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Is PK automatically indexed?
Yes a primary key is always an index.
Which types of constraint require an index?
Unique and Primary Key constraints are implemented using indexes. In most cases, Foreign Key constraints can also be implemented with associated indexes.
For which constraints indexes are created when the constraint is added?
For primary key and unique constraints, the index is built on the constrained columns as a mechanism to detect duplicates as rows are added or updated. For referential constraints, the index is built on the referencing columns of the constraint.
What does a foreign key constraint do?
A foreign key is a column (or combination of columns) in a table whose values must match values of a column in some other table. FOREIGN KEY constraints enforce referential integrity, which essentially says that if column value A refers to column value B, then column value B must exist.
What is the difference between index and constraint?
A unique index ensures that the values in the index key columns are unique. A unique constraint also guarantees that no duplicate values can be inserted into the column(s) on which the constraint is created. When a unique constraint is created a corresponding unique index is automatically created on the column(s).
How to find index associated with PK constraint?
An enabled PK constraint is always associated with an index. The associated index can be unique or non-unique (discussed later). The corresponding index can be find by querying: Also, if we have an enabled PK constraint, the corresponding column (s) will be “ NOT NULL “.
How to enforce the PK constraint in Oracle?
So, to enforce the PK constraint, Oracle has to search through the already present records and this is where the index comes handy. If you have an index on that column, the search will be quite fast. The unique index will be the best but a non-unique index will also be a better option as compared to a full table scan.
Can a primary key be associated with an index?
A primary key is a constraint, not an index. An index can be associated with a primary key, but it’s not required. And unless using SQL Server, an index doesn’t ensure uniqueness.
Why do you use PK instead of null?
1. In the future releases of Oracle , PK is the one going to stay and we should use PK in our tables. 2. In case of replication, if the table does not have PK we cant replicate the that table. and we said… why use a unique index + not null when you could use a primary key? A primary key has semantic meaning, it TELLS people things.