2007/4/16, Ottó Havasvölgyi <[EMAIL PROTECTED]>:
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 ---------------------------(end of broadcast)--------------------------- TIP 9: In versions below 8.0, the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
