itemid.h introduces the struct ItemIdData as follows:
/*
* An item pointer (also called line pointer) on a buffer page
Meanwhile, itemptr.h introduces the struct ItemPointerData as follows:
/*
* ItemPointer:
*
* This is a pointer to an item within a disk page of a known file
* (for example, a cross-link from an index to its parent table).
It doesn't seem reasonable to assume that you should know the
difference based on context. The two concepts are closely related. An
ItemPointerData points to a block, as well as the, uh, item pointer
within that block.
This ambiguity is avoidable, and should be avoided. ISTM that the
least confusing way of removing the ambiguity would be to no longer
refer to ItemIds as item pointers, without changing anything else.
--
Peter Geoghegan