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

Marcelo Fernandes <marcefern7(at)gmail(dot)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