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What is a covering index?

A An index that covers every column of the table
B An index containing all columns a query needs, so the table itself is never read
C An index on a foreign key
D An index that covers multiple tables

Answer

An index containing all columns a query needs, so the table itself is never read

When all required columns are in the index, MySQL answers from the index alone. EXPLAIN shows 'Using index' in the Extra column.

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