MySQL + SQL · Lesson 89
Primary Index and Secondary Index
Two Kinds of Index
| Type | Meaning |
|---|---|
| Primary (clustered) | Built on the primary key; decides the physical row order. One per table. |
| Secondary (non-clustered) | Extra index on other columns; points to the row location. Many allowed. |
Key Point
In InnoDB, the primary key IS the clustered index — data is physically stored in primary-key order. Secondary indexes store the primary key to find the full row.
Summary
- Primary/clustered index = one per table, sets row order (the primary key).
- Secondary index = additional, on other searched columns.
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