Is it possible in general to have two primary indices on the same relation for different search keys?
The correct answer and explanation is :
No, in general, it is not possible to have two primary indices on the same relation for different search keys. The primary index is defined on a relation’s primary key, which must be unique for each record. Since a table can only have one primary key, it can have only one primary index.
Explanation:
A primary index is an index on a primary key, which uniquely identifies each row in a table. It is typically implemented as a clustered index, meaning that the data in the table is physically organized in the order of the primary key. Because the physical order of rows in a database can follow only one sequence, a table can have only one primary index.
If we were to define two different primary indices on the same relation with different search keys, we would face several problems:
- Conflicting Data Organization: Since a primary index typically dictates how data is stored on disk, having two different primary indices would require two conflicting physical organizations, which is not feasible.
- Uniqueness Constraint: A primary index enforces uniqueness on its key, meaning that only one such index can exist per table.
- Alternative Approaches: While a table cannot have two primary indices, it can have multiple secondary indices. A secondary index is an additional index that allows efficient searching on other attributes without affecting the physical organization of the table.
Conclusion:
A table can have only one primary index, but multiple secondary indices can be created to support searches on different attributes. If performance improvements are needed for multiple search keys, composite indexes or secondary indexes are the preferred solutions.