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Re: Altering a column (increasing size) in Postgres takes long time

From: Jaime Casanova <jcasanov(at)systemguards(dot)com(dot)ec>
To: "Tomeh, Husam" <HTomeh(at)facorelogic(dot)com>
Cc: "pgsql-admin(at)postgresql(dot)org" <pgsql-admin(at)postgresql(dot)org>
Subject: Re: Altering a column (increasing size) in Postgres takes long time
Date: 2010-02-25 23:40:55
Message-ID: 3073cc9b1002251540s8b91e7cw844b3bb6da2a0cd@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-admin
On Thu, Feb 25, 2010 at 6:04 PM, Tomeh, Husam <HTomeh(at)facorelogic(dot)com> wrote:
> We have a huge table with hundred million records. We need to increase the
> size of an existing column with varchar type, and it has been running for
> more than 12 hours.
>
>
>
> We’re using: ALTER TABLE Property ALTER COLUMN "situs-number" TYPE
> varchar(30);
>

it's not better to have the field beign type text and don't worry
about length but just in check constraints if really necesary?

>
> 1)       Can you explain what’s happening internally that make this a very
> long process? Does the table get re-created?
>

yes, and all it's indexes rebuild not just the one you dropped
and the FK's rechecked (don't think so but can't remember right now)?

>
>
> 2)       Assuming the Alter statement finished successfully, And if I didn’t
> drop the index (on that column), do I have to rebuild the index? Does the
> index get invalidated for just alter the indexed column?
>

it's get rebuilt

>
>
> 3)       Some folks referred to directly updating Postgres internal tables
> (pg_attribute) which takes seconds to make the column change happen. How
> safe if this and would potentially cause any corruption?
>

no, that's insane

>
> 4)       Is there a more practical and safe method  to alter a huge table
> with reasonable amount of time?
>
>

use text fields instead of varchar(n)

-- 
Atentamente,
Jaime Casanova
Soporte y capacitación de PostgreSQL
Asesoría y desarrollo de sistemas
Guayaquil - Ecuador
Cel. +59387171157

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