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 @@ end:
   */
   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 @@ end_share:
 
 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();