There was a problem on MacOSX with a shared object ha_example.so.
  It used hash_search(). During build of ha_example.so no libmysys
  was specified. Since MacOSX had a hash_search() in the system
  library, it built the shared object so that the dynamic linker
  linked hash_search() to the system library, which caused a crash
  when called. To come around this, we renamed hash_search() to
  my_hash_search(), as we did long ago with hash_insert() and
  hash_reset(). However, this time we made the move complete with
  all names. To keep compatibility, we redefine the old names.
  Since every C and C++ file, that uses HASH, needs to include
  this file, the change is complete. Both names could be used
  in the code, but the my_* versions are recommended now.