On 2021-Nov-24, Peter Geoghegan wrote: > TIDs (ItemPointerData structs) are of course not the same thing as > line pointers (ItemIdData structs). There is a tendency to refer to > the latter as "item pointers" all the same, which was confusing. I > personally corrected/normalized this in commit ae7291ac in 2019. I > think that it's worth being careful about precisely because they're > closely related (but distinct) concepts. And so FWIW "LP_DEAD item > pointer" is not a thing. I agree that an LP_DEAD item pointer has no > tuple storage, and so you could say that it points to nothing (though > only in heapam). I probably would just say that it has no tuple > storage, though.
OK, this makes a lot more sense. I wasn't aware of ae7291ac (and I wasn't aware of the significance of 8523492d either, but that's not really relevant here.) > I agree with others that the term "item" is vague, but I don't think > that that's necessarily a bad thing here -- I deliberately changed the > comments to say either "TIDs" or "LP_DEAD items", emphasizing whatever > the important aspect seemed to be in each context (they're LP_DEAD > items to the heap structure, TIDs to index structures). I think we could say "LP_DEAD line pointer" and that would be perfectly clear. Given how nuanced we have to be if we want to be clear about this, I would rather not use "LP_DEAD item"; that seems slightly contradictory, since the item is the storage and such a line pointer does not have storage. Perhaps change that define in progress.h to PROGRESS_VACUUM_NUM_DEAD_LPS, and, in the first comment in vacuumlazy.c, use wording such as + * The major space usage for LAZY VACUUM is storage for the array of TIDs + * of dead line pointers that are to be removed from indexes. or + * The major space usage for LAZY VACUUM is storage for the array of TIDs + * of LP_DEAD line pointers that are to be removed from indexes. (The point being that TIDs are not dead themselves, only the line pointers that they refer to.) -- Álvaro Herrera 39°49'30"S 73°17'W — https://www.EnterpriseDB.com/ "Most hackers will be perfectly comfortable conceptualizing users as entropy sources, so let's move on." (Nathaniel Smith)
