Hi, On Mon, Jul 24, 2023 at 08:57:05AM +0800, jian he wrote: > hi. > > https://www.postgresql.org/docs/current/pageinspect.html#id-1.11.7.34.6 > > > This is a B-tree leaf page. All tuples that point to the table happen to be posting list tuples (all of which store a total of 100 6 byte TIDs). > > There is also a “high key” tuple at itemoffset number 1. ctid is used to store encoded information about each tuple in this example, though > > leaf page tuples often store a heap TID directly in the ctid field instead. tids is the list of TIDs stored as a posting list. > > (all of which store a total of 100 6 byte TIDs) > I think the meaning is something like: > (all of which store a total of 100 TIDs, each TID is 6 byte long.) Yes that's what it means. > What's the meaning of (16, 8292)? After looking around. I'm still confused. > Would it be better to add an explanation about (16, 8292) to the docs? I'm not sure how easy it would be to document it. If you want more detail about "encoded information about each tuple" you need to look at the btree implementation, in src/include/access/nbtree.h. For instance, to get the number of items in a posting list, you see the BTreeTupleGetNPosting() function, which basically returns ctid & 0X0FFF. And 8292 & 0X0FFF is 100, which is the mentioned number of tids. If you look around line 462 of the same file (near the BT_OFFSET_MASK) you will see all the other usage for the ctid in case of a posting list, like knowing whether the IndexTuple is a posting list of not.
