Floating-point precision loss caused by distinct

after removing distinct in query,col_3's Floating-point precision loss happen

mysql> SELECT DISTINCT LENGTH(dzg62.c10) AS col_1, ABS(dzg62.c14) AS col_3 FROM t2 AS dzg62 USE INDEX (idx_t2_c3_c10) ORDER BY dzg62.c4 DESC UNION SELECT hkb92.c1 AS col_1, hkb92.c1 AS col_3 FROM t1 AS hkb92 WHERE (hkb92.c4 < 47);
+-------+-------------------+
| col_1 | col_3             |
+-------+-------------------+
|     6 | 90.91000366210938 |
|     6 | 97.70999908447266 |
|  3584 |            3584.0 |
+-------+-------------------+
3 rows in set (0.00 sec)

mysql> SELECT LENGTH(dzg62.c10) AS col_1,ABS(dzg62.c14) AS col_3 FROM t2 AS dzg62 USE INDEX (idx_t2_c3_c10) ORDER BY dzg62.c4 DESC UNION SELECT hkb92.c1 AS col_1, hkb92.c1 AS col_3 FROM t1 AS hkb92 WHERE (hkb92.c4 < 47);
+-------+--------+
| col_1 | col_3  |
+-------+--------+
|     6 |  90.91 |
|     6 |  97.71 |
|  3584 | 3584.0 |
+-------+--------+
3 rows in set (0.00 sec)

How to repeat

DROP DATABASE IF EXISTS test;
CREATE DATABASE IF NOT EXISTS test;
USE test;
CREATE TABLE t1 (
    c1 INT NOT NULL AUTO_INCREMENT,
    c2 VARCHAR(255) NOT NULL,
    c3 VARCHAR(255) NULL,
    c4 INT NULL,
    c5 DATE NOT NULL,
    c6 VARCHAR(10) NOT NULL,
    PRIMARY KEY (c1)
);

CREATE TABLE t2 (
    c1 INT NOT NULL AUTO_INCREMENT,
    c2 INT NOT NULL,
    c3 DECIMAL(10,2) NOT NULL,
    c4 VARCHAR(50) NOT NULL,
    c5 DATE NOT NULL,
    c6 MEDIUMTEXT NULL,
    c7 LONGTEXT NULL,
    c8 MEDIUMBLOB NULL,
    c9 LONGBLOB NULL,
    c10 ENUM('value1','value2','value3') NULL,
    c11 SET('a','b','c','d') NULL,
    c12 BIT(8) NULL,
    c13 DATETIME NULL,
    c14 FLOAT(8,2) NULL,
    c15 DOUBLE(12,4) NULL,
    PRIMARY KEY (c1)
);

INSERT INTO t1 (c1, c2, c3, c4, c5, c6) VALUES (3584, 'sample_AQJP3FCb7PX46hF2d3VoKsPIHtIgzieVgOK1Ue7iSbNProjjh', 'sample_v', 36, '2025-09-07', 'sample_J');

