MySQL Bugs: #80793: EXTEND EXPLAIN to cover ALTER TABLE

Bug #80793	EXTEND EXPLAIN to cover ALTER TABLE
Submitted:	18 Mar 2016 16:03	Modified:	19 Jul 2016 8:06
Reporter:	Simon Mudd (OCA)	Email Updates:
Status:	Verified	Impact on me:	None
Category:	MySQL Server: DDL	Severity:	S4 (Feature request)
Version:	8.0	OS:	Any
Assigned to:		CPU Architecture:	Any
Tags:	ALTER TABLE, explain

Description:
ALTER TABLE now has many ways of working and that can be a bit confusing even for a seasoned MySQL DBA especially as things change all the time.

How to repeat:
Run ALTER TABLE and see which behaviour happens depending on the statement you execute.

There are various possible behaviours:

1. METADATA change, no change to data
2. INLINE change, no full rebuild and apps can use the table while the change is happening (e.g. new indexes being added)
3. TABLE REBUILD, traditional full rebuild of the table

Suggested fix:
I'd like to see EXPLAIN EXTENDED to show what it would do with the ALTER TABLE statement.

There are other subtle things that may trigger a full rebuild of the table including changes due to data type changes (e.g. the "old" and "new" timestamp or datetime formats), so having a way to see what MySQL would do without actually triggering the action would be really good.

Posted by developer:
 
Reported version value updated to reflect release name change from 5.8 to 8.0

Hello Simon,

Thank you for the feature request!

Thanks,
Umesh

Duplicate of BUG #34354