Subject: Re: Alter or rename enum value
To: "David G. Johnston" <david.g.johnston@gmail.com>
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Cc: Matthias Kurz <m.kurz@irregular.at>, Tom Lane <tgl@sss.pgh.pa.us>,
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From: Andrew Dunstan <andrew@dunslane.net>
Message-ID: <56F6827F.9040104@dunslane.net>
Date: Sat, 26 Mar 2016 08:37:19 -0400
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On 03/26/2016 12:35 AM, David G. Johnston wrote:
> On Friday, March 25, 2016, Andrew Dunstan <andrew@dunslane.net 
> <mailto:andrew@dunslane.net>> wrote:
>
>
>     On 03/25/2016 04:13 AM, Matthias Kurz wrote:
>
>
>         Hopefully at the commitfest at least the transaction
>         limitation will/could be tackled - that would help us a lot
>         already.
>
>
>     I don't believe anyone knows how to do that safely. Enums pose
>     special problems here exactly because unlike all other types the
>     set of valid values is mutable.
>
>
> Yeah, I'm not sure there is much blue sky here as long as the 
> definition of an enum is considered system data.  It probably needs to 
> be altered so that a user can create a table of class enum with a 
> known layout that PostgreSQL can rely upon to perform optimizations 
> and provide useful behaviors - at least internally.  The most visible 
> behavior being displaying the label while ordering using its position.
>
> The system, seeing a data type of that class, would have an implicit 
> reference between columns of that type and the source table.
> You have to use normal cascade update/delete/do-nothing while 
> performing DML on the source table.
>
> In some ways it would be a specialized composite type, and we could 
> leverage that to you all the syntax available for those - but without 
> having a different function for each differently named enum classed 
> table since they all would share a common structure, differing only in 
> name.  But the tables would be in user space and not a preordained 
> relation in pg_catalog.  Maybe require they all inherit from some 
> template but empty table...
>
>

We don't have the luxury of being able to redesign this as a green 
fields development.

cheers

andrew


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