Created by Oleg Tsarev and last modified
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208. By Oleg Tsarev

Return back Percona's "innodb_stats_method=nulls_ignored" algorithm.

Related Launchpad bug #892405
Related MySQL bug http://bugs.mysql.com/bug.php?id=63320

I added new value "nulls_ignored_exact" for variable innodb_stats_method.

Current upsteam "nulls_ignored" algorithm calculate not_null values on every estimated (random) page, and after that try to approximates total value count.
Unfortunately, this algorithm works incorrectly on mostly null indexes (see MySQL Bug #63320).

Percona'a algorithm excludes from estimation pages with nulls, calculates the count of different values and approximates total value count.
This algorithm works correctly on mostly null indexes, but has limitation - it works correctly just on one-column-key indexes. On compound-keys algorithm can works incorrectly (I didn't test this).
Unfortunately, I can't imagine now how to discard this limitation.

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