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#3275 ✓stale
Jordan Brough

[PATCH] Optimization for dropping multiple columns in MySQL

Reported by Jordan Brough | September 27th, 2009 @ 01:46 AM

Dropping multiple columns on a large table in MySQL (at least version 5.0) can be slow proportional to the number of columns being dropped. That can be a drag on a large production database:

...MySQL, for almost all ALTER TABLE commands, copies the entire table into a temporary table, then renames the new table and drops the old one. ... The only type of ALTER TABLE call that doesn’t copy the entire table is a table rename.


MySQL offers an extension to SQL that allows multiple columns to be dropped at once, so at least you only have to go through that once:

You can issue multiple ADD, ALTER, DROP, and CHANGE clauses in a single ALTER TABLE statement, separated by commas. This is a MySQL extension to standard SQL, which allows only one of each clause per ALTER TABLE statement. For example, to drop multiple columns in a single statement, do this:



Here's a patch that overrides MysqlAdapter#remove_column to issue multiple drops in a single statement.

I'm assuming this doesn't need additional tests since it's an implementation detail optimization of a feature that's already covered by tests but I'm happy to write some if someone has an idea about what to write.

The patch was applied to Rails 3.0 but the same diff should apply to Rails 2.3 as well.

Comments and changes to this ticket

  • CancelProfileIsBroken

    CancelProfileIsBroken September 27th, 2009 @ 11:25 AM

    • Tag changed from activerecord, optimization to activerecord, bugmash, optimization
  • Elomar França

    Elomar França September 27th, 2009 @ 05:53 PM

    +1, verified.

    I think that optimizations like this (tiny but significative) are always welcome. The patch applies on 2-3-stable and master.

  • sr.iniv.t

    sr.iniv.t September 27th, 2009 @ 06:35 PM

    +1 verified.

    The patch applies cleanly on 2-3-stable and master. But the patch lacks tests.

  • Jordan Brough

    Jordan Brough September 28th, 2009 @ 02:48 PM

    What would new tests look like? Verify that execute was only called once for multiple drops? Wouldn't that be getting a little bit too concerned with the 'how' instead of the 'what'? Functionality-wise it seems to be covered pretty well already but I'm happy to add some tests if there's some ideas on what tests to add.

  • Rizwan Reza

    Rizwan Reza February 12th, 2010 @ 12:46 PM

    • Tag changed from activerecord, bugmash, optimization to activerecord, optimization
  • Santiago Pastorino

    Santiago Pastorino February 9th, 2011 @ 12:31 AM

    • State changed from “new” to “open”

    This issue has been automatically marked as stale because it has not been commented on for at least three months.

    The resources of the Rails core team are limited, and so we are asking for your help. If you can still reproduce this error on the 3-0-stable branch or on master, please reply with all of the information you have about it and add "[state:open]" to your comment. This will reopen the ticket for review. Likewise, if you feel that this is a very important feature for Rails to include, please reply with your explanation so we can consider it.

    Thank you for all your contributions, and we hope you will understand this step to focus our efforts where they are most helpful.

  • Santiago Pastorino

    Santiago Pastorino February 9th, 2011 @ 12:31 AM

    • State changed from “open” to “stale”
  • bingbing

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