Currently, we restrict btree index tuples to a size that ensures three of
them will fit on a page. The motivation for this is the following two
considerations:
1. In a non-rightmost page, we need to include a "high key", or page
boundary key, that isn't one of the useful data keys.
2. In a non-leaf page, there had better be at least two child pages
(downlink entries), else we have failed to subdivide the page's key
range at all, and thus there would be a nonterminating recursion.
However: a non-leaf page actually has one more pointer than key,
eg a page with three children needs only two data keys:
---------------- entire key range assigned to page ------------------
-- range 1 -- boundary key -- range 2 -- boundary key -- range 3 -- | |
| v v v
child page 1 child page 2 child page 3
We implement this by having the first data "tuple" on a non-leaf page
contain only a downlink TID and no key data, ie it's just the header.
So it appears to me that we could allow the maximum size of a btree
entry to be just less than half a page, rather than just less than
a third of a page --- the worst-case requirement for a non-leaf page
is not three real tuples, but one tuple header and two real tuples.
On a leaf page we might manage to fit only one real data item, but
AFAICS that doesn't pose any correctness problems.
Obviously a tree containing many such pages would be awfully inefficient
to search, but I think a more common case is that there are a few wide
entries in an index of mostly short entries, and so pushing the hard
limit up a little would add some flexibility with little performance
cost in real-world cases.
Have I missed something? Is this worth changing?
regards, tom lane