Comment 1 for bug 994002

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Baron Schwartz (baron-xaprb) wrote : Re: pt-online-schema-change 2.1.1-cannot alter table with mutli field PK where 2nd field is char

Looks to me like the relevant part of the output is this:

Error copying rows from `app`.`CityPopularity` to `app`.`___CityPopularity_new`: Possible infinite loop detected! The lower boundary for chunk 1 is <ar, ar, ae, ar, ae, 7> and the lower boundary for chunk 2 is also <ar, ar, ae, ar, ae, 7>. This usually happens when using a non-unique single column index. The current chunk index for table app.CityPopularity is guest_language which is not unique and covers 3 columns.

So the tool is not using the primary key for some reason. This might duplicate an existing report. To be investigated.