Hi!
>>>>> "Guilhem" == Guilhem Bichot <guilhem@stripped> writes:
Guilhem> Hello,
Guilhem> I see the default value for this variable is 62 (i.e. exhaustive search
Guilhem> for the best query plan, as we allow maximum ~64 tables in a join). It's
Guilhem> so in 5.0, 5.1, 6.0-codebase.
I think we only allow 61 tables in the join.
3 bits in the table map is reserved as markers:
#define MAX_TABLES (sizeof(table_map)*8-3) /* Max tables in join */
#define PARAM_TABLE_BIT (((table_map) 1) << (sizeof(table_map)*8-3))
#define OUTER_REF_TABLE_BIT (((table_map) 1) << (sizeof(table_map)*8-2))
#define RAND_TABLE_BIT (((table_map) 1) << (sizeof(table_map)*8-1))
Guilhem> I remember Timour explaining that indeed beyond 7 tables, exhaustive
Guilhem> search becomes very costly.
Guilhem> --optimizer-search-depth=0, documented as "make a reasonable choice",
Guilhem> chooses min(number of tables, 7) (see determine_search_depth() in
Guilhem> sql_select.cc).
Guilhem> Why is exhaustive (62) the default? I guess most users use the default
Guilhem> (naturally), which is why choosing a good default is important...
Guilhem> Does anyone have an idea or some background about this default?
I think the default should be MAX_TABLES, not MAX_TABLES+1
(don't understand the +1)
For the answer about the default value, we have wait for Timour.
Regards,
Monty
