[6 Feb 2008 17:04] Paul Dubois

Description:

In MySQL 5.1, work was done to make some ALTER TABLE operations faster, with no need for a temporary table (such as changes that affect only metadata), as described here:

http://dev.mysql.com/doc/refman/5.1/en/alter-table.html

However, a user cannot tell in advance whether a given operation will be fast or not, and thus must actually execute ALTER TABLE and hope that it will be fast.

An EXPLAIN-style statement would be useful for ALTER TABLE. I envision that EXPLAIN ALTER TABLE would provide information to the user that would indicate the "cost" of a given alteration.

How to repeat:

N/A

[6 Feb 2008 19:32] Valeriy Kravchuk

Thank you for a reasonable feature request.

[18 Oct 2016 8:51] Daniël van Eeden

A duplicate of this bug:
Bug #80793 	EXTEND EXPLAIN to cover ALTER TABLE

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