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From James Taylor <jamestay...@apache.org>
Subject Re: Emulating a true INSERT or UPDATE
Date Thu, 28 Jul 2016 22:45:03 GMT
James,
Your logic looks correct, assuming that you have the complete row keys in
your SELECT statement. FYI, the transaction will start automatically at
(2). You can optimize this slightly by just doing a COUNT(*) instead of
returning the rows back to the client. For the UPDATE case, you'd throw if
the count doesn't match the number of rows you have. You'll also have the
added benefit that another client attempting to INSERT or UPDATE the same
rows at the same time would fail (that's the conflict detection piece that
Thomas mentioned).
Thanks,
James

On Thu, Jul 28, 2016 at 2:46 PM, Thomas D'Silva <tdsilva@salesforce.com>
wrote:

> If the table is transactional, you are guaranteed that if there are
> overlapping transactions that try to commit the same row one will succeed
> and the others will fail with an exception. There is also an additional
> cost to doing conflict detection at commit time.
>
>
> On Thu, Jul 28, 2016 at 8:18 AM, Heather, James (ELS) <
> james.heather@elsevier.com> wrote:
>
>> What would I need to do in order to emulate an INSERT or UPDATE in
>> Phoenix, as opposed to an UPSERT?
>>
>>
>> Suppose I had a TRANSACTIONAL table. To do an INSERT, I then:
>>
>>    1. Start a transaction
>>    2. SELECT the relevant rows, and throw an error if the SELECT is
>>    non-empty
>>    3. UPSERT
>>    4. Commit the transaction
>>
>> To do an UPDATE, I do the same, except that in step 2 I throw an error if
>> the SELECT is empty.
>>
>>
>> If all of the possible writes to those rows are enclosed in transactions,
>> will this avoid the race conditions and give me a true INSERT and UPDATE
>> (at a cost of having to make multiple queries, of course)?
>>
>>
>> The case I have in mind is where we might have DELETE and PATCH queries
>> coming into our API. With a back end that supports UPDATE, it's not a
>> problem if a DELETE and a PATCH come in at the same time: either the DELETE
>> succeeds and then the PATCH fails, or the PATCH succeeds and then the
>> DELETE succeeds. Either way, you end up with the row's being deleted. But
>> if we use an UPSERT statement for the PATCH, we have a problem: the DELETE
>> can succeed, and then the PATCH will still succeed, but it'll insert the
>> row back in again.
>>
>>
>> I'm unclear as to how to use transactions to guarantee the right
>> behaviour here.
>>
>>
>> James
>>
>> ------------------------------
>>
>> Elsevier Limited. Registered Office: The Boulevard, Langford Lane,
>> Kidlington, Oxford, OX5 1GB, United Kingdom, Registration No. 1982084,
>> Registered in England and Wales.
>>
>
>

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