Basic Data Structures
Terminology clarification
Terms have obtained different meanings, depending on context.
Term |
SQL |
Database implementation |
Key |
Usually understood to mean primary key. Key is unique. |
The key of a hash table or tree. Key may or may not be unique. |
Primary key |
Set of columns that uniquely identifies rows. Also, the target
of foreign keys. |
— |
Index |
Data structure for fast lookup by key |
Primary index |
— |
Not an option in Postgres. |
Secondary index |
— |
Neither clustered nor sparse. All indexes in Postgres are secondary. |
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