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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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- MariaDB\ 71.8%
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82bd9b6b61
When only one row was present, the subtraction of nearly the same number resulted in catastropic cancellation, introducing an error in the VARIANCE calculation near 1e-15. That was sqrt()ed to get STDDEV, the error was escallated to near 1e-8. The simple fix of testing for a row count of 1 and forcing that to yield 0.0 is insufficient, as two rows of the same value should also have a variance of 0.0, yet the error would be about the same. So, this patch changes the formula that computes the VARIANCE to be one that is not subject to catastrophic cancellation. In addition, it now uses only (faster-than-decimal) floating point numbers to calculate, and renders that to other types on demand. mysql-test/r/func_group.result: Test that the bug is fixed, and that no unexpected behavior arises from the changes. mysql-test/t/func_group.test: Test that the bug is fixed, and that no unexpected behavior arises from the changes. sql/item_sum.cc: Serg's suggestion: Force all VARIANCE calculations to be done with floating- point types. It's faster, and the SQL standard says we may implement these functions any way we want. Additionally, use a form of variance calculation that is not subject to catastrophic cancellation. http://static.flickr.com/108/311308512_5c4e1c0c3d_b.jpg sql/item_sum.h: Remove unused members and add a comment describing the recurrence relation. |
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| README | ||
This is a release of MySQL, a dual-license SQL database server. MySQL is brought to you by the MySQL team at MySQL AB. License information can be found in these files: - For GPL (free) distributions, see the COPYING file. - For commercial distributions, see the MySQLEULA.txt file. For further information about MySQL or additional documentation, see: - The latest information about MySQL: http://www.mysql.com - The current MySQL documentation: http://dev.mysql.com/doc Some manual sections of special interest: - If you are migrating from an older version of MySQL, please read the "Upgrading from..." section first! - To see what MySQL can do, take a look at the features section. - For installation instructions, see the Installation chapter. - For future plans, see the TODO appendix. - For the new features/bugfix history, see the News appendix. - For the currently known bugs/misfeatures (known errors) see the problems appendix. - For a list of developers and other contributors, see the Credits appendix. A local copy of the MySQL Reference Manual can be found in the Docs directory in GNU Info format. You can also browse the manual online or download it in any of several formats at the URL given earlier in this file. ************************************************************ IMPORTANT: Bug or error reports should be sent to http://bugs.mysql.com.