Table Elimination in MariaDB
The first thing the MariaDB optimizer does is to merge the
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
- and if there won't be a match,
LEFT JOINwill generate a NULL-complemented “row”
- and we don't care what the matching record is, as table
ac_dobis 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.
- This page is based on the following blog post about table elimination: http://s.petrunia.net/blog/?p=58
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