Frank van Vugt <[EMAIL PROTECTED]> writes: > When the 'article.id < 50' constraint is added, it follows that > 'foo.article_id < 50' is a constraint as well. Why is this constraint not > used to avoid the seqscan on package?
We don't attempt to make every possible inference (and I don't think you'd like it if we did). The current code will draw inferences about transitive equality, for instance given a = b and b = c it will infer a = c, if all three operators involved are mergejoinable. But given a = b and some arbitrary other constraint on b, it won't consider substituting a into that other constraint. This example doesn't persuade me that it would be worth expending the cycles to do so. Aside from the sheer cost of planning time, there are semantic pitfalls to consider. In some datatypes there are values that are "equal" according to the = operator but are distinguishable by other operators --- for example, zero and minus zero in IEEE-standard float arithmetic. We'd need a great deal of caution in determining what inferences can be drawn. regards, tom lane ---------------------------(end of broadcast)--------------------------- TIP 8: explain analyze is your friend