Table Elimination in MariaDB

The first thing the MariaDB optimizer does is to merge the VIEW definition into the query to obtain:

select ACRAT_rating
from
  ac_anchor
  left join ac_name on ac_anchor.AC_ID=ac_name.AC_ID
  left join ac_dob on ac_anchor.AC_ID=ac_dob.AC_ID
  left join ac_rating on (ac_anchor.AC_ID=ac_rating.AC_ID and
                          ac_rating.ACRAT_fromdate = 
                            (select max(sub.ACRAT_fromdate)
                             from ac_rating sub where sub.AC_ID = ac_rating.AC_ID))
where
 ACNAM_name='Gary Oldman'

It's important to realize that the obtained query has a useless part:

• left join ac_dob on ac_dob.AC_ID=... will produce exactly one matching record:
  • primary key(ac_dob.AC_ID) guarantees that there will be at most one match for any value of ac_anchor.AC_ID,
  • and if there won't be a match, LEFT JOIN will generate a NULL-complemented “row”
• and we don't care what the matching record is, as table ac_dob is not used anywhere else in the query.

This means that the left join ac_dob on ... part can be removed from the query and this is what Table Elimination module does. The detection logic is rather smart, for example it would be able to remove the left join ac_rating on ... part as well, together with the subquery (in the above example it won't be removed because ac_rating used in the selection list of the query). The Table Elimination module can also handle nested outer joins and multi-table outer joins.

See Also

• This page is based on the following blog post about table elimination: http://s.petrunia.net/blog/?p=58