Re: Is it necessary to rewrite table while increasing the scale of datatype numeric？

On Thu, Sep 05, 2013 at 10:41:25AM -0700, Jeff Janes wrote:
> In order to avoid the rewrite, the code would have to be changed to
> look up the column definition and if it specifies the scale, then
> ignore the per-row n_header, and look at the n_header only if the
> column is NUMERIC with no precision or scale. That should
> conceptually be possible, but I don't know how hard it would be to
> implement--it sounds pretty invasive to me. Then if the column was
> altered from NUMERIC with scale to be a plain NUMERIC, it would have
> to rewrite the table to enforce the row-wise scale to match the old
> column-wise scale. Where as now that alter doesn't need a re-write.
> I don't know if this would be an overall gain or not.

Invasive indeed. The type-supplementary data would need to reach essentially
everywhere we now convey a type OID. Compare the invasiveness of adding
collation support. However, this is not the first time it would have been
useful. We currently store a type OID in every array and composite datum.
That's wasteful and would be unnecessary if we reliably marshalled similar
information to all the code needing it. Given a few more use cases, the
effort would perhaps start to look credible relative to the benefits.

--
Noah Misch
EnterpriseDB http://www.enterprisedb.com