Re: Eliminating unnecessary left joins

2007/4/16, Ottó Havasvölgyi <havasvolgyi(dot)otto(at)gmail(dot)com>:

> Eliminate the table T from the query/subquery if the following requirements
> are satisfied:
> 1. T is left joined
> 2. T is referenced only in the join expression where it is left joined
> 3. the left join's join expression is a simple equality expression like
> T1.C1=T2.C2; T1!=T2 and (T==T1 or T==T2)
> 4. the column of T in the join exression is the primary key of T

Condition 4 should be: the column of T in the join expression is a key
of T (i.e. it doesn't need to be the PK, a UNIQUE constraint would be
enough).

This process can be done recursively (implementation doesn't have to
be recursive, of course), to eliminate whole sub-trees of the join
tree.

Nicolas

--
Nicolas Barbier
http://www.gnu.org/philosophy/no-word-attachments.html