From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
---|---|
To: | Simon Riggs <simon(at)2ndQuadrant(dot)com> |
Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: ALTER TABLE .. SET SCHEMA lock strength |
Date: | 2011-01-01 18:17:46 |
Message-ID: | 22825.1293905866@sss.pgh.pa.us |
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Lists: | pgsql-hackers |
Simon Riggs <simon(at)2ndQuadrant(dot)com> writes:
> On Sat, 2011-01-01 at 11:06 -0500, Robert Haas wrote:
>> While reviewing the SQL/MED patch, I happened to notice that
>> ExecAlterObjectSchemaStmt calls AlterTableNamespace with a lock mode
>> argument of AccessExclusiveLock. Does anyone see a reason why
>> ShareUpdateExclusiveLock would be insufficient?
> It seemed unsafe to me to do that while an object was being accessed,
> since it effectively changes the search_path, which is dangerous.
ALTER RENAME and ALTER SET SCHEMA are both in the nature of changing the
object's identity. Consider the fairly typical use-case where you are
renaming an "old" instance out of the way and renaming another one into
the same schema/name. Do you really want that to be a low-lock
operation? I find it really hard to envision a use case where it'd be
smart to allow some concurrent operations to continue using the the old
instance while others start using the new one.
regards, tom lane
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