revno: 2876.47.174
revision-id: jorgen.loland@oracle.com-20110519120355-qn7eprkad9jqwu5j
parent: mayank.prasad@oracle.com-20110518143645-bdxv4udzrmqsjmhq
committer: Jorgen Loland <jorgen.loland@oracle.com>
branch nick: mysql-trunk-11765831
timestamp: Thu 2011-05-19 14:03:55 +0200
message:
  BUG#11765831: 'RANGE ACCESS' MAY INCORRECTLY FILTER 
                        AWAY QUALIFYING ROWS
        
  The problem was that the ranges created when OR'ing two 
  conditions could be incorrect. Without the bugfix, 
  "I <> 6 OR (I <> 8 AND J = 5)" would create these ranges:
  
  "NULL < I < 6",
  "6 <= I <= 6 AND 5 <= J <= 5",
  "6 < I < 8",
  "8 <= I <= 8 AND 5 <= J <= 5",
  "8 < I"
  
  While the correct ranges is
  "NULL < I < 6",
  "6 <= I <= 6 AND 5 <= J <= 5",
  "6 < I"
  
  The problem occurs when key_or() ORs
  (1) "NULL < I < 6, 6 <= I <= 6 AND 5 <= J <= 5, 6 < I" with 
  (2) "8 < I AND 5 <= J <= 5"
  
  The reason for the bug is that in key_or(), SEL_ARG *tmp is 
  used to point to the range in (1) above that is merged with 
  (2) while key1 points to the root of the red-black tree of 
  (1). When merging (1) and (2), tmp refers to the "6 < I" 
  part whereas the root is the "6 <= ... AND 5 <= J <= 5" part. 
  
  key_or() decides that the tmp range needs to be split into
  "6 < I < 8, 8 <= I <= 8, 8 < I", in which next_key_part of the 
  second range should be that of tmp. However, next_key_part is
  set to key1->next_key_part ("5 <= J <= 5") instead of 
  tmp->next_key_part (empty). Fixing this gives the correct but
  not optimal ranges:
  "NULL < I < 6",
  "6 <= I <= 6 AND 5 <= J <= 5",
  "6 < I < 8",
  "8 <= I <= 8",
  "8 < I"
  
  A second problem can be seen above: key_or() may create 
  adjacent ranges that could be replaced with a single range. 
  Fixes for this is also included in the patch so that the range
  above becomes correct AND optimal:
  "NULL < I < 6",
  "6 <= I <= 6 AND 5 <= J <= 5",
  "6 < I"
  
  Merging adjacent ranges like this gives a slightly lower cost 
  estimate for the range access.