MySQL Bugs: #51680: "ALTER TABLE ... DELAY_KEY_WRITE=1" causes a full table copy

Bug #51680	"ALTER TABLE ... DELAY_KEY_WRITE=1" causes a full table copy
Submitted:	3 Mar 2010 8:36	Modified:	3 Mar 2010 9:30
Reporter:	Max Sheehan	Email Updates:
Status:	Duplicate	Impact on me:	None
Category:	MySQL Server: DDL	Severity:	S3 (Non-critical)
Version:	5.1.43	OS:	IBM AIX
Assigned to:		CPU Architecture:	Any
Tags:	alter, DELAY_KEY_WRITE

Description:
If I do "ALTER TABLE dbname.tblname DELAY_KEY_WRITE=1", I would expect a flag to be changed on the table and that's about it. However, MySQL is doing a very slow copy (about 1MB/s) as if I had inserted a column or something.

How to repeat:
Create a table.
Insert a couple of million records.
ALTER TABLE dbname.tblname DELAY_KEY_WRITE=1.

Suggested fix:
I can't imagine DELAY_KEY_WRITE needs to modify each row so shouldn't it be considered a "Fast" alteration like renaming a column would?

Please do not submit the same bug more than once. An existing bug report already describes this very problem. Even if you feel that your issue is somewhat different, the resolution is likely
to be the same. Because of this, we hope you add your comments to the original bug instead.

Thank you for your interest in MySQL.

Duplicate of bug #45492