Some useful things for debugging optimizer code with gdb.

Useful Print Functions

• dbug_print_item() prints the contents of an Item object into a buffer and returns pointer to it.
• dbug_print_sel_arg() prints an individual SEL_ARG object (NOT the whole graph or tree) and returns pointer to the buffer holding the printout.
• dbug_print_table_row prints the current row buffer of the given table.
• There are more dbug_print_XX functions for various data structures

Printing the Optimizer Trace

The optimizer trace is collected as plain text. One can print the contents of the trace collected so far as follows:

printf "%s\n", thd->opt_trace->current_trace->current_json->output.str.Ptr

Printing Current Partial Join Prefix

best_access_path() is a function that adds another table to the join prefix.

When in or around that function, the following can be useful:

A macro to print the join prefix already constructed:

define bap_print_prefix
  set $i=0
  while ($i < idx)
    p join->positions[$i++].table->table->alias.Ptr
  end
end

Other Settings

• May need to set innodb_fatal_semaphore_wait_threshold to be high enough?

See Also

• How to collect large optimizer traces