[yimiaoxiehou]

Problem

On openGauss, every metadata command fails outright:

$ usql postgres://host:5432/db
pg:root@db=> \dt
error: failed to list tables: sql: Scan error on column index 5,
name "Description": converting NULL to string is unsupported

\d, \df, \dp and \l fail the same way.

Root cause

The readers guard nullable catalog columns with COALESCE(x, '') because
database/sql cannot convert NULL into a string or int. That guard is
useless under openGauss' Oracle-compatible mode (sql_compatibility=A),
where the empty string literal '' is itself NULL:

select ('' is null);             -- t
select coalesce(null::text,'');   -- NULL  (even ''::text does not help)

so COALESCE(x, '') evaluates to COALESCE(x, NULL) and the NULL still
reaches the scanner. An empty string cannot be represented at all under
these semantics, so the fix has to happen on the scan side rather than in
the SQL.

Fix

Add nullStr / nullInt / nullBool / nullBoolVal scan targets that
absorb NULL, and route every scan in the information_schema and
postgres metadata readers through them. The SQL is unchanged, so the
queries stay correct under both PostgreSQL and openGauss semantics.

nullInt deliberately replicates database/sql's asString conversion so
it accepts exactly the value types the plain *int destinations it replaces
accepted — otherwise a driver returning a number as text or as uint64
would start failing on the other databases sharing this reader (MySQL,
SQL Server, ...), which never had this problem. nullStr and nullBool
already matched the standard library, whose *string branch accepts only
string and []byte.

Also accept int64 for is_grantable: information_schema.yes_or_no is a
domain over boolean in PostgreSQL but is reported as a numeric YES/NO by
openGauss, which otherwise broke \dp with cannot scan int64 into bool.

Verification

Against openGauss 6.0.3 (sql_compatibility=A), 23 metadata commands —
\l \dn \dt \dt+ \di \d \d+ \dT \dtT \df \dp \dv \ds \z \dC \dD \dx \dew \deu \sf and pattern variants — all pass with no Scan errors. Output was also
checked for correctness, not just absence of errors: comments, column
defaults and nullability all render correctly.

nullscan_test.go asserts the targets accept exactly the value types
database/sql converts while absorbing NULL. Note that it cannot run
without a working Docker daemon, as TestMain in this package
unconditionally starts database containers.

Target branch

Opened against release-21 because main no longer contains this code:
drivers/metadata has been removed there (no obj_description or
COALESCE(column_default remain anywhere in the repository). The
0.21.x line is what still needs this fix.

[kenshaw]

@yimiaoxiehou there will not be any further releases on the release-21 branch, barring a security issue/fix. There is a new database meta model being used on the main branch that uses the dbmeta project/repo.

If openGauss needs its own metadata model, please point a coding agent at that repo and instruct (prompt) it to build a DIALECT for that database. If it connects using the PG driver, it would be another "flavor" of the PostgreSQL DIALECT, and is well doxumented. I will make a point to add this, but otherwise rejecting this PR as significantly different plans are in motion already.

That said, I do appreciate the contribution!