to metadata locks, so to avoid
                                            them we should not wait in case if
@@ -3356,6 +3363,7 @@ static int fill_schema_table_from_frm(THD *thd, TABLE_LIST *tables,
                                       LEX_STRING *db_name,
                                       LEX_STRING *table_name,
                                       enum enum_schema_tables schema_table_idx,
+                                      Open_tables_backup *open_tables_state_backup,
                                       bool can_deadlock)
 {
   TABLE *table= tables->table;
@@ -3501,13 +3509,27 @@ static int fill_schema_table_from_frm(THD *thd, TABLE_LIST *tables,
 
 end_unlock:
   mysql_mutex_unlock(&LOCK_open);
-  /*
-    Don't release the MDL lock, it can be part of a transaction.
-    If it is not, it will be released by the call to
-    MDL_context::rollback_to_savepoint() in the caller.
-  */
 
 end:
+  /*
+    Release metadata lock we might have acquired.
+
+    Without this step metadata locks acquired for each table processed
+    will be accumulated. In situation when a lot of tables are processed
+    by I_S query this will result in transaction with too many metadata
+    locks. As result performance of acquisition of new lock will suffer.
+
+    Of course, the fact that we don't hold metadata lock on tables which
+    were processed till the end of I_S query makes execution less isolated
+    from concurrent DDL. Consequently one might get 'dirty' results from
+    such a query. But we have never promised serializability of I_S queries
+    anyway.
+
+    We don't have any tables open since we took backup, so rolling back to
+    savepoint is safe.
+  */
+  DBUG_ASSERT(thd->open_tables == NULL);
+  thd->mdl_context.rollback_to_savepoint(open_tables_state_backup->mdl_system_tables_svp);
   thd->clear_error();
   return res;
 }
@@ -3758,6 +3780,7 @@ int get_all_tables(THD *thd, TABLE_LIST *tables, COND *cond)
               int res= fill_schema_table_from_frm(thd, tables, schema_table,
                                                   db_name, table_name,
                                                   schema_table_idx,
+                                                  &open_tables_state_backup,
                                                   can_deadlock);
 
               thd->pop_internal_handler();