Re: DDL Damage Assessment

On Thu, Oct 2, 2014 at 1:30 PM, Dimitri Fontaine <dimitri(at)2ndquadrant(dot)fr>
wrote:
>
> Hi fellow hackers,
>
> I would like to work on a new feature allowing our users to assess the
> amount of trouble they will run into when running a DDL script on their
> production setups, *before* actually getting their services down.
>
> The main practical example I can offer here is the ALTER TABLE command.
> Recent releases are including very nice optimisations to it, so much so
> that it's becoming increasingly hard to answer some very basic
> questions:
>
> - what kind of locks will be taken? (exclusive, shared)
> - on what objects? (foreign keys, indexes, sequences, etc)
> - will the table have to be rewritten? the indexes?
>
> Of course the docs are answering parts of those, but in particular the
> table rewriting rules are complex enough that “accidental DBAs” will
> fail to predict if the target data type is binary coercible to the
> current one.
>
> Questions:
>
> 1. Do you agree that a systematic way to report what a DDL command (or
> script, or transaction) is going to do on your production database
> is a feature we should provide to our growing user base?
>
> 2. What do you think such a feature should look like?
>
> 3. Does it make sense to support the whole set of DDL commands from the
> get go (or ever) when most of them are only taking locks in their
> own pg_catalog entry anyway?
>
> Provided that we are able to converge towards a common enough answer to
> those questions, I propose to hack my way around and send patches to
> have it (the common answer) available in the next PostgreSQL release.
>

What you are proposing is some kind of "dry-run" with verbose output?

Regards,

--
Fabrízio de Royes Mello
Consultoria/Coaching PostgreSQL
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