`STD()`/`STDDEV()`/`STDDEV_POP()`/`STDDEV_SAMP()`/`VAR_POP()`/`VAR_SAMP()`/`VARIANCE()` result precision is not correct

[actiontech-bot]

Raised by: @FlyingMao Steps:

1. Dble:
2. MySQL:

[actiontech-bot]

By @ditdb

Analysis

1. MySQL uses recurrence formula, while Dble use the sum of squares, which causes the precision misbehave on two similar number subtraction (refer to <数值分析，第8页，清华大学出版社，冯有前>). In MySQL, refer to sql/item_sum.h

Computing standard deviation refers to https://www.johndcook.com/blog/2008/09/26/comparing-three-methods-of-computing-standard-deviation/

2. MySQL returns data in string format, which causes precision lose too.
3. Floating calculation implementation differs between MySQL and Java.

Cannot figures out a better way.

BTW: The method mentions in http://www.cnblogs.com/yoyaprogrammer/p/delta_variance.html also causes precision lose.