On 08.01.25 17:38, Tom Lane wrote:
Peter Eisentraut <pe...@eisentraut.org> writes:
On 03.12.24 15:15, jian he wrote:
SELECT attrelid, attname, attgenerated FROM pg_attribute WHERE
attgenerated IN ('v') and (attnotnull or not atthasdef);
I don't understand what the purpose of testing attnotnull is. That is
independent of attgenerated, I think.
Does it make any sense to set NOT NULL on a generated column (virtual
or otherwise, but especially virtual)? What is the system supposed
to do if the expression evaluates to null? That concern generalizes
to any constraint really. Even if we checked it at row storage time,
there's no real guarantee that the expression is immutable enough
to pass the constraint later.
The generation expression is required to be immutable. So a table
definition like
a int,
b int generated always as (a * 2) virtual,
check (b > 0)
is not very different from
a int,
check (a * 2 > 0)
in terms of the constraint execution.
The current patch does not support not-null constraints, but that's
mostly because it's not implemented yet. Maybe that's what Jian was
thinking about.
