The 64 table limit on joins was discussed with a proposed solution based on the change to
key_map in the thread 16713 some time ago:
This has become much more of a constraint when implementing a partly schemaless design
with an attribute table (ala couchDb etc.) where you might need a self-join for every
field in the logical table. I noticed the declaration ulonglong table_map is still in
both 5.1 and 5.4 vs. key_map which is Bitmap<64>. I wonder if anyone has an
opinion if this is a safe change to make for table_map?