Re: DDL Damage Assessment

On Thu, Oct 2, 2014 at 1:46 PM, Fabrízio de Royes Mello
<fabriziomello@gmail.com> wrote:
> On Thu, Oct 2, 2014 at 1:30 PM, Dimitri Fontaine <dimitri@2ndquadrant.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?


EXPLAIN ALTER TABLE ?