from MariaDB 10.0.
  
The bug in mdev-3948 was an instance of the problem fixed by Sergey's patch
in 10.0 - namely that the range optimizer could change table->[read | write]_set,
and not restore it.
  
revno: 3471
committer: Sergey Petrunya <psergey@askmonty.org>
branch nick: 10.0-serg-fix-imerge
timestamp: Sat 2012-11-03 12:24:36 +0400
message:
  # MDEV-3817: Wrong result with index_merge+index_merge_intersection, InnoDB table, join, AND and OR conditions
  Reconcile the fixes from:
  #
  # guilhem.bichot@oracle.com-20110805143029-ywrzuz15uzgontr0
  # Fix for BUG#12698916 - "JOIN QUERY GIVES WRONG RESULT AT 2ND EXEC. OR
  # AFTER FLUSH TABLES [-INT VS NULL]"
  #
  # guilhem.bichot@oracle.com-20111209150650-tzx3ldzxe1yfwji6
  # Fix for BUG#12912171 - ASSERTION FAILED: QUICK->HEAD->READ_SET == SAVE_READ_SET
  # and
  #
  and related fixes from: BUG#1006164, MDEV-376:
  
  Now, ROR-merged QUICK_RANGE_SELECT objects make no assumptions about the values
  of table->read_set and table->write_set.
  Each QUICK_ROR_SELECT has (and had before) its own column bitmap, but now, all 
  QUICK_ROR_SELECT's functions that care: reset(), init_ror_merged_scan(), and 
  get_next()  will set table->read_set when invoked and restore it back to what 
  it was before the call before they return.

  This allows to avoid the mess when somebody else modifies table->read_set for 
  some reason.