Version: '5.6.20-68.0'  socket: '/var/run/mysqld/mysqld.sock'  port: 3306  Percona Server (GPL), Release 68.0, Revision 656
14:51:06 UTC - mysqld got signal 11 ;
This could be because you hit a bug. It is also possible that this binary
or one of the libraries it was linked against is corrupt, improperly built,
or misconfigured. This error can also be caused by malfunctioning hardware.
We will try our best to scrape up some info that will hopefully help
diagnose the problem, but since we have already crashed, 
something is definitely wrong and this may fail.
Please help us make Percona Server better by reporting any
bugs at http://bugs.percona.com/

key_buffer_size=16777216
read_buffer_size=131072
max_used_connections=1
max_threads=153
thread_count=1
connection_count=1
It is possible that mysqld could use up to 
key_buffer_size + (read_buffer_size + sort_buffer_size)*max_threads = 77370 K  bytes of memory
Hope that's ok; if not, decrease some variables in the equation.

Thread pointer: 0x2d4a150
Attempting backtrace. You can use the following information to find out
where mysqld died. If you see no messages after this, something went
terribly wrong...
stack_bottom = 7f4758299e30 thread_stack 0x30000
/usr/sbin/mysqld(my_print_stacktrace+0x2c)[0x90608c]
/usr/sbin/mysqld(handle_fatal_signal+0x352)[0x662992]
/lib/x86_64-linux-gnu/libpthread.so.0(+0xfcb0)[0x7f47730c6cb0]
/lib/x86_64-linux-gnu/libc.so.6(+0x13ce78)[0x7f477283ce78]
/usr/sbin/mysqld(_ZNSt8_Rb_treeIP14dict_foreign_tS1_St9_IdentityIS1_E20dict_foreign_compareSaIS1_EE11equal_rangeERKS1_+0x9d)[0xa83bdd]
/usr/sbin/mysqld[0xa8360a]
/usr/sbin/mysqld[0x945912]
/usr/sbin/mysqld(_Z17mysql_alter_tableP3THDPcS1_P24st_ha_create_informationP10TABLE_LISTP10Alter_infojP8st_orderb+0x1f81)[0x747641]
/usr/sbin/mysqld(_ZN19Sql_cmd_alter_table7executeEP3THD+0x482)[0x83b9c2]
/usr/sbin/mysqld(_Z21mysql_execute_commandP3THD+0x1509)[0x6ef099]
/usr/sbin/mysqld(_Z11mysql_parseP3THDPcjP12Parser_state+0x5c8)[0x6f4dc8]
/usr/sbin/mysqld(_Z16dispatch_command19enum_server_commandP3THDPcj+0x21f6)[0x6f7606]
/usr/sbin/mysqld(_Z24do_handle_one_connectionP3THD+0x20d)[0x6bd2ed]
/usr/sbin/mysqld(handle_one_connection+0x39)[0x6bd369]
/usr/sbin/mysqld(pfs_spawn_thread+0x140)[0xb61950]
/lib/x86_64-linux-gnu/libpthread.so.0(+0x7e9a)[0x7f47730bee9a]
/lib/x86_64-linux-gnu/libc.so.6(clone+0x6d)[0x7f47727f24bd]

Trying to get some variables.
Some pointers may be invalid and cause the dump to abort.
Query (7f4720004f50): is an invalid pointer
Connection ID (thread ID): 1
Status: NOT_KILLED

You may download the Percona Server operations manual by visiting
http://www.percona.com/software/percona-server/. You may find information
in the manual which will help you identify the cause of the crash.