INSERT INTO t2 (c1, c2, c3, c4, c5, c6, c7, c8, c9, c10, c11, c12, c13, c14, c15) VALUES (994, 3423, 181.05, 'sample_wfxp8dzD7XrFnPLzr', '2024-12-16', 'sample_pVtuVRHgHyCTAdMjTX5uDmgDFTTcRtYK4MA3VzWGhxyRAthAan095ltHnWC2wZqFzmNIz2BhrrY2h0fxo8UzijtqT3Qzyc7ydCZ760AbwzyWxKRhxPNIiXBNmglw9ylwoKoF0Q5QMAuWKKHLT07TyALEjZEq9SXDPPHZkG637npBmt2TKOxONpcXFVVrsW3jgaSAVhmwZMYON8YAyhzkhs0fsDZAohNdcrXeb43HI5de5EWnnY2lj8xyjRyfvWyVnBrhfeRiVnwUHuk0bcwoN3ueyVTRnXwQSF56ZbxUsw4hUmzUQ4GGRNJqlHOzrdMFLZHnsp5SamGZ0WPFJC1Vx1RuAnB5RdThXSIL8o2Fgebw5VIlETIWBvsuSQqrlbUxl0DbKa4xwcIiP23T8cVxdUgU4xKNvx6nab1kO47gdMcGoSGyXyvr23yKaVpjtw6XNe0uwE6zNt4nqZ1IMHxo', 'sample_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', X'4DD6A6D687C6A9C08CDA93E9B48429DAA8ECBF93E891B67DDEBCDEBCE79C8A20', X'CF8CCBA2C880ED9DA9CF8644ECA5BD53E6B0AAED898E10C398D0AD', 'value1', 'a,c,b,d', b'01101010', '2025-04-25 18:28:30', 90.91, 53.96);
INSERT INTO t2 (c1, c2, c3, c4, c5, c6, c7, c8, c9, c10, c11, c12, c13, c14, c15) VALUES (9219, 8711, 166.61, 'sample_b7C1trecK6yntdY4x3tkM8epFQtYVnoEfz17wiJ0h80', '2025-04-12', 'sample_lM7ywI1JmA9kqk0PwSZZlxesXZdCBedJvwYKBw9dR56w7WO7DzbB1ZVsW2uBiEHEd2wR0zfA4sx55xrLO89GGMf4F2mz0UofYA4LoqJgyFeasOQ0xqixQuPntbtFFaU0qHyeuRqR7GpKm67SSQeJQd5eZ7q71APzN3n0zFxL9bC0jVQUAA2zi4LuWSd0Py4RHM12FEZBN1KuHU5ZozMhLHirnlRKFd0CHuHyUguieTnEjt89dxMMgaDrETZaKHVR1ScgTZefc8hb2Y5a2zsDfzRCQbzUBuZhyxWPDkLMg5eJaLJRAGHMN71OJrpgGZCWwFkmvr3GquST0vH8CnT59cLY9O9v9udhzAtGCiQXbwB5Lj7KzeYHgjKr8X09e6EGFI4JxrFhFkCd8iyB8xSq8NysRpDP91m9AphkU', 'sample_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', X'3E00CE8341C791EE918038EE949521C58A15D0B301D3BBE09EADEBBDA75BE890B0D58D53', X'37', 'value1', 'c,b,a', b'11010111', '2025-01-23 17:58:04', 97.71, 79.57);

CREATE  INDEX idx_t2_c3_c10 ON t2 (c3, c10);
SELECT DISTINCT LENGTH(dzg62.c10) AS col_1, ABS(dzg62.c14) AS col_3 FROM t2 AS dzg62 USE INDEX (idx_t2_c3_c10) ORDER BY dzg62.c4 DESC UNION SELECT hkb92.c1 AS col_1, hkb92.c1 AS col_3 FROM t1 AS hkb92 WHERE (hkb92.c4 < 47);
SELECT LENGTH(dzg62.c10) AS col_1,ABS(dzg62.c14) AS col_3 FROM t2 AS dzg62 USE INDEX (idx_t2_c3_c10) ORDER BY dzg62.c4 DESC UNION SELECT hkb92.c1 AS col_1, hkb92.c1 AS col_3 FROM t1 AS hkb92 WHERE (hkb92.c4 < 47);

Version:

mysql> select polardb_version();
+---------+--------------+---------------------+
| TYPE    | VERSION      | RELEASE_DATE        |
+---------+--------------+---------------------+
| Product | PolarDB V2.0 | Distributed Edition |
| CN      | 2.4.0.5.4.19 | SNAPSHOT            |
| DN      | 2.4.0.8.4.19 | 20240430            |
| GMS     | 2.4.0.8.4.19 | 20240430            |
+---------+--------------+---------------------+
4 rows in set (0.01 sec)

mysql> select @@version;
+-------------------------+
| @@version               |
+-------------------------+
| 8.0.32-X-Cluster-8.4.19 |
+-------------------------+
1 row in set (0.00 sec)

Start by running the SQL reproduction against the stated PolarDB-X and MySQL-compatible versions, comparing the DISTINCT and non-DISTINCT UNION results. Trace the query execution path for ABS on FLOAT values and DISTINCT handling; done means the reproduced query no longer loses floating-point precision and regression coverage exists for this case.