enum optimization

[rspadim]

hi guys, could anyone help me developing this MDEV (mariadb, maybe usefull for twitter too, since it's a nice feature that optimize many queries using ENUM fields)

https://mariadb.atlassian.net/browse/MDEV-4419

it's a enum optimization without index (but can help index too)
example:
a field with => enum ('a','b') NOT NULL

WHERE enum = 'c' => rewrite to WHERE 0 (impossible where)
WHERE enum != 'c' => rewrite to enum IN ('','a','b') OR WHERE 1 (always true)
WHERE enum!='b' => rewrite to enum IN ('','a') (COND_OK)

check that we can use any operator (LIKE for example) the key is:
read all enum possible values, execute operator for each enum value, if we have all false return it's a impossible where, if we have all true we have a always true, if we don't have all true and all false we can rewrite to IN () operator and use very optimized index queries (using HASH or BTREE for example)