Bug #119807 Generated default values bypass BLOB/TEXT/JSON default value restriction

• Submitted: 29 Jan 9:05
• Modified: 29 Jan 12:20
• Reporter: huanlong wang
• Status: Not a Bug
• Category: MySQL Server: DDL
• Severity: S3 (Non-critical)
• Version: 8.0.34
• OS: Any
• CPU Architecture: Any

[29 Jan 9:05] huanlong wang

Description:
The check for default values on BLOB/TEXT/JSON columns only applies to constant defaults, not expression defaults, allowing bypass of the restriction.

How to repeat:
1. Connect to MySQL 8.0.34
2. Run:
   CREATE TABLE t1 (a TEXT DEFAULT '');  -- Fails (good)
   CREATE TABLE t2 (a TEXT DEFAULT (concat('', '')));  -- Succeeds (bug!)

Expected Result:
All statements should fail with ERROR 1101.

Actual Result:
Only constant defaults are caught; expression defaults bypass the check.

Suggested fix:
Analysis:
In Create_field::init(), the check only tests fld_default_value but not fld_default_val_expr. This creates an inconsistency.

Suggested Fix:
Extend the check to cover both default value types.

[29 Jan 12:20] MySQL Verification Team

Please see this section of the manual:

https://dev.mysql.com/doc/refman/8.4/en/data-type-defaults.html
 The BLOB, TEXT, GEOMETRY, and JSON data types can be assigned a default value only if the value is written as an expression, even if the expression value is a literal:

    This is permitted (literal default specified as expression):

    CREATE TABLE t2 (b BLOB DEFAULT ('abc'));

    This produces an error (literal default not specified as expression):

    CREATE TABLE t2 (b BLOB DEFAULT 'abc');

Expression default values must adhere to the following rules. An error occurs if an expression contains disallowed constructs.