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It may looks a naive question but I am wondering about the relationship between primary keys and indexes in most common SQL databases.
Is it a standard rule for a SQL database to create automatically an index for every primary key?
I am asking that because I am designing a model with Django and I am wondering if it is redundant to set both primary_key=True and db_index=True.
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The primary key is a special unique index. Only one primary key index can be defined in a table. The primary key is used to uniquely identify a record and is created using the keyword PRIMARY KEY. Indexes can cover multiple data columns, such as index like INDEX (columnA, columnB), which is a joint index.
A primary index is automatically created for the primary key and ensures that the primary key is unique. You can use the primary index to retrieve and access objects from the database. The unique index is a column, or an ordered collection of columns, for which each value identifies a unique row.
You can define a primary key in SQL Server by using SQL Server Management Studio or Transact-SQL. Creating a primary key automatically creates a corresponding unique clustered index, or a nonclustered index if specified as such.
A primary key is unique, whereas an index does not have to be unique. Therefore, the value of the primary key identifies a record in a table, the value of the index not necessarily. Primary keys usually are automatically indexed - if you create a primary key, no need to create an index on the same column(s).
Yes, you can assume it is a standard rule that RDBMSes require an index for the primary key.
Mainly, the database engine needs the index to enforces data uniqueness for the primary key, and to do this quickly requires an index.
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