From: Chao Li <li.evan.chao@gmail.com>
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Subject: Fix domain fast defaults on empty tables
Message-Id: <7033D663-DDB4-4B35-922C-F33DE53B1502@gmail.com>
Date: Fri, 5 Jun 2026 15:48:00 +0800
Cc: Andrew Dunstan <andrew@dunslane.net>,
 jian he <jian.universality@gmail.com>
To: Postgres hackers <pgsql-hackers@lists.postgresql.org>
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 <https://www.postgresql.org/message-id/7033D663-DDB4-4B35-922C-F33DE53B1502%40gmail.com>
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Hi,

I tested "[a0b6ef29a] Enable fast default for domains with non-volatile =
constraints". After tracing some cases from the regression tests, I came =
up with this test case and found a bug:
```
evantest=3D# create domain d_div as int check (1 / (value - 1) > 0);
CREATE DOMAIN
evantest=3D# create table t (a int);
CREATE TABLE
evantest=3D# alter table t add column b d_div default 1;
ERROR:  division by zero
```

It looks like errors inside the CHECK expression itself, such as the =
int4div division-by-zero in this test, are still hard errors that can =
fail the ALTER TABLE command.

For the fix, I worked out two solutions:

Solution 1
=3D=3D=3D=3D=3D=3D=3D=3D

We can add PG_TRY/PG_CATCH to capture those hard errors. But as =
a0b6ef29a's commit message mentions, the fallback should only apply =
while evaluating the CoerceToDomain expression. To avoid broadly =
suppressing hard errors, I only catch hard errors from =
domain_check_safe(), which verifies the default value against the =
domain. The default expression itself is still evaluated with hard =
errors.

My concern with this solution is that there might be some error from =
domain_check_safe() that I am not aware of and that would be hidden by =
the try-catch. But that may be acceptable, because it matches the =
behavior before a0b6ef29a, so at least it is not a regression.

Solution 2
=3D=3D=3D=3D=3D=3D=3D=3D

This solution is simpler and is based on the purpose of the original =
feature. a0b6ef29a's commit message says the purpose of the feature is =
to avoid table rewriting. Since an empty table has no data to rewrite, =
we can bypass the fast path for empty tables.

The problem with this solution is that I currently use =
RelationGetNumberOfBlocks(rel) to decide whether the table is empty, so =
it cannot handle cases like:
```
Begin;
Delete from t;
Alter table t add column =E2=80=A6
```

See the attached patches for details. v1-s1 is solution 1, and v1-s2 is =
solution 2. Please let me know which solution is preferred.

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/


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Content-Disposition: attachment;
	filename=v1-s1-0001-Handle-throwing-domain-checks-when-probing-fas.patch
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TABLE=20test_add_domain_col=20ADD=20COLUMN=20g=20domain9=20DEFAULT=2014;=0A=
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TABLE=20test_add_domain_col=20ADD=20COLUMN=20g=20domain10=20DEFAULT=20=
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CREATE=20DOMAIN=20domain5=20AS=20int=20CHECK(value=20>=2010)=20DEFAULT=20=
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overrides=20domain's=20default=0A@@=20-325,8=20+337,8=20@@=20ALTER=20=
TABLE=20test_add_domain_col=20ADD=20COLUMN=20f=20domain7;=0A=20=0A=20--=20=
domain=20with=20both=20volatile=20and=20non-volatile=20CHECK=20=
constraints:=20the=0A=20--=20volatile=20one=20forces=20a=20table=20=
rewrite=0A-CREATE=20DOMAIN=20domain9=20AS=20int=20CHECK(value=20>=2010)=20=
CHECK((value=20+=20random(min=3D>1::int,=20max=3D>1))=20>=200);=0A-ALTER=20=
TABLE=20test_add_domain_col=20ADD=20COLUMN=20g=20domain9=20DEFAULT=2014;=0A=
+CREATE=20DOMAIN=20domain10=20AS=20int=20CHECK(value=20>=2010)=20=
CHECK((value=20+=20random(min=3D>1::int,=20max=3D>1))=20>=200);=0A+ALTER=20=
TABLE=20test_add_domain_col=20ADD=20COLUMN=20g=20domain10=20DEFAULT=20=
14;=0A=20=0A=20--=20virtual=20generated=20columns=20cannot=20have=20=
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--Apple-Mail=_919D82ED-2F95-4B87-A045-048DE2A3192A--