[PATCH] Rebuild CHECK constraints after generated column SET EXPRESSION

Hi,

I think I may have run into a small gap left by f80bedd52b1 ("Allow ALTER
COLUMN SET EXPRESSION on virtual columns with CHECK constraints"), and
wanted to share a patch and hear what others think.

After that commit, ALTER TABLE ... ALTER COLUMN ... SET EXPRESSION rebuilds
CHECK constraints that depend on the generated column. The rebuild list is
populated by RememberAllDependentForRebuilding(), which scans dependencies
on the column's attnum. That works well for ordinary column references,
but CHECK constraints that reference the table through a whole-row Var (for
example CHECK (tab IS NOT NULL)) do not record a per-column dependency on
the generated column, so they don't seem to be picked up. As a result,
SET EXPRESSION can change the generation expression in a way that makes
existing rows no longer satisfy the whole-row CHECK constraint, and the
ALTER TABLE still succeeds.

The attached patch handles this by remembering all CHECK constraints on the
relation during SET EXPRESSION, in addition to the objects found by the
per-column dependency scan. It's fairly conservative, but it keeps the
logic simple, and RememberConstraintForRebuilding() already de-duplicates
constraints found through both paths.

A more targeted alternative would be to inspect each CHECK expression and
only add constraints that contain a whole-row Var. I went with the simpler
approach here, but I'd be happy to switch to the narrower scan if that
seems preferable.

The patch adds regression coverage for both stored and virtual generated
columns.

- on unpatched master, SET EXPRESSION succeeds and leaves a row for which
the whole-row CHECK expression evaluates to false
- with the patch, the same SET EXPRESSION command is rejected with:

ERROR: check constraint "whole_row_check" of relation "..." is violated
by some row

Thoughts?

Regards,
Ayush