by Markus Winand.

Primary Keys

We start with the simplest yet most common where clause: the primary key lookup. For the examples throughout this chapter we use the EMPLOYEES table defined as follows:

CREATE TABLE employees (
   employee_id   NUMBER         NOT NULL,
   first_name    VARCHAR2(1000) NOT NULL,
   last_name     VARCHAR2(1000) NOT NULL,
   date_of_birth DATE           NOT NULL,
   phone_number  VARCHAR2(1000) NOT NULL,
   CONSTRAINT employees_pk PRIMