pure virtual method called
terminate called without an active exception
160826 10:14:21 [ERROR] mysqld got signal 6 ;
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.

To report this bug, see https://mariadb.com/kb/en/reporting-bugs

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.

Server version: 10.1.16-MariaDB
key_buffer_size=134217728
read_buffer_size=131072
max_used_connections=33
max_threads=1026
thread_count=33
It is possible that mysqld could use up to 
key_buffer_size + (read_buffer_size + sort_buffer_size)*max_threads = 2384644 K  bytes of memory
Hope that's ok; if not, decrease some variables in the equation.

Thread pointer: 0x0x7fc12a497008
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 = 0x7fc14c080d30 thread_stack 0x40000
/usr/sbin/mysqld(my_print_stacktrace+0x2e)[0x7fc15b61beee]
/usr/sbin/mysqld(handle_fatal_signal+0x2d5)[0x7fc15b143265]
/lib64/libpthread.so.0(+0xf100)[0x7fc15a761100]
/lib64/libc.so.6(gsignal+0x37)[0x7fc158abb5f7]
/lib64/libc.so.6(abort+0x148)[0x7fc158abcce8]
/lib64/libstdc++.so.6(_ZN9__gnu_cxx27__verbose_terminate_handlerEv+0x165)[0x7fc1591aa9b5]
/lib64/libstdc++.so.6(+0x5e926)[0x7fc1591a8926]
/lib64/libstdc++.so.6(+0x5e953)[0x7fc1591a8953]
/lib64/libstdc++.so.6(+0x5f4bf)[0x7fc1591a94bf]
/usr/sbin/mysqld(_Z18get_datetime_valueP3THDPPP4ItemS3_16enum_field_typesPb+0x3c)[0x7fc15b17587c]
/usr/sbin/mysqld(_ZN14Arg_comparator16compare_temporalE16enum_field_types+0x59)[0x7fc15b175c29]
/usr/sbin/mysqld(_ZN12Item_func_ge7val_intEv+0x2a)[0x7fc15b16ed5a]
/usr/sbin/mysqld(_ZN4Item8val_boolEv+0x9c)[0x7fc15b15476c]
/usr/sbin/mysqld(_ZN14Item_func_case9find_itemEP6String+0x182)[0x7fc15b178312]
/usr/sbin/mysqld(_ZN14Item_func_case6int_opEv+0x75)[0x7fc15b178425]
/usr/sbin/mysqld(_ZN27Item_func_hybrid_field_type7val_intEv+0x154)[0x7fc15b19f074]
/usr/sbin/mysqld(_ZN4Item13save_in_fieldEP5Fieldb+0x15c)[0x7fc15b152f0c]
/usr/sbin/mysqld(_Z21update_virtual_fieldsP3THDP5TABLE21enum_vcol_update_mode+0x82)[0x7fc15b06fc42]
/usr/sbin/mysqld(_Z13rr_sequentialP11READ_RECORD+0x88)[0x7fc15b24b228]
/usr/sbin/mysqld(_Z10sub_selectP4JOINP13st_join_tableb+0x1fb)[0x7fc15affa7cb]
/usr/sbin/mysqld(+0x47764d)[0x7fc15b00964d]
/usr/sbin/mysqld(_ZN4JOIN10exec_innerEv+0xb4c)[0x7fc15b01957c]
/usr/sbin/mysqld(_ZN4JOIN4execEv+0x54)[0x7fc15b01b574]
/usr/sbin/mysqld(_Z12mysql_selectP3THDPPP4ItemP10TABLE_LISTjR4ListIS1_ES2_jP8st_orderSB_S2_SB_yP13select_resultP18st_select_lex_unitP13st_select_lex+0x112)[0x7fc15b017c22]
/usr/sbin/mysqld(_Z13handle_selectP3THDP3LEXP13select_resultm+0x245)[0x7fc15b0186f5]
/usr/sbin/mysqld(+0x427b41)[0x7fc15afb9b41]
/usr/sbin/mysqld(_Z21mysql_execute_commandP3THD+0x5f8f)[0x7fc15afc5fff]
/usr/sbin/mysqld(_Z11mysql_parseP3THDPcjP12Parser_state+0x28e)[0x7fc15afc98de]
/usr/sbin/mysqld(_Z16dispatch_command19enum_server_commandP3THDPcj+0x25db)[0x7fc15afccd9b]
/usr/sbin/mysqld(_Z10do_commandP3THD+0x169)[0x7fc15afcd619]
/usr/sbin/mysqld(_Z24do_handle_one_connectionP3THD+0x18a)[0x7fc15b091c1a]
/usr/sbin/mysqld(handle_one_connection+0x40)[0x7fc15b091df0]
/lib64/libpthread.so.0(+0x7dc5)[0x7fc15a759dc5]
/lib64/libc.so.6(clone+0x6d)[0x7fc158b7cced]

Trying to get some variables.
Some pointers may be invalid and cause the dump to abort.
Query (0x7fc12a584020): SELECT * from custom_903
Connection ID (thread ID): 42
Status: NOT_KILLED

Optimizer switch: index_merge=on,index_merge_union=on,index_merge_sort_union=on,index_merge_intersection=on,index_merge_sort_intersection=off,engine_condition_pushdown=off,index_condition_pushdown=on,derived_merge=on,derived_with_keys=on,firstmatch=on,loosescan=on,materialization=on,in_to_exists=on,semijoin=on,partial_match_rowid_merge=on,partial_match_table_scan=on,subquery_cache=on,mrr=off,mrr_cost_based=off,mrr_sort_keys=off,outer_join_with_cache=on,semijoin_with_cache=on,join_cache_incremental=on,join_cache_hashed=on,join_cache_bka=on,optimize_join_buffer_size=off,table_elimination=on,extended_keys=on,exists_to_in=on,orderby_uses_equalities=off

The manual page at http://dev.mysql.com/doc/mysql/en/crashing.html contains
information that should help you find out what is causing the crash.