Re: Savepoints in transactions for speed?

On Thu, Nov 29, 2012 at 10:14 AM, Mike Blackwell <mike(dot)blackwell(at)rrd(dot)com> wrote:
>
>
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> On Thu, Nov 29, 2012 at 12:09 PM, Jeff Janes <jeff(dot)janes(at)gmail(dot)com> wrote:
>>
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>> But If you do keep the drop index inside the transaction, then you
>> would probably be better off using truncate rather than delete, and
>> rebuild the index non-concurrently and move that inside the
>> transaction as well.
>>
>
>
> Hmm.... From the 9.2 manual it seems that might not work out so well:
>
> TRUNCATE is not MVCC-safe (see Chapter 13 for general information about
> MVCC). After truncation, the table will appear empty to all concurrent
> transactions, even if they are using a snapshot taken before the truncation
> occurred.
>
> It looks like other transactions could find an empty table while it was
> being reloaded under that approach.

They would block during the load, it is just after the load that they
would see the table as empty. I thought that that would only be a
problem for repeatable read or higher, but a test shows that read
committed has that problem as well. But yeah, that could definitely
be a problem with that method.

Cheers,

Jeff