Re: Virtual generated columns

-- check constraints
CREATE TABLE gtest20 (a int PRIMARY KEY, b int GENERATED ALWAYS AS (a
* 2) VIRTUAL CHECK (b < 50));
INSERT INTO gtest20 (a) VALUES (10); -- ok
INSERT INTO gtest20 (a) VALUES (30); -- violates constraint

ALTER TABLE gtest20 ALTER COLUMN b SET EXPRESSION AS (a * 100); --
violates constraint
ALTER TABLE gtest20 ALTER COLUMN b SET EXPRESSION AS (a * 3); -- ok
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The above content is in src/test/regress/sql/generated_virtual.sql,
the last two query comments
seem to conflict with the error message for now.

i add some regress tests for your v10 changes in
src/backend/commands/statscmds.c.
please check attached.

the sql tests,
"sanity check of system catalog" maybe place it to the end of the sql
file will have better chance of catching some error.
for virtual, we can also check attnotnull, atthasdef value.
like:
SELECT attrelid, attname, attgenerated FROM pg_attribute WHERE
attgenerated IN ('v') and (attnotnull or not atthasdef);