what is RELY constraint in hive external tables?

Question

managed table vs external table

in above link they mentioned

"Only the RELY constraint is allowed on external tables."

But no info that what is RELY constraint. Can somebody help me to understand what is Only the "RELY constraint". thanks in advance.

Answer 1

CBO is expected to take into account constraint, trust it if it is defined with RELY option. CBO may produce better plan and eliminate unnecessary joins.
See also Jira HIVE-13350