Date: Sun, 12 Jan 2025 22:19:01 +0100
From: "Peter J. Holzer" <hjp-pgsql@hjp.at>
To: pgsql-general@lists.postgresql.org
Subject: Re: Alter table fast
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On 2025-01-09 20:52:27 +0100, shammat@gmx.net wrote:
> Am 09.01.25 um 20:17 schrieb veem v:
> > > Out of curiosity, why NUMERIC(15,0) instead of BIGINT?
> >=20
> > It's for aligning the database column types to the data model and
> > it's happening across all the upstream downstream systems. I was
> > thinking if this can be made  faster with the single line alter
> > statement "Alter table <table_name> alter column <column_name> type
> > numeric(15,0) USING <column_name>::NUMERIC(15,0);"
>=20
> Hmm, I would rather change numeric(15,0) to bigint if I had to "align" ty=
pes across systems.

I'm also wondering what "the data model" is.

If I have numeric(15,0) in an abstract data model, that means that I
expect values larger than 99,999,999,999,999 but at most
999,999,999,999,999. That seems to be oddly specific and also somewhat
at odds with reality when until now there apparently haven't been any
values larger than 2,147,483,647. What kind of real world value could
suddenly jump by more than 5 orders of magnitude but certainly not by 7?

A bigint is much less precise (more than 2,147,483,647 but not more
than 9,223,372,036,854,775,807) and therefore more suitable for values
where you don't really know the range.

However, for the problem at hand, I doubt it makes any difference.
Surely converting a few million values takes much less time than
rewriting a 50 GB table and all its indexes.

So there isn't really a faster way to do what Veem wants. There may
however be less disruptive way: He could create a new column with the
new values (which takes at least as long but can be done in the
background) and then switch it over and drop the old column.

        hp

--=20
   _  | Peter J. Holzer    | Story must make more sense than reality.
|_|_) |                    |
| |   | hjp@hjp.at         |    -- Charles Stross, "Creative writing
__/   | http://www.hjp.at/ |       challenge!"

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