GROUP BY merges distinct DECIMAL values after generated-column substitution

CREATE DATABASE decimal_groups;

USE decimal_groups;

CREATE TABLE amounts (a DECIMAL(4,2), b DECIMAL(5,4) AS (a), KEY by_generated_value(b)) ENGINE=InnoDB;

INSERT IGNORE INTO amounts(a) VALUES (11.11),(22.22);

SHOW WARNINGS;

SELECT 'base_rows' AS section;

SELECT a,b FROM amounts ORDER BY a+0;

SELECT 'indexed_groups' AS section;

SELECT a,COUNT(*) AS n FROM amounts GROUP BY a;

SELECT 'unindexed_groups' AS section;

SELECT a,COUNT(*) AS n FROM amounts IGNORE INDEX(by_generated_value) GROUP BY a;

EXPLAIN EXTENDED SELECT a,COUNT(*) AS n FROM amounts GROUP BY a;

SHOW WARNINGS;

DROP DATABASE decimal_groups;

SELECT COUNT(*) AS remaining_schemas FROM information_schema.SCHEMATA WHERE SCHEMA_NAME='decimal_groups';

Level  Code  Message

Warning  1264  Out of range value for column 'b' at row 1

Warning  1264  Out of range value for column 'b' at row 2

section

base_rows

a      b

11.11  9.9999

22.22  9.9999

section

indexed_groups

a      n

11.11  2

section

unindexed_groups

a      n

11.11  1

22.22  1

select `decimal_groups`.`amounts`.`a` AS `a`,count(0) AS `n` from `decimal_groups`.`amounts` group by `decimal_groups`.`amounts`.`b`