postgresql-12-first-last-agg binary package in Ubuntu Focal s390x

 first-last-agg is a simple extension providing two aggregate functions, last
 and first, operating on any element type and returning the last or the first
 value of the group.
 .
 Since by default the ordering inside groups created by a GROUP BY expression
 is not defined, it is advisable to use an ORDER BY clause inside the aggregate
 expression (see the aggregate function syntax).

Source package