## Code Analysis

### The catalog advertises a default but does not expose its expression
`server/tables/pgcatalog/pg_attribute.go:120-132`
```go
hasDefault := col.Default != nil

attr := &pgAttribute{
	attname:   col.Name,
	attnotnull: !col.Nullable,
	atthasdef: hasDefault,
}
```
The catalog sets `atthasdef` from the presence of `col.Default`, so a column with `DEFAULT 9` is reported as having a default.

`server/tables/pgcatalog/pg_attrdef.go:103-118`
```go
// TODO: Implement adbin when pg_node_tree exists
return sql.Row{
	col.OID.AsId(),                  // oid
	tableOid,                        // adrelid
	int16(col.Item.ColumnIndex + 1), // adnum
	nil,                             // adbin
}, nil
```
The `pg_attrdef` row iterator unconditionally returns `nil` for `adbin`, so the stored default expression is not exposed through this catalog row.

`server/functions/pg_get_expr.go:32-52`
```go
// TODO: Implement this when the pg_node_tree type exists
return nil, errors.Errorf("pg_get_expr is not yet supported")
```
Both registered `pg_get_expr` overloads return the unsupported error instead of converting a catalog default expression to text. Together, these paths explain how `atthasdef=true` can coexist with an empty catalog expression.

### Observed catalog readback
The test created a named default and queried the relevant PostgreSQL-compatible catalogs:

```sql
DROP TABLE IF EXISTS catalog_named;
CREATE TABLE catalog_named (
  nn_col integer CONSTRAINT named_nn NOT NULL,
  default_col integer CONSTRAINT named_default DEFAULT 9,
  unique_col integer CONSTRAINT named_unique UNIQUE
);
```

The captured catalog output showed the default flag but no expression:

```text
pg_attribute:
   attname   | attnotnull | atthasdef
-------------+------------+-----------
 nn_col      | t          | f
 default_col | f          | t
 unique_col  | f          | f

pg_attrdef via pg_get_expr:
   attname   | default_expr
-------------+--------------
 default_col | (null)
```

Raw `pg_attrdef` readback also returned an empty `adbin` and empty `pg_get_expr` result. PostgreSQL-compatible behavior for `DEFAULT 9` requires the expression to read back as `9`; the observed result therefore supports the reported catalog metadata failure.

### Result
The executed catalog comparison failed for the named default: `atthasdef` was true, but the corresponding default expression was empty instead of `9`. The source excerpts identify the unconditional `nil` `adbin` value and unsupported `pg_get_expr` implementation as the production-code path behind the observed result.

### Test context
The other named constraint checks in the same comparison, including the unique constraint and not-null metadata, matched their expected forms.