Hi, I have encountered an unexpected behavior where the DEFERRABLE and INITIALLY DEFERRED properties of foreign keys are lost after toggling them from NOT ENFORCED to ENFORCED.
Background
In the Ruby on Rails framework, there is a built-in mechanism to
temporarily bypass foreign key checks while loading test data.
Currently, this is implemented using: ALTER TABLE ... DISABLE TRIGGER
ALL; ALTER TABLE ... ENABLE TRIGGER ALL;
However, this requires superuser privileges. With the newly introduced
support for "NOT ENFORCED" foreign keys in PostgreSQL 18, I am
interested in switching to: ALTER TABLE ... ALTER CONSTRAINT ... NOT
ENFORCED; ALTER TABLE ... ALTER CONSTRAINT ... ENFORCED;
This would allow the operation to be performed by the table owner
without superuser rights. However, I discovered that switching the
state back to ENFORCED unexpectedly strips away the DEFERRABLE
property.
Problem
When re-enforcing a constraint, there is a discrepancy between the
constraint definition and its underlying triggers.
While the flags in pg_constraint remain correct, the corresponding
triggers in pg_trigger (tgdeferrable and tginitdeferred) are reset to
defaults ('f') when they are reconstructed during the ENFORCED
operation.
I have attached a reproduction SQL script that demonstrates this by
comparing the values in pg_constraint and pg_trigger. In the current
PostgreSQL 18.3, you can see that the triggers lose their
deferrability even though the constraint itself is still defined as
DEFERRABLE. This causes "SET CONSTRAINTS ... DEFERRED" to fail.
After Patch
* The tgdeferrable and tginitdeferred flags in pg_trigger are
correctly preserved to match pg_constraint after the toggle, and
deferred execution works as expected.
I've attached the reproduction SQL script and a patch to fix this in
src/backend/commands/tablecmds.c. The patch and reproduction SQL
script were developed with the assistance of Claude Code. I have
reviewed and verified the code myself.
Any feedback is appreciated.
Regards,
--
Yasuo Honda
0001-Restore-tgdeferrable-and-tginitdeferred-after-NOT-EN.patch
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tgdeferrable-tginitdeferred-not-restored.sql
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