Consolidate discussion of how VACUUM places pages in the FSM for recycling by adding a new section that comes after discussion of page deletion. This structure reflects the fact that page recycling is explicitly decoupled from page deletion in Lanin & Shasha's paper. Page recycling in nbtree is an implementation of what the paper calls "the drain technique".
This decoupling is an important concept for nbtree VACUUM. Searchers have to detect and recover from concurrent page deletions, but they will never have to reason about concurrent page recycling. Recycling can almost always be thought of as a low level garbage collection operation that asynchronously frees the physical space that backs a logical tree node. Almost all code need only concern itself with logical tree nodes. (Note that "logical tree node" is not currently a term of art in the nbtree code -- this all works implicitly.)
This is preparation for an upcoming patch that teaches nbtree VACUUM to remember the details of pages that it deletes on the fly, in local memory. This enables the same VACUUM operation to consider placing its own deleted pages in the FSM later on, when it reaches the end of btvacuumscan().
b071a31149 Add nbtree README section on page recycling.
src/backend/access/nbtree/README | 73 ++++++++++++++++++++++++++++------------
1 file changed, 52 insertions(+), 21 deletions(-)