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Re: Rename a database that has connections

From: Mark Kirkwood <mark(dot)kirkwood(at)catalyst(dot)net(dot)nz>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Rename a database that has connections
Date: 2011-11-22 04:24:24
Message-ID: 4ECB23F8.1040403@catalyst.net.nz (view raw or flat)
Thread:
Lists: pgsql-hackers
On 22/11/11 16:41, Tom Lane wrote:
> Mark Kirkwood<mark(dot)kirkwood(at)catalyst(dot)net(dot)nz>  writes:
>> I've been helping out several customers recently who all seem to be
>> wrestling with the same issue: wanting to update/refresh non-production
>> databases from the latest corresponding prod version. Typically they
>> have (fairly complex) scripts that at some point attempt to restore a
>> dump into new database and then rename the to-be-retired db out of the
>> way and rename the newly restored one to take over.
>> In many cases such scripts would be simplified if a database could be
>> renamed without requiring its connections terminated. I've been asked
>> several times if this could be added... so I've caved in a done a patch
>> that allows this.
>> The default behavior is unchanged - it is required to specify an
>> additional trailing FORCE keyword to elicit the more brutal behavior.
>> Note that existing connections to the renamed database are unaffected,
>> but obviously SELECT current_database() returns the new name (in the
>> next transaction).
> This patch seems to me to be pretty thoroughly misguided.  Either
> renaming a database with open connections is safe, or it isn't.  If it
> is safe, we should just allow it.  If it isn't, making people write an
> extra FORCE keyword does not make it safe.  It's particularly silly
> to allow someone to rename the database out from under other sessions
> (which won't know what happened) but not rename it out from under his
> own session (which would or at least could know it).
>
> What you need to be doing is investigating whether the comments about
> this in RenameDatabase() are really valid concerns or not.
>

The reason I added FORCE was to preserve backwards compatibility - for 
any people out there that like the way it behaves right now. I am 
certainly willing to be convinced that such a concern is unneeded.

You are quite right about the patch being inconsistent with respect to 
the renaming the current database, it should allow that too (will change 
if this overall approach makes sense).

With respect to the concerns in RenameDatabase(), that seems to boil 
down to applications stashing the current dbname somewhere and caring 
about it. This was not viewed as a issue by any of the folks who I 
talked to about this (they are all application developers/architects etc 
so they understand that issue). However there may well be application 
frameworks out there that do care... which seemed to me to be another 
reason for making the forced rename require an extra keyword.

I have not been able to find any other problems caused by this... 
renaming a db (many times) with hundreds of pgbench connections does not 
give rise to any issues.

regards

Mark

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Subject: Re: Rename a database that has connections
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