Re: Optimizing TransactionIdIsCurrentTransactionId()

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От Simon Riggs
Тема Re: Optimizing TransactionIdIsCurrentTransactionId()
Дата
Msg-id CANP8+jKzMEBLkSpCAKeny7ahq5JR2uyV1Xs4UtV5JEyRqNuU+w@mail.gmail.com
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Ответ на Re: Optimizing TransactionIdIsCurrentTransactionId()  (Robert Haas <robertmhaas@gmail.com>)
Ответы Re: Optimizing TransactionIdIsCurrentTransactionId()  (Robert Haas <robertmhaas@gmail.com>)
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On Thu, 19 Dec 2019 at 19:27, Robert Haas <robertmhaas@gmail.com> wrote:
On Wed, Dec 18, 2019 at 5:07 AM Simon Riggs <simon@2ndquadrant.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.

That's not the point of the patch.

If the TopTransactionId is not assigned, we can leave the whole function more quickly, not just avoid a test.

Read only transactions should have a fast path thru this function since they frequently read more data than write transactions.

--
Simon Riggs                http://www.2ndQuadrant.com/
PostgreSQL Solutions for the Enterprise

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