MySQL Bugs: #121231: Prepared statement uses stale div_precision_increment value after SESSION variable change

Bug #121231	Prepared statement uses stale div_precision_increment value after SESSION variable change
Submitted:	4 Sep 14:57
Reporter:	Xiaoyuan Xie	Email Updates:
Status:	Open	Impact on me:	None
Category:	MySQL Server: Prepared statements	Severity:	S2 (Serious)
Version:	9.7.0	OS:	Any
Assigned to:		CPU Architecture:	Any

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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