Re: Optimizing TransactionIdIsCurrentTransactionId()

On Thu, Dec 19, 2019 at 02:27:01PM -0500, Robert Haas wrote:
>On Wed, Dec 18, 2019 at 5:07 AM Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
>> TransactionIdIsCurrentTransactionId() doesn't seem to be well optimized for the case when an xid has not yet been assigned, so for read only transactions.
>>
>> A patch for this is attached.
>
>It might be an idea to first call TransactionIdIsNormal(xid), then
>GetTopTransactionIdIfAny(), then TransactionIdIsNormal(topxid), so
>that we don't bother with GetTopTransactionIdIfAny() when
>!TransactionIdIsNormal(xid).
>
>But it's also not clear to me whether this is actually a win. You're
>dong an extra TransactionIdIsNormal() test to sometimes avoid a
>GetTopTransactionIdIfAny() test. TransactionIdIsNormal() is pretty
>cheap, but GetTopTransactionIdIfAny() isn't all that expensive either,
>and adding more branches costs something.
>

I think "optimization" patches should generally come with some sort of
quantification of the gains - e.g. a benchmark with somewhat realistic
workload (but even synthetic is better than nothing). Or at least some
explanation *why* it's going to be an improvement.

regards

--
Tomas Vondra http://www.2ndQuadrant.com
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