by copying only a prefix of the row that covers the requested
columns.


innobase/include/row0mysql.h:
  row_prebuilt_struct: Add field mysql_prefix_len
  in order to reduce memcpy() time in
  row_sel_pop_cached_row_for_mysql().
innobase/row/row0sel.c:
  row_sel_pop_cached_row_for_mysql(): memcpy() only
  mysql_prefix_len bytes instead of mysql_row_len.
sql/ha_innodb.cc:
  build_template(): Initialize prebuilt->mysql_prefix_len.