here is an interesting question (to me)
suppose you wanted to be certain that either one of two or more columns
were present
Like in a user table, either a username or an email need to be present
to create a row.
You can't use not null because it's an either or situation.
what's the best method to accomplish this sort of constraint?
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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
