Re: BUG #11638: Transaction safety fails when constraints are dropped and analyze is done

On Sat, Oct 11, 2014 at 4:14 AM, Andres Freund <andres@2ndquadrant.com>
wrote:
>> When, within a transaction:
>> * I drop a foreign key (or any) constraint.
>> * Load some data to the table which won't fit the constraint.
>> * Analyze the table.
>> * Attempt to re-add the constraint which fails and should roll back the
>> whole transaction.
>>
>> The constraint is still missing after rollback.
> That's, like, a *seriously* bad idea. The current xact doesn't see a
> trigger, so we remove relhastriggers (and similarly other relhas*
> stuff). The kicker is that we do so *nontransactionally*.
>
> That's fine enough for VACUUM because that doesn't run in a
> transaction. But vac_update_relstats is also run for ANALYZE.
>
> I've no time to think this through right now, but my current thinking is
> that we should use a transactional update for ANALYZE.

The comments on top of vac_update_relstats rely on heap_inplace_update:
 *              Note another assumption: that two VACUUMs/ANALYZEs on a
table can't
 *              run in parallel, nor can VACUUM/ANALYZE run in parallel
with a
 *              schema alteration such as adding an index, rule, or
trigger.  Otherwise
 *              our updates of relhasindex etc might overwrite uncommitted
updates.
I am not sure what would be the side effects of such a change, but it seems
dangerous to add any control mechanism able to choose if ctup is updated
with either heap_inplace_update or simple_heap_update, especially something
like (GetTopTransactionIdIfAny() == InvalidTransactionId) to determine if
this code path is taken in an xact that has already done a transactional
update. Perhaps a solution would be to document properly that analyze
should not be run in the same transaction as schema changes.
Regards,
--
Michael