But if we could find a way to represent that it would make a lot of common use
cases much more convenient to use.
(But that sounds rather like pie in the sky, actually. Which other
databases can do that, and how do they do it?)
Oracle does it, by building a big index. Few people use it.
The people that use it are the people stuck by dogmatic rules about "every
table must have a primary key" or "every logical constraint must be protected
by a database constraint". Ie, database shops run by the CYA principle.
Or ones that actually believe that every table where possible should
have a primary key.
There are very, very few instances in good design where a table does not
have a primary key.
It has nothing to do with CYA.
Joshua D. Drake
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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
