atomic.alter_table became more than 2x slow in after MDEV-20912, commit 7fcaab7aaac.

The reason is the server startup, that became much slower, in particular, the function init_compiled_charsets(). That line

649

my_bool add_alias_for_collation(LEX_CSTRING *collation_name, LEX_CSTRING *alias,

650

                                uint alias_id)

651

{

652

  char *coll_name, *comment;

653

  struct charset_info_st *new_ci;

654

  CHARSET_INFO *org;

655

  MY_CHARSET_LOADER loader;

656

  char comment_buff[64+15];

657

  size_t comment_length;

658

  uint org_id= get_collation_number_internal(collation_name->str);

659

  DBUG_ASSERT(org_id);

660

  DBUG_ASSERT(all_charsets[org_id]);

is called for every 0900 collation and it iterates the list of collations, so it's O(N²). If I comment this single function call away, the execution time of atomic.alter_table goes back to normal.

Note that this function is a complete waste of time, because collation ids are fixed and can never change, there is no need to look them up at runtime.