For numeric constants we only need to add, since the parser doesn't produce
negative numbers.
For strings we only add (we actually could substract 1 if given string is a constant
and it has '-number' form but we're not doing that because
* we set max_length bigger then necessary in other cases as well.
* the current solution is simpler and safer (bigger max_length is better then cutting out)
the result takes its charset/collation
attributes from the character string,
e.g. SELECT func(NULL, _latin2'string')
now returns a latin2 result. This is
done by introducing a new derivation
(aka coercibility) level DERIVATION_IGNORABLE,
which is used with Item_null.
2. 'Pure' NULL is now BINARY(0), not CHAR(0).
I.e. NULL is now more typeless.
TIMESTAMP columns should be unsigned to preserve compatibility with 4.0
(Or else InnoDB will return different internal TIMESTAMP values when user upgrades to 4.1).
Altough this fix will introduce problems with early 4.1 -> 4.1 upgrades (tables with
TIMESTAMP field should be reloaded using mysqldump) it will allow easy 4.0 -> 4.1
upgrade (which is more important since 4.1 is still beta).
