How pt-online-schema-change Handles Foreign Keys

pt-online-schema-changeForeign key related issues are very common when dealing with DDL changes in MySQL using Percona toolkit. In this blog post, I will explain how the tool (pt-online-schema-change) handles foreign key constraints when executing a DDL change.

First of all, I would like to explain why foreign keys have to be handled at all before writing more about the “How”. Foreign key constraints are aware of table rename operations. In other words, if the parent table is renamed, the child table automatically knows it and changes the foreign key constraint accordingly. Please have a look at the below example, and you can see the table name is automatically updated in the child table after the rename operation on the parent table:

Well, that is indeed very nice and to be expected. But please allow me to explain how this becomes a problem when dealing with DDL changes using pt-online-schema-change. The tool implements the DDL changes as mentioned below. Please keep in mind that these are just to give an idea of how the tool works, as there would be more internal things going on in reality.

Let’s take an example ALTER for this case:

Query:

pt-online-schema-change steps for the above alter:

  1. Create a similar table _T1_new
  2. Modify the column c1 to BIGINT in the table _T1_new
  3. Define triggers on table T1 so that changes to data on the original table will be applied to _T1_new as well.
  4. Copy the data from table T1 to _T1_new.
  5. Swap the tables
  6. Drop triggers.

All looks good so far. Now let’s see why these steps create a problem, with a close look at Step #5 (Swap the tables).

Without foreign keys: Swapping of these tables is done as below, which looks nice.

  • Rename T1 —> T1_old
  • Rename _T1_new –> T1
  • If everything right, drop the table T1_old
  • Drop triggers on the new T1 table

With foreign keys: As I mentioned earlier, if there are any child tables with foreign keys to table T1, after renaming, they would automatically point to T1_old but not the new T1. Pt online schema change has to ensure the child table refers to a correct parent table by the end of this DDL change.

  • Rename T1 —> T1_old       =====➤ The child table refers to T1_old automatically.
  • Rename _T1_new —> T1

In this case, the foreign keys in the child table are still referring to the old table T1_old which don’t have the schema change in place. If you drop T1_old, child table CT1 ends up pointing to a table that doesn’t exist. That’s a very bad situation. Now let’s talk about how the tool handles this.

How does pt-online-schema-change handle this?

The tool comes up with an option named --alter-foreign-keys-method This option supports two values at a high level and below you can see what are those and how they will work.

alter-foreign-keys-method=drop_swap

With this value, it won’t swap as mentioned in the steps. Rather, it drops the old table and then renames the new table with the change in place.

  • Disable foreign key checks for the session (FOREIGN_KEY_CHECKS=0)
  • Drop the table T1_old
  • Rename the new table _T1_new –> T1

The good thing here is that it is quick, but the bad thing is that it’s not reliable. If something goes wrong with renaming, it ends up with the same problem of referring to an unexisting table.

alter-foreign-keys-method=rebuild_constraints

This is the preferred approach for the reason it maintains the consistency of the schema and its relations. In this approach, before dropping the old table, it runs ALTER on all the child tables to drop existing FK and re-add new FK constraints that points to the columns from the new table (with the schema change in place).  Below sequence of bullet points explains the same.

  • Rename T1 –> T1_old
  • Rename _T1_new –> T1
  • ALTER on child table to adjust the foreign key so that it points to T1 rather T1_old.

  • Drop the table T1_old
  • Drop triggers from the new T1 table.

I would like to mention that the current implementation to rebuild the child table can be improved by making use of the INPLACE ALTER which I hope would probably be available in upcoming releases. You can see more information about this in the existing bug report here. I will discuss in brief about the two other options available, which are derived based on the above two. Let’s have a quick look.

auto:  If this value is used, it leaves the decision up to the tool itself to choose from the two (drop_swap/rebuild_constraints) options available. If the number of rows in the child table is small, it uses rebuild_constraints; otherwise, it goes with the drop_swap approach. For this reason, this option should always be chosen carefully as it can end up with unexpected results when choosing drop_swap. Below is an example log snippet which explains this behavior:

none: If this value is used, it is similar to drop_swap but without swapping. In other words, it just drops the original table and leaves the child tables in a state which they point to a table that doesn’t exist. In this case, DBA’s have need to fix the leftover job.

Photo by Silas Köhler on Unsplash

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  • Nick Postma Reply

    Hi Uday Varagani,

    Interesting post!

    I am experiencing some problems with Percona toolkit 3.0.6 regarding your topic. When creating a FK referencing to the same table i keep having a reference to a _old table on success when rebuild_constraints is used. Use DROP_SWAP and there is no issue. a FK to itself is created. Can you explain why this is happening? Your article states that this can occur on error. But i am getting a ‘Successfully altered ‘ message.

    Looking forward to your insight,

    Nick

    June 13, 2019 at 9:20 am

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