| From: | Andres Freund <andres(at)anarazel(dot)de> |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | Andrew Dunstan <andrew(dot)dunstan(at)2ndquadrant(dot)com>, David Rowley <david(dot)rowley(at)2ndquadrant(dot)com>, Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: ALTER TABLE ADD COLUMN fast default |
| Date: | 2018-03-29 21:31:29 |
| Message-ID: | 20180329213129.peqeam7t2a3ab2xk@alap3.anarazel.de |
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| Lists: | pgsql-hackers |
Hi,
On 2018-03-29 17:27:47 -0400, Tom Lane wrote:
> Andres Freund <andres(at)anarazel(dot)de> writes:
> > There's plenty databases with pg_attribute being many gigabytes large,
> > and this is going to make that even worse.
>
> Only if you imagine that a sizable fraction of the columns have fast
> default values, which seems somewhat unlikely.
Why is that unlikely? In the field it's definitely not uncommon to
define default values for just about every column. And in a lot of cases
that'll mean we'll end up with pg_attribute containing default values
for most columns but the ones defined at table creation. A lot of
frameworks make it a habit to add columns near exclusively in
incremental steps. You'd only get rid of them if you force an operation
that does a full table rewrite, which often enough is impractical.
Greetings,
Andres Freund
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