bzr commit into mysql-next-mr-bugfixing branch (jorgen.loland:3240) WL#4800

[Jorgen Loland]

#At file:///export/home/jl208045/mysql/wl4800/mysql-next-mr-opt-backporting-wl4800/ based on revid:guilhem@stripped

 3240 Jorgen Loland	2010-12-06
      WL#4800:
      The optimizer may have second thoughts about which access method
      to use for a table. This needs to be traced, so added item to 
      WL4800 TODO list.
     @ WL4800_TODO.txt
        Added todo-item

    modified:
      WL4800_TODO.txt
=== modified file 'WL4800_TODO.txt'
--- a/WL4800_TODO.txt	2010-11-24 18:54:26 +0000
+++ b/WL4800_TODO.txt	2010-12-06 13:38:41 +0000
@@ -47,3 +47,11 @@ http://lists.mysql.com/commits/124599
 
 check save_in_field_no_warnings() in opt_range.cc: do we need to
 disable tracing there? see optimizer_trace_no_prot.result.
+
+The optimizer may have second thoughts about which access method to
+use for a table. This should be traced. See example query (1) where
+the trace says that ref-access is best but we change our mind and use
+range access anyway.
+(1)CREATE TABLE t1 (a int, b int, PRIMARY KEY (a,b), KEY b (b));
+   INSERT INTO t1 VALUES (1,1),(1,2),(1,0),(1,3);
+   SELECT MAX(b), a FROM t1 WHERE b < 2 AND a = 1 GROUP BY a;


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