Re: bug when apply fast default mechanism for adding new column over domain with default value

On Sat, Mar 1, 2025 at 2:43 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> I do not believe that case should require a table rewrite.
> Both the default and the check constraint are immutable,
> so we ought to be able to apply the check once and then
> use the default as the attmissingval.
>
> > Attach a patch to fix this issue by cause it to table rewrite.
>
> I see no attached patch, but in any case forcing a table rewrite
> seems like the wrong direction...
>

forcing table rewrite would be an easier fix.
but not forcing table write seems doable.

 \d pg_attrdef
              Table "pg_catalog.pg_attrdef"
 Column  |     Type     | Collation | Nullable | Default
---------+--------------+-----------+----------+---------
 oid     | oid          |           | not null |
 adrelid | oid          |           | not null |
 adnum   | smallint     |           | not null |
 adbin   | pg_node_tree | C         | not null |
Indexes:
    "pg_attrdef_oid_index" PRIMARY KEY, btree (oid)
    "pg_attrdef_adrelid_adnum_index" UNIQUE CONSTRAINT, btree (adrelid, adnum)

pg_attrdef_adrelid_adnum_index means
a column can only have one default expression adbin.
if we store domain's default expression in pg_attrdef it may have error like:

CREATE DOMAIN int_domain AS int DEFAULT 11;
ALTER TABLE t2 ADD COLUMN b int_domain default 3;
ERROR:  duplicate key value violates unique constraint
"pg_attrdef_adrelid_adnum_index"
DETAIL:  Key (adrelid, adnum)=(18102, 2) already exists.

currently function StoreAttrDefault will
1. set pg_attribute.atthasdef
2. compute and set atthasmissing, attmissingval
3. insert an entry in pg_attrdef.
but we only want 2.

So I duplicated StoreAttrDefault and removed pg_attribute.atthasdef,
pg_attrdef related code.
and it works fine.