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Topic: unique the index across the field email, whether costs?

Good afternoon. At me a little vast question.
Whether costs in tables of users, to do unique an index across the field email (naturally, it is supposed that email is unique). Or it is better to do a normal index of certain length? Whether has value at a choice the size of the table, an amount of users?

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Re: unique the index across the field email, whether costs?

Victor256;
Restriction of uniqueness is necessary or it is not necessary is the task defines. If value has to be unique under the task - the unique index means should be.

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Re: unique the index across the field email, whether costs?

Melkij;
But if under a field 100-200 characters are selected? Where that boundary, more which it is not necessary to do an index?

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Re: unique the index across the field email, whether costs?

Victor256 wrote:

where that boundary, more which it is not necessary to do an index?

On field boundary. Otherwise uniqueness of an index loses meaning, and uniqueness of value is not guaranteed.