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I'm considering using pt-online-schema-change to ALTER a primary key (changing it from one column, to a composite primary index) of a table. Are there any limitations to doing this? The documentation states

In most cases the tool will refuse to operate unless a PRIMARY KEY or UNIQUE INDEX is present in the table. See --alter for details.

Ztyx
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Well, don't do that. Running pt-online-schema-change on a table w/o a single column unique index, may result in a loss of data. See #3 here: http://www.pythian.com/blog/important-caveats-when-using-pt-online-schema-change/

Moshe Kaplan
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    Thanks! I posted a follow-up question regarding a workaround for this: http://stackoverflow.com/q/30798499/260805 – Ztyx Jun 12 '15 at 08:20