Bug #121231 Prepared statement uses stale div_precision_increment value after SESSION variable change

• Submitted: 4 Sep 14:57
• Modified: 23 Sep 7:16
• Reporter: Xiaoyuan Xie
• Status: Open
• Category: MySQL Server
• Severity: S2 (Serious)
• Version: 9.7.2
• OS: Any
• CPU Architecture: Any

[4 Sep 14:57] Xiaoyuan Xie

Description:
A prepared statement continues to use the value of div_precision_increment that was active when the statement was prepared, even after the SESSION variable is restored to its default value. 
After restoration, @@SESSION.div_precision_increment is 4, but executing the prepared statement still produces a result whose scale corresponds to div_precision_increment=8.

How to repeat:
SET SESSION div_precision_increment = 8;

PREPARE s FROM
    'SELECT CAST(1 AS DECIMAL(10,2)) / 3 AS result';

SET SESSION div_precision_increment = DEFAULT;

SELECT @@SESSION.div_precision_increment;
-- 4

EXECUTE s;

Actual Result: 0.3333333333
Expected Result: 0.333333

[10 Sep 7:51] Xiaoyuan Xie

New version: 9.7.2

[23 Sep 7:16] Xiaoyuan Xie

This bug appears to be related to session configuration handling rather than specifically to prepared statements.