uuid_generate_v4() evaluated at table creation instead of runtime

[PabloPerezAguilo]

Describe the bug

noticed that when defining a table with a column like:

CREATE TABLE "organizations" (
"organization_id" uuid NOT NULL DEFAULT uuid_generate_v4(),
...
)

pg-mem calls the registered uuid_generate_v4 immediately during table creation, and logs a UUID. This means the column’s DEFAULT is set to that single fixed UUID, rather than a function that should be executed on each INSERT.

As a result:

The first insert works.

Any subsequent insert fails with a duplicate key, because the default UUID is reused.

In PostgreSQL real, DEFAULT uuid_generate_v4() executes at runtime for each insert, generating a new UUID every time.

It would be helpful if this behavior were either:

Adjusted to match PostgreSQL (call the function at runtime per insert), or

Explicitly documented as a known limitation, so users don’t get unexpected duplicate PKs.

Expected (not working)

import { randomUUID } from 'crypto';

db.public.registerFunction({
  name: 'uuid_generate_v4',
  returns: DataType.uuid,
  implementation: randomUUID,
});

It works:

import { randomUUID } from 'crypto';

db.public.registerFunction({
  name: 'uuid_generate_v4',
  returns: DataType.uuid,
  implementation: ()=>randomUUID,
});

pg-mem version

3.0.5