[YSQL] Adding stored generated column causes a table rewrite

[arpang]

Jira Link: DB-17268

Description

create table test(amount int);
insert into test values (1), (2), (3);
ALTER TABLE test ADD COLUMN simple int GENERATED ALWAYS AS ( amount * 3 ) STORED;

Rewriting large tables can be very expensive, and consequently, the ALTER may not even succeed.

Issue Type

kind/enhancement

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