Pgstatindex and leaf fragmentation.

There is a Postgres extension in the contrib directory, named
pgstattuple, which collects valuable statistics for tables and indexes.
One of the calculated numbers is called "leaf_fragmentation". How exactly
is id defined, does anybody have a formula or a simple explanation? Is it
related to the photosynthesis? Here is an example:

testtrack=# select * from pgstatindex('public.defects_pkey');
version | tree_level | index_size | root_block_no | internal_pages |
leaf_pages | empty_pages | deleted_pages |
avg_leaf_density|leaf_fragmentation
------------+----------------+----------------+-----------------------+-------------------------+-----------------+---------------------+-----------------------+--------------------------+--------------------
2 | 1 | 647168 | 3
| 0 | 78 | 0
| 0 | 89.67 | 0
(1 row)

What do numbers "leaf_density" and "leaf_fragmentation" mean? I googled
but was unable to come up with a decent explanation. I am looking for
the formula or, at least, a good heuristic explanation.

--

Mladen Gogala
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