Re: TransactionIdIsInProgress() cache

Simon Riggs <simon(at)2ndquadrant(dot)com> writes:
> We currently have a single item cache of the last checked TransactionId,
> which optimises the call to TransactionIdDidCommit() during
> HeapTupleSatisfiesMVCC() and partners.

> Before we call TransactionIdDidCommit() we always call
> TransactionIdIsInProgress().

> TransactionIdIsInProgress() doesn't check the single item cache, so even
> if we have just checked for this xid, we will check it again. Since this
> function takes ProcArrayLock and may be called while holding other locks
> it will improve scalability if we can skip the call, for the cost of an
> integer comparison.

> Following patch implements fastpath in TransactionIdIsInProgress() to
> utilise single item cache.

Applied with minor adjustments and some desultory comment-improvement.

regards, tom lane