Re: Technical validation on altering atttypmod for numeric column in PostgreSQL

On Mon, 2026-03-23 at 13:02 +0000, Mauricio Martini wrote:
> I am evaluating an approach to change the scale of a numeric column from numeric(18,2)
> to numeric(18,4) in a large table, aiming to avoid the cost of a full table rewrite.
>
> The proposed approach involves directly updating the PostgreSQL system catalog:
>
> UPDATE pg_attribute SET atttypmod = (18 << 16 | 4) + 4 WHERE attrelid = 'table'::regclass
>    AND attname  = 'column';
>
> Before considering this in practice, I would like to validate a few points:
>  * Is this approach considered safe from a data integrity perspective?
>  * Is there a risk of inconsistencies in the internal representation of the numeric
>    type (especially regarding scale)?
>  * Could this impact indexes, functions, or aggregation operations?
>  * Is there any official recommendation or prior experience using this approach in
>    production environments?
>  * Are there additional risks related to rollback, maintenance, or future operations
>    (e.g., dump/restore, upgrades)?
>
> The goal is to determine whether this alternative is viable, or if we should stick with more
> standard approaches (e.g., shadow column, incremental migration, etc.).
> If anyone has experience with a similar scenario, your insights would be appreciated.

It should be safe to perform this catalog modification, but I won't take any
liability.  "If it breaks, you get to keep both pieces."

Two things come to my mind when considering such a dangerous activity:

- LOCK the table in ACCESS EXCLUSIVE node while you perform the operation

- make sure there is no view that depends on the column, or if there is,
  drop the view first and re-create it afterwards

I myself would be a tad afraid to perform something like that.  Test well.

Yours,
Laurenz Albe