To: pgsql-sql@postgresql.org
From: Seb <spluque@gmail.com>
Subject: Re: enum data type vs table
Date: Wed, 25 May 2011 18:08:10 -0500
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On Wed, 25 May 2011 17:23:26 -0500,
Peter Koczan <pjkoczan@gmail.com> wrote:

> On Tue, May 17, 2011 at 11:23 PM, Seb <spluque@gmail.com> wrote:
>> Are there any guidelines for deciding whether to 1) create an enum
>> data type or 2) create a table with the set of values and then have
>> foreign keys referencing this table?  Some fields in a database take
>> a small number of values, and I'm not sure which of these routes to
>> take.  The enum data type seems like a clean way to handle this
>> without creating a constellation of tables for all these values, but
>> if one wants to add a new label to the enum or make changes to it at
>> some point, then the tables using it have to be recreated, so it's
>> quite rigid.  Have I got this right?  Thanks.

> I think your choice depends on a few things:

> 1 - How do you want to interact with the tables? What I mean is, are
> you planning on querying, inserting, or updating data to those tables
> via text or will you need to join to your reference table? If you
> don't want to join, you'll either need to use enum types, use views
> (which can be a pain if you want to update a view), or
> duplicate/reference the text directly (which is slow and a bad idea
> for several reasons).

> 2 - How much can you tolerate downtime or a busy database? Changing
> types is a single transaction and requires an exclusive lock. On small
> tables this is negligible, but on big tables it can require downtime.

> 3 - How often do you really expect changes to the enum type? If adding
> a new value to an enum type is truly a rare event, it's . If it's
> frequent or regular, you should probably have a table.

> I've used both of these approaches and I've found enum types to be
> well worth any trouble to drop/recreate types. The changes I've made
> have been rare, and I've been able to schedule downtime pretty easily,
> so it made the most sense for me.

> Also, Postgres 9.1 allows adding values to enum types, so you could
> always use that when it is finally released.

These are great guidelines, thanks.


-- 
Seb