On Mon, Dec 26, 2005 at 15:12:48 -0500, Gregory Maxwell <[EMAIL PROTECTED]> wrote: > On 12/26/05, Pavel Stehule <[EMAIL PROTECTED]> wrote: > > (1,1) <* (1,2) = true > > (1,2) <* (2,1) is NULL > > (2,3) <* (1,2) = false > > > > it's usefull for multicriterial optimalisation > > This is indeed a sane and useful function which should be adopted by > the SQL standard.. in postgresql this would easily enough be > implemented as a user function so I'm not sure we need special support > for it. > > The idea is that in a multidimension comparison you can only sometimes > say when one tuple is strictly less than (or greater than) another > because differing dimensions are incomparable. So, like his example, > we can not say if (1,2) is lesser or greater than (2,1) because saying > so would require some priority of the dimensions which may not be > known or may not exist, it is only clear that they are not equal..
That's normally called a partial order. From CS classes, I don't remember ever seeing < and > combined that way. Normally you just looked at whether or not < was true or whether or not > was true. Is it common in practice for people to want the answer to both of these at once? Also if you are really dealing with <= and >= (as your example above seems to imply), then there are 4 possible states (the new one being a = b) and you can't represent that with just 3 possible results. ---------------------------(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