pgsql: Forbid generated columns in FOR PORTION OF

Forbid generated columns in FOR PORTION OF

With virtual generated columns there is no column to assign to, and we
shouldn't assign directly to stored generated columns either. (Once
we have PERIODs, we will allow a stored generated column here, but we
will assign to its start/end inputs.)

We can't do this in parse analysis, because views haven't yet been
rewritten, so they mask generated columns.

Author: Paul A. Jungwirth <pj(at)illuminatedcomputing(dot)com>
Discussion: https://www.postgresql.org/message-id/agOOykf2HV26yVfU%40nathan

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e994f956e4864f424320f5243b9af11e173ad398

Modified Files
--------------
doc/src/sgml/ddl.sgml | 4 +-
src/backend/optimizer/plan/planner.c | 26 ++++++++++
src/test/regress/expected/for_portion_of.out | 73 ++++++++++++++++++++++++++++
src/test/regress/sql/for_portion_of.sql | 57 ++++++++++++++++++++++
4 files changed, 159 insertions(+), 1 deletion(-)