pgstattuple extension for indexes

This is an extension of pgstattuple to query information from indexes.
It supports btree, hash and gist. Gin is not supported.
It scans only index pages and does not read corresponding heap tuples.
Therefore, 'dead_tuple' means the number of tuples with LP_DELETE flag.

Also, I added an experimental feature for btree indexes. It checks
fragmentation factor of indexes. If an leaf has the right link on the next
adjacent page in the file, it is assumed to be continuous (not fragmented).
It will help us to decide when to REINDEX.

Suggestions welcome.

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$ pgbench -i
$ pgbench -n -t 100 -c 10
# select * from pgstattuple('accounts_pkey');
NOTICE: 0.36% fragmented
HINT: continuous=273, forward=1, backward=0
-[ RECORD 1 ]------+--------
table_len | 2260992
tuple_count | 100996 -- 996 tuples are dead practically,
tuple_len | 1615936 but no LP_DELETE yet.
tuple_percent | 71.47
dead_tuple_count | 4
dead_tuple_len | 64 -- 64 tuples are marked as LP_DELETE.
dead_tuple_percent | 0
free_space | 208188
free_percent | 9.21
----

Regards,
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ITAGAKI Takahiro
NTT Open Source Software Center