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

On 11/3/25 00:24, Marcelo Fernandes wrote:
> 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?

Short version:

https://www.postgresql.org/docs/current/datatype-numeric.html#DATATYPE-NUMERIC-DECIMAL

"
Numeric values are physically stored without any extra leading or
trailing zeroes. Thus, the declared precision and scale of a column are
maximums, not fixed allocations. (In this sense the numeric type is more
akin to varchar(n) than to char(n).) The actual storage requirement is
two bytes for each group of four decimal digits, plus three to eight
bytes overhead.
"

Long version, numeric.c:

https://github.com/postgres/postgres/blob/master/src/backend/utils/adt/numeric.c

>
> Thank you,
> Marcelo.
>
> - [0] https://www.postgresql.org/docs/current/datatype-numeric.html#DATATYPE-NUMERIC-DECIMAL
>
>

--
Adrian Klaver
adrian(dot)klaver(at)aklaver(dot)com