pgsql: Fix ALTER TABLE / INHERIT with generated columns

Fix ALTER TABLE / INHERIT with generated columns

When running ALTER TABLE t2 INHERIT t1, we must check that columns in
t2 that correspond to a generated column in t1 are also generated and
have the same generation expression.  Otherwise, this would allow
creating setups that a normal CREATE TABLE sequence would not allow.

Discussion: https://www.postgresql.org/message-id/22de27f6-7096-8d96-4619-7b882932ca25@2ndquadrant.com

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/64190d65f2995203df401738b77adc5ebd4d2fce

Modified Files
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src/backend/commands/tablecmds.c        | 60 +++++++++++++++++++++++++++++++++
src/test/regress/expected/generated.out | 21 ++++++++++++
src/test/regress/sql/generated.sql      | 14 ++++++++
3 files changed, 95 insertions(+)