Re: Adding a stored generated column without long-lived locks

On 2026-Sep-21, Alberto Piai wrote:

> Repro:
>
> create table tgen.t_repro_1 (a numeric, b numeric);
> insert into tgen.t_repro_1 values ('1.0', '1.00'), ('1.0', '1.0');
> create unique index on tgen.t_repro_1 ((b::text));
> alter table tgen.t_repro_1
> add constraint chk_gen check (b is not distinct from a);
>
> alter table tgen.t_repro_1
> alter b add generated using constraint chk_gen stored;
>
> update tgen.t_repro_1 set a = a;
> ERROR: duplicate key value violates unique constraint "t_repro_1_b_idx"
> DETAIL: Key ((b::text))=(1.0) already exists.

Does this _matter_?

Does anybody want to have a generated numeric column that's identical to
the base column except it has more zeroes in the decimal part? Or to
generate a text column that's not binary identical to another text
column but compares equal when viewed through an nondeterministic
collation?

I think the answer is no.

I'm pretty okay with saying that if you want to add a new column that's
generated in such a way, then you have to go the normal route of using
the blocking command.

--
Álvaro Herrera Breisgau, Deutschland — https://www.EnterpriseDB.com/
"La experiencia nos dice que el hombre peló millones de veces las patatas,
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las patatas pelarían al hombre" (Ijon Tichy)