pg_class.relnatts excludes dropped columns despite retained pg_attribute entries

Describe the problem
Hi, CockroachDB developer. Thanks for reading my report. I find a bug in pg_class.relnatts.
After DROP COLUMN, CockroachDB retains the corresponding pg_attribute row with attisdropped = true and its original attnum, but decreases pg_class.relnatts to the number of live columns.

This is inconsistent with PostgreSQL catalog semantics, where relnatts includes dropped-column placeholders and therefore equals the number of positive-numbered pg_attribute entries.

To reproduce

DROP DATABASE IF EXISTS test_relnatts CASCADE;
CREATE DATABASE test_relnatts;
USE test_relnatts;

CREATE TABLE t0 (c0 INT PRIMARY KEY, c1 INT, c2 INT);
ALTER TABLE t0 DROP COLUMN c2;

SELECT c.relname, c.relnatts, COUNT(a.attnum) AS attr_cnt
FROM pg_catalog.pg_class AS c
LEFT JOIN pg_catalog.pg_attribute AS a
  ON a.attrelid = c.oid AND a.attnum > 0
WHERE c.relname = 't0'
GROUP BY c.oid, c.relname, c.relnatts;
relname | relnatts | attr_cnt
--------+----------+---------
t0      |        2 |        3

SELECT attname, attnum, attisdropped
FROM pg_catalog.pg_attribute
WHERE attrelid = 't0'::regclass AND attnum > 0
ORDER BY attnum;
            attname            | attnum | attisdropped
-------------------------------+--------+---------------
  c0                           |      1 |      f
  c1                           |      2 |      f
  ........pg.dropped.3........ |      3 |      t

Actual behavior
The dropped column remains as ........pg.dropped.3........ with attnum = 3 and attisdropped = true, while relnatts is 2.

relname | relnatts | attr_cnt
--------+----------+---------
t0      |        2 |        3

Expected behavior
pg_class.relnatts should be 3, matching the three positive-numbered pg_attribute entries, including the dropped-column placeholder:

relname | relnatts | attr_cnt
--------+----------+---------
t0      |        3 |        3

Environment

• CockroachDB: CCL v26.3.1, built 2026-08-24
• Deployment: official cockroachdb/cockroach:v26.3.1 Docker image
• Host: Linux x86-64, Ubuntu 20.04
• Clients: built-in CockroachDB SQL CLI and JDBC

Additional context
CockroachDB documents that its pg_catalog tables correspond to PostgreSQL system catalogs. In PostgreSQL, pg_class.relnatts includes dropped-column placeholders, which remain in pg_attribute with attisdropped = true. Therefore, PostgreSQL returns:

relname | relnatts | attr_cnt
--------+----------+---------
t0      |        3 |        3

CockroachDB instead returns 2 | 3, making pg_class inconsistent with pg_attribute. This may cause PostgreSQL-compatible schema-introspection tools, drivers, ORMs, migration utilities, and catalog-consistency checks to miscount columns or report malformed catalog metadata after a column is dropped.

References:

• https://docs.cockroachlabs.com/docs/v26.3/pg-catalog
• https://www.postgresql.org/docs/current/catalog-pg-class.html
• https://www.postgresql.org/docs/current/catalog-pg-attribute.html

Jira issue: CRDB-68547

Start by running the supplied SQL reproduction and compare pg_class.relnatts with the positive-numbered pg_attribute rows after DROP COLUMN. Trace the implementation of these pg_catalog values and check how PostgreSQL handles dropped-column placeholders. Done means relnatts reports 3 for the example and remains consistent with the retained pg_attribute entries.