`generation_expression` in `information_schema.columns` is NULL for a generated column

[mattchristenson]

On DoltgreSQL 1.3.1, information_schema.columns answers NULL in generation_expression for the stored
generated column b int GENERATED ALWAYS AS (a + 1) STORED, although the column computes its values and
is_generated answers ALWAYS. PostgreSQL 18.6 answers the expression, (a + 1).

Reproduction

repro.sql:

-- Print NULL as (null), unlike an empty string.
\pset null '(null)'

-- A generated column whose expression needs no brackets.
CREATE TABLE t (
    a int,
    b int GENERATED ALWAYS AS (a + 1) STORED
);

-- The generated column works.
INSERT INTO t (a) VALUES (1);
SELECT a, b FROM t;

-- The generated column in information_schema.columns.
SELECT is_generated, generation_expression,
       column_default
FROM information_schema.columns
WHERE table_name = 't' AND column_name = 'b';

Expected behavior

The generated column computes b = 2 for a = 1, and information_schema.columns answers its expression
in generation_expression, next to is_generated ALWAYS and a NULL column_default. This is what
PostgreSQL 18.6 does:

-- The generated column works.
INSERT INTO t (a) VALUES (1);
INSERT 0 1
SELECT a, b FROM t;
 a | b 
---+---
 1 | 2
(1 row)

-- The generated column in information_schema.columns.
SELECT is_generated, generation_expression,
       column_default
FROM information_schema.columns
WHERE table_name = 't' AND column_name = 'b';
 is_generated | generation_expression | column_default 
--------------+-----------------------+----------------
 ALWAYS       | (a + 1)               | (null)
(1 row)

Actual behavior

The generated column computes the same value, and is_generated and column_default answer the same, but
generation_expression is NULL. This is what DoltgreSQL 1.3.1 does:

-- The generated column works.
INSERT INTO t (a) VALUES (1);
INSERT 0 1
SELECT a, b FROM t;
 a | b 
---+---
 1 | 2
(1 row)

-- The generated column in information_schema.columns.
SELECT is_generated, generation_expression,
       column_default
FROM information_schema.columns
WHERE table_name = 't' AND column_name = 'b';
 is_generated | generation_expression | column_default 
--------------+-----------------------+----------------
 ALWAYS       | (null)                | (null)
(1 row)

Run it

A runnable reproduction is at https://github.com/Reliable-Collaboration/repro-doltgresql-bug-generation-expression. Its script runs the test on PostgreSQL and DoltgreSQL in throwaway containers and prints the two outputs side by side:

git clone https://github.com/Reliable-Collaboration/repro-doltgresql-bug-generation-expression.git
cd repro-doltgresql-bug-generation-expression
./repro.sh

Other observations

Each was checked on DoltgreSQL 1.3.1 and PostgreSQL 18.6 with the same kind of test:

• generation_expression is NULL on DoltgreSQL for every generated column tried, while is_generated
  answers ALWAYS and column_default NULL on both servers: (upper(s)), (length(s)), (a),
  (a::text), (a * 2) added with ALTER TABLE ... ADD COLUMN, and (a + 1) in a table in another
  schema. PostgreSQL answers upper(s), length(s), a, (a)::text, (a * 2) and (a + 1).
• It stays NULL from a new connection and, on DoltgreSQL, after SELECT dolt_commit('-Am', 'generated').
• ((a + 1) * 2) answers NULL too, where PostgreSQL answers ((a + 1) * 2). The test uses an expression
  without brackets because that column stores 3 for a = 1 on DoltgreSQL 1.3.1, where PostgreSQL stores 4,
  a separate bug.
• In pg_attribute, attgenerated is s on both servers, but atthasdef is f on DoltgreSQL and t on
  PostgreSQL. pg_attrdef has no row for the generated column on DoltgreSQL; on PostgreSQL its row answers
  (a + 1) from pg_get_expr(adbin, adrelid).
• For a plain DEFAULT 7, pg_attrdef has a row on DoltgreSQL, but pg_get_expr(adbin, adrelid) answers
  NULL, where PostgreSQL answers 7. column_default answers 7 on both.
• dtd_identifier is NULL for both columns of the table on DoltgreSQL, where PostgreSQL answers 1 and
  2.
• For an identity column, id int GENERATED ALWAYS AS IDENTITY, DoltgreSQL answers is_generated ALWAYS,
  is_identity NO and a NULL identity_generation, where PostgreSQL answers NEVER, YES and ALWAYS.
• PostgreSQL 18.6 creates GENERATED ALWAYS AS (a + 1) VIRTUAL, and the same clause without STORED, as
  virtual columns (attgenerated v). DoltgreSQL refuses both, with
  ERROR: at or near "virtual": syntax error and ERROR: at or near ")": syntax error.
• Another report on the same view: dolthub/doltgresql#3244,
  where information_schema.columns answers blank metadata for the columns of a view.

Possibly related

#3244 (open), about information_schema.columns returning blank metadata for view columns, concerns the same view but a different fault.

Environment

• DoltgreSQL 1.3.1, the newest release when this was written: image dolthub/doltgresql:1.3.1, digest
  sha256:6c85cb1f35beabf47f094336a420255130b841b1645f36d79ef046276af36851, built for linux/amd64 and
  linux/arm64. Its bundled psql is 17.11.
• PostgreSQL 18.6: image postgres:18.6-bookworm, digest
  sha256:1c59e2c3c818eaa0f0628f695b36e7c9e362d6b219b36a54a32df645cbd7e1af. Its psql is 18.6.
• Reproduced on 2026-09-11 (UTC) with Docker 29.7.2 on Ubuntu 26.04.1 LTS under WSL2 (Linux 6.18.33.2,
  x86_64).