Subject: Re: logical replication syntax (was DROP SUBSCRIPTION,
 query cancellations and slot handling)
To: Tom Lane <tgl@sss.pgh.pa.us>, Robert Haas <robertmhaas@gmail.com>
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Cc: Michael Paquier <michael.paquier@gmail.com>,
 PostgreSQL mailing lists <pgsql-hackers@postgresql.org>
From: Petr Jelinek <petr.jelinek@2ndquadrant.com>
Message-ID: <9174dc31-787a-a2f6-818f-cb4a5c1dfddb@2ndquadrant.com>
Date: Tue, 2 May 2017 15:22:34 +0200
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On 02/05/17 15:14, Petr Jelinek wrote:
> On 02/05/17 15:10, Tom Lane wrote:
>> Robert Haas <robertmhaas@gmail.com> writes:
>>>> On Thu, Apr 20, 2017 at 7:46 AM, Petr Jelinek
>>>> <petr.jelinek@2ndquadrant.com> wrote:
>>>>> DROP SUBSCRIPTION mysub NODROP SLOT;
>>
>>>> I'm pretty uninspired by this choice of syntax.
>>
>> Actually, this command has got much worse problems than whether you like
>> the spelling of its option: it shouldn't have an option in the first
>> place.  I put it to you as an inviolable rule that no object DROP command
>> should ever have any options other than RESTRICT/CASCADE and IF EXISTS.
>> If it does, then you don't know what to do when the object is recursed
>> to by a cascaded drop.
>>
>> It's possible that we could allow exceptions to this rule for object types
>> that can never have any dependencies.  But a subscription doesn't qualify
>> --- it's got an owner, so DROP OWNED BY already poses this problem.  Looks
>> to me like it's got a dependency on a database, too.  (BTW, if
>> subscriptions are per-database, why is pg_subscription a shared catalog?
>> There were some pretty schizophrenic decisions here.)
> 
> Because otherwise we would need launcher process per database, not pretty.
> 
>>
>> So ISTM we need to get rid of the above-depicted syntax.  We could instead
>> have what-to-do-with-the-slot as a property of the subscription,
>> established at CREATE SUBSCRIPTION and possibly changed later by ALTER
>> SUBSCRIPTION.  Not quite sure what to call the option, maybe something
>> based on the concept of the subscription "owning" the slot.
>>
> 
> So what do you do if the upstream does not exist anymore when you are
> dropping subscription?
> 

Let me expand, if we don't drop the slot by default when dropping
subscription, we'll have a lot of users with dead slots laying around
holding back WAL/catalog_xmin, that's really bad. If we do drop by
default like now, we need option to not do that, neither RESTRICT, nor
CASCADE fit that.

-- 
  Petr Jelinek                  http://www.2ndQuadrant.com/
  PostgreSQL Development, 24x7 Support, Training & Services


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