INFORMATIVE

Issue:
------
When memory allocation by range optimizer exceeds
"range_optimizer_max_mem_size", optimizer chooses a table
scan.

This can also happen with data truncation when column with
index is a varchar and value is an integer.

In the DELETE/UPDATE code, the check for SQL_SAFE_UPDATES
interprets a table scan as the absence of a WHERE clause.
So it rejects the DML statement with the error indicating
a violation of the safe update mode.

Solution:
---------
Append the warning (data truncation /
'range_optimizer_max_mem_size' exceeded) to the
safe update error. This will allow the user to understand
why index couldn't be used for the DELETE / UPDATE.

A few other changes:
1) No error will be thrown for EXPLAIN. This allows the
   user to understand why an index isn't used.
2) For Multi-Delete / Multi-Update throw the error only if
   the target table chooses a table scan.