# Pt-online-schema-change says unique index is missing but it is defined

**URL:** <https://forums.percona.com/t/pt-online-schema-change-says-unique-index-is-missing-but-it-is-defined/11008>\
**Category:** MySQL & MariaDB\
**Tags:** mysql, percona\
**Created:** [June 23, 2021, 7:56pm UTC](https://forums.percona.com/t/pt-online-schema-change-says-unique-index-is-missing-but-it-is-defined/11008 "2021-06-23T19:56:22Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![zylo47](https://avatars.discourse-cdn.com/v4/letter/z/5fc32e/32.png) [@zylo47](https://forums.percona.com/u/zylo47)\
**Post date:** [June 23, 2021, 7:56pm UTC](https://forums.percona.com/t/pt-online-schema-change-says-unique-index-is-missing-but-it-is-defined/11008/1 "2021-06-23T19:56:22Z")

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I am using pt-online-schema-change to convert some large tables to the utf8mb4 character set. For a number of tables I am getting an error message similar to the following

> The new table `my_db`.`_my_table_new` does not have a PRIMARY KEY or a unique index which is required for the DELETE trigger.

In each case, I have confirmed that a composite unique key is, in fact, defined on the original table. The error message is referencing the \_new version of the table

Does pt-online-schema-change not work with composite unique keys? Is there a specific switch I should be using for it to take effect?

I’m using Aurora MySQL 5.6 and pt-online-schema-change version 3.1.0

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<div class="post-metadata">

**Author:** ![zylo47](https://avatars.discourse-cdn.com/v4/letter/z/5fc32e/32.png) [@zylo47](https://forums.percona.com/u/zylo47)\
**Post date:** [June 24, 2021, 2:26pm UTC](https://forums.percona.com/t/pt-online-schema-change-says-unique-index-is-missing-but-it-is-defined/11008/2 "2021-06-24T14:26:00Z")

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Someone gave me the answer to this in a Slack discussion so I wanted to post the answer here for future reference if anyone else encounters this obscure problem.

It turns out the problem is that at least one of the columns in the UNIQUE KEY are NULLable. According [to the docs](https://dev.mysql.com/doc/refman/8.0/en/create-index.html):

> A `UNIQUE` index permits multiple `NULL` values for columns that can contain `NULL`

Other RDBMS don’t behave this way. The effect is that pt-online-schema-change will not use the UNIQUE KEY defined on at least one NULLable column as a unique constraint so it is bypassed, causing the error.