Fix new assertion for MERGE view_name ... DO NOTHING.
Such queries don't expand automatically updatable views, and ModifyTable
uses the wholerow attribute unconditionally. The user-visible behavior
is fine, so change to more-specific assertions. Commit
d5f788b41dc2cbdde6e7694c70dda54d829a5ed5 added the wrong assertion.
Back-patch to v17, where commit
5f2e179bd31e5f5803005101eb12a8d7bf8db8f3
introduced MERGE view_name.
Reported by Alexander Lakhin.
Discussion: https://postgr.es/m/
e4b40a88-c134-6926-3196-
bc4501cb87a2@gmail.com