Answer
A non-clustered index stores pointers to table rows and is used on non-primary key columns.
Quick Answer
In InnoDB, all indexes other than the primary key are non-clustered (secondary indexes). They store the indexed columns plus the primary key value for row lookup. A query using a secondary index often needs two lookups: find the PK in the secondary index, then look up the full row in the clustered index. Covering indexes avoid the second lookup by including all needed columns.
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SugharaIQ Editorial Team
Verified Answer
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Source: SugharaIQ