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Corresponds to the SQL DEFAULT expression.

This Expr is unsafe for numerous reasons, and should be used with care:

1. This Expr only makes sense in an INSERT or UPDATE statement.
2. Rel8 is not able to verify that a particular column actually has a DEFAULT value. Trying to use unsafeDefault where there is no default will cause a runtime crash
3. DEFAULT values cannot be transformed. For example, the innocuous Rel8 code unsafeDefault + 1 will crash, despite type checking.

Also note, PostgreSQL's syntax rules mean that DEFAULT can only appear in INSERT expressions whose rows are specified using VALUES. This means that if the rows field of your Insert record doesn't look like values [..], then unsafeDefault won't work.

Given all these caveats, we suggest avoiding the use of default values where possible, instead being explicit. A common scenario where default values are used is with auto-incrementing identifier columns. In this case, we suggest using nextval instead.