Re: Pgstatindex and leaf fragmentation.

Mladen Gogala wrote:
> 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.
>
>

To answer my own question, the "leaf_fragmentation" measures the number
of free space fragments in the index. It is a synthetic value. I am
still not sure about the significance of this value on the performance
of an index access. Fragmented free space should only affect the insert
performance. I am not sure what amount of fragmentation should be taken
as an indication for re-indexing.

snprintf(values[j++], 32, "%.2f", (double) indexStat.fragments /
(double) indexStat.leaf_pages * 100.0);

--

Mladen Gogala
Sr. Oracle DBA
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