On 25/01/2021 18:56, Robert Haas wrote:
I attach a series of proposed patches to slightly improve some minor
things related to the CLOG code.
[patches 0001 - 0003]
Makes sense.
0004 - In StartupCLOG(), correct an off-by-one error. Currently, if
the nextXid is exactly a multiple of the number of CLOG entries that
fit on a page, then the value we compute for
XactCtl->shared->latest_page_number is higher than it should be by 1.
That's because nextXid represents the first XID that hasn't yet been
allocated, not the last one that gets allocated.
Yes.
So, the CLOG page gets created when nextXid advances from the first
value on the page to the second value on the page, not when it
advances from the last value on the previous page to the first value
on the current page.
Yes. It took me a moment to understand that explanation, though. I'd
phrase it something like "nextXid is the next XID that will be used, but
we want to set latest_page_number according to the last XID that's
already been used. So retreat by one."
Having a separate FullTransactionIdToLatestPageNumber() function for
this seems like overkill to me.
Some of the other SLRUs have similar issues as a result of
copy-and-paste work over the years. I plan to look at tidying that
stuff up, too. However, I wanted to post (and probably commit) these
patches first, partly to get some feedback, and also because all the
cases are a little different and I want to make sure to do a proper
analysis of each one.
Yeah, multixact seems similar at least.
- Heikki
