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

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


Nicolas Barbier

---------------------------(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

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