GUC for cleanup indexes threshold.

Hi and happy new year.

The lazy vacuum calls lazy_cleanup_index to update statistics of
indexes on a table such as relpages, reltuples at the end of the
lazy_scan_heap. In all type of indexes the lazy_cleanup_index scans
all index pages. It happens even if table has not been updated at all
since previous vacuum invoked. Freeze map reduces the execution time
and cost of table vacuuming much if almost table has been frozen. But
it doesn't work for cleaning up indexes. If a very large static table
has index then because the cleaning up index is called and it always
scans all index pages, it takes time to scan all pages of index as
reported[1].

Attached patch introduces new GUC parameter parameter
vacuum_cleanup_index_scale_factor which specifies the fraction of the
table pages containing dead tuple needed to trigger a cleaning up
indexes. The default is 0.0, which means that the cleanup index is not
invoked if no update on table. In other word, if table is completely
frozen then lazy vacuum can skip the index scans as well. Increasing
this value could reduce total time of lazy vacuum but the statistics
and the free space map of index are not updated.

[1] https://www.postgresql.org/message-id/MWHPR20MB142177B86D893C946FAFC9A4A18C0%40MWHPR20MB1421.namprd20.prod.outlook.com

Regards,

--
Masahiko Sawada
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center