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Always use indexes. They allow faster look up of columns. The lookups can be done with a b-tree rather than a full scan leading to O(logn) instead of O(n).
A way to uniquely identify a row. It must be unique, and there can only be one primary key per table. It can consist of multiple columns.
All primary keys are indexes.
EXPLAIN can be used to describe the query optimization plan.
A few key things to look out for:
type
The type is important for determining the access. Some common ones are ref, specifying index lookups. all specifying full scan (normally bad). range specifying range query
Shows better sequence of query.
Executes query and reports back statistics.
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