Re: Increasing a NUMERIC column precision doesn't cause a table rewrite. Why?

From: Marcelo Fernandes <marcefern7@gmail.com>

Date: Monday, November 3, 2025 at 5:24 AM

To: pgsql-general@lists.postgresql.org <pgsql-general@lists.postgresql.org>

Subject: Increasing a NUMERIC column precision doesn't cause a table rewrite. Why?

Hi everyone,

A numeric field is defined as: NUMERIC(precision, scale) [0].

After doing some playing around with different operations against a numeric

column, namely: increasing/decreasing the precision or increasing/decreasing

the scale, I noticed that the table is rewritten in all cases except when

increasing the precision number.

Take the following snippet as example (tested on postgres 15):

```sql

CREATE DATABASE test_precision_field;

-- \c test_precision_field

SET client_min_messages=debug1;

DROP TABLE IF EXISTS foo;

CREATE TABLE foo (id SERIAL PRIMARY KEY, bar NUMERIC(10, 2));

-- Only increase precision but keep the scale:

-- This does not rewrite the table

ALTER TABLE foo ALTER COLUMN bar TYPE numeric(20, 2);

-- Only decrease the precision but keep the scale:

-- This rewrites the table.

ALTER TABLE foo ALTER COLUMN bar TYPE numeric(10, 2);

-- DEBUG:  rewriting table "foo"

-- DEBUG:  building index "foo_pkey" on table "foo" serially

-- DEBUG:  index "foo_pkey" can safely use deduplication

-- Only increase the scale, but keep the precision:

-- This rewrites the table.

ALTER TABLE foo ALTER COLUMN bar TYPE numeric(10, 4);

-- DEBUG:  rewriting table "foo"

-- DEBUG:  building index "foo_pkey" on table "foo" serially

-- DEBUG:  index "foo_pkey" can safely use deduplication

-- Only decrease the scale, but keep the precision:

-- This rewrites the table.

ALTER TABLE foo ALTER COLUMN bar TYPE numeric(10, 2);

-- DEBUG:  rewriting table "foo"

-- DEBUG:  building index "foo_pkey" on table "foo" serially

-- DEBUG:  index "foo_pkey" can safely use deduplication

```

My assumption is that once the precision is increased, Postgres can still

interpret the old values that were generated with a lower precision, whereas

this is not possible for any other types of changes on the numeric field.

Is that assumption correct? How can I verify it?

For example, is there a catalog table I can have a look to see how these values

are defined, along with some form of documentation to understand how they are

interpreted by Postgres?

Thank you,

Marcelo.

- [0] https://nam10.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.postgresql.org%2Fdocs%2Fcurrent%2Fdatatype-numeric.html%23DATATYPE-NUMERIC-DECIMAL&data=05%7C02%7Cluis.weck%40pismo.io%7C27632e42b9a1441b1e5d08de1ab275e8%7C38305e12e15d4ee888b9c4db1c477d76%7C0%7C0%7C638977550961297908%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=KTGfeCLZXuC8s7hk%2B5K%2BDNNu4BCuAjnCo9VDSSdxfZ0%3D&reserved=0