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What is the limit to the length of primary keys

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Does MySQL have any limitations on the length of primary keys?

In MySQL by default the maximum lenght for any index is 500 bytes for MyISAM tables and 1024 bytes for InnoDB. An index can contain from 1 to 16 columns or parts of columns. The default maximum lenght of index can be changed at compile. We just keep it limited to avoid the need to have a large thread stack.