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in MySQL, If I create a unique key on two columns, say
UNIQUE KEY my_key (column1, column2)
is it necessary to build another key on column1? My goal is to have column1 and column2 to be unique, and I’ll do lots of selects keyed by column1.
No, it is not necessary because an index on (column1, column2) can be used instead of an index of column1 alone.
You might want to make a new index on just column two though as the index (column1, column2) is not good when searching only for (column2).