create table ten(a int primary key);

insert into ten values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);

create table one_k(a int primary key);

insert into one_k select A.a + B.a* 10 + C.a * 100 from ten A, ten B, ten C;

create table t21 (

  a varchar(500) COLLATE utf8mb4_bin

);

insert into t21 select A.a+1000*B.a  from one_k A, one_k B;

set max_heap_table_size=32000; # just some low value to overflow the heap table.

select conc_val, count(*) from  t21 group by conc_val ;

(gdb) p maria_max_key_length()

  $53 = 1000

(gdb) p this

  $55 = (ha_maria * const) 0x7fff6c165f38

(gdb) p table->key_info[0]

  $57 = {key_length = 2000, flags = 2083, user_defined_key_parts = 1, ...