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

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.klaver@aklaver.com

Marcelo Fernandes <marcefern7@gmail.com> writes:
> 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.

Yup.

> 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?

You'd have to look into the source code:

https://github.com/postgres/postgres/blob/c4067383cb2c155c4cfea2351036709e2ebb3535/src/backend/utils/adt/numeric.c

The first few hundred lines of numeric.c are largely comments that
you'd find relevant.  Also see numeric_support():


https://github.com/postgres/postgres/blob/c4067383cb2c155c4cfea2351036709e2ebb3535/src/backend/utils/adt/numeric.c#L1179

ALTER TABLE skips table rewrite if the datatype conversion expression
is simplified to a no-op by a support function such as
numeric_support.

            regards, tom lane