deparser: hash partition with REMAINDER 0 loses its entire FOR VALUES clause #350

Summary

For a hash partition with REMAINDER 0, the deparser omits the whole FOR VALUES WITH (...) clause, producing invalid SQL. REMAINDER 1, 2, … are fine.

Every hash-partitioned table has exactly one remainder-0 partition, so this affects every hash partitioning scheme.

Versions

• pgsql-deparser@18.3.6
• plpgsql-parser@18.5.8
• libpg-query@18.1.4
• Node 24

Reproduction

import { parse } from 'plpgsql-parser'
import { deparse } from 'pgsql-deparser'
import { loadModule } from 'libpg-query'
await loadModule()

for (const r of [0, 1, 2]) {
  const sql = `ALTER TABLE ONLY o ATTACH PARTITION o_p FOR VALUES WITH (MODULUS 4, REMAINDER ${r});`
  console.log(await deparse((await parse(sql)).sql))
}

Expected

ALTER TABLE ONLY o ATTACH PARTITION o_p FOR VALUES WITH (MODULUS 4, REMAINDER 0);

Actual

REMAINDER 0 -> ALTER TABLE ONLY o ATTACH PARTITION o_p;                                  <-- invalid
REMAINDER 1 -> ALTER TABLE ONLY o ATTACH PARTITION o_p FOR VALUES WITH (modulus 4, remainder 1);
REMAINDER 2 -> ALTER TABLE ONLY o ATTACH PARTITION o_p FOR VALUES WITH (modulus 4, remainder 2);

Cause

The AST loses the field, because protobuf JSON encoding omits zero-valued scalars:

REMAINDER 0 -> {"strategy":"h","modulus":4,"location":52}
REMAINDER 1 -> {"strategy":"h","modulus":4,"remainder":1,"location":52}

So remainder being absent is not distinguishable from remainder: 0 at the AST level — that part is arguably inherent to the encoding.

The actionable part is the deparser's response to it. Given strategy: "h", a missing remainder should be treated as 0 and rendered, rather than causing the entire FOR VALUES clause to be dropped. A hash bound is never valid without one, so the current behaviour cannot be correct in any case.

Aside

The clause is emitted in lower case (modulus / remainder). That is accepted by PostgreSQL, so it is cosmetic, but it differs from pg_dump's output if anyone is diffing rendered SQL as text.