MariaDB server is a community developed fork of MySQL server. Started by core members of the original MySQL team, MariaDB actively works with outside developers to deliver the most featureful, stable, and sanely licensed open SQL server in the industry.
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Varun Gupta b1ab2ba583 MDEV-20519: Query plan regression with optimizer_use_condition_selectivity > 1
The issue here is the wrong estimate of the cardinality of a partial join,
the cardinality is too high because the function table_cond_selectivity()
returns an absurd number 100 while selectivity cannot be greater than 1.

When accessing table t by outer reference t1.a via index we do not perform any
range analysis for t. Yet we see TABLE::quick_key_parts[key] and
TABLE->quick_rows[key] contain a non-zero value though these should have been
remained untouched and equal to 0.

Thus real cause of the problem is that TABLE::init does not clean the arrays
TABLE::quick_key_parts[] and TABLE::>quick_rows[].
It should have done it because the TABLE structure created for any
instance of a table can be reused for many queries.
2019-11-07 15:24:21 +05:30
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VERSION

MariaDB is designed as a drop-in replacement of MySQL(R) with more
features, new storage engines, fewer bugs, and better performance.

MariaDB is brought to you by the MariaDB Foundation.
Please read the CREDITS file for details about the MariaDB Foundation,
and who is developing MariaDB.

MariaDB is developed by many of the original developers of MySQL who
now work for the MariadB Foundation and the MariaDB Corporation, and by many people in
the community.

MySQL, which is the base of MariaDB, is a product and trademark of Oracle
Corporation, Inc. For a list of developers and other contributors,
see the Credits appendix.  You can also run 'SHOW authors' to get a
list of active contributors.

A description of the MariaDB project and a manual can be found at:
http://mariadb.org/
https://mariadb.com/kb/en/
https://mariadb.com/kb/en/mariadb-vs-mysql-features/
https://mariadb.com/kb/en/mariadb-versus-mysql-features/
https://mariadb.com/kb/en/mariadb-versus-mysql-compatibility/

As MariaDB is a full replacement of MySQL, the MySQL manual at
http://dev.mysql.com/doc is generally applicable.

More help is available from the Maria Discuss mailing list
https://launchpad.net/~maria-discuss
and the #maria IRC channel on Freenode.

***************************************************************************

NOTE: 

MariaDB is specifically available only under version 2 of the GNU
General Public License (GPLv2). (I.e. Without the "any later version"
clause.) This is inherited from MySQL. Please see the README file in
the MySQL distribution for more information.

License information can be found in the COPYING, COPYING.LESSER,
and COPYING.thirdparty files.

***************************************************************************

IMPORTANT:

Bug and/or error reports regarding MariaDB should be submitted at:
https://jira.mariadb.org

For reporting security vulnerabilities see:
https://mariadb.org/about/security-policy/

Bugs in the MySQL code can also be submitted at:
http://bugs.mysql.com

The code for MariaDB, including all revision history, can be found at:
https://github.com/MariaDB/server

***************************************************************************