are primary keys automatically indexed
If you require more explicit control over the indexes associated with UNIQUE and PRIMARY KEY constraints, the database lets you: 1  Specify an existing index that the database is to use to enforce the constraint. 2  Specify a CREATE INDEX statement that the database is to use to create the index and enforce the constraint. Scenario 1 : Primary Key will default to Clustered Index. In this case we will create only Primary Key and when we check the kind of index created on the table we will notice that it has created clustered index automatically over it. -- Case 1 Primary Key Defaults to Clustered Index. USE TempDB. Re: Are Indices automatically generated for primary keys? Clemens Eisserer <linuxhippy(at)gmail(dot)com> wrote: > Are indices for columns marked with PRIMARY KEY automatically > generated by postgresql, or do I have to do it manually? 1.8.3.1 PRIMARY KEY and UNIQUE Index Constraints. Normally, errors occur for data-change statements (such as INSERT or UPDATE) that would violate primary-key, unique-key, or foreign-key constraints. If you are using a transactional storage engine such as InnoDB, MySQL automatically rolls back the statement. Scenario 3: Primary Key defaults to Non-Clustered Index with another column defined as a Clustered Index. In this case we will create clustered index on another column, SQL Server will automatically create a Primary Key as a non-clustered index as clustered index is specified on another column. -- Case 3 Primary Key Defaults to Non-clustered Index. USE TempDB. MySQL supports an IGNORE keyword for INSERT, UPDATE, and so forth. If you use it, MySQL ignores primary-key or unique-key violations and continues processing with the next row. See the section for the statement that you are using (Section 14.2.5, “INSERT Syntax”, Section 14.2.11, “UPDATE Syntax”, and so forth). Determine the best creation method. Indexes are created in the following ways: By defining a PRIMARY KEY or UNIQUE constraint on a column by using CREATE TABLE or ALTER TABLE. The SQL Server Database Engine automatically creates a unique index to enforce the uniqueness requirements of a PRIMARY KEY or UNIQUE constraint. Indexes can be created in any tablespace. An index can be created in the same or different tablespace as the table it indexes. If you use the same tablespace for a table and its index, it can be more convenient to perform database maintenance (such as tablespace or file backup) or to ensure application availability. If you look at the documentation page for CREATE TABLE, you'll see the following: | PostgreSQL automatically creates an index for each unique | constraint and primary key constraint to enforce uniqueness. Thus, | it is not necessary to create an index explicitly for primary key | columns. Use the following guidelines for determining when to create an index: 1  Create an index if you frequently want to retrieve less than 15% of the rows in a large table. The percentage varies greatly according to the relative speed of a table scan and how the distribution of the row data in relation to the index key.
Yes, primary keys automatically indexed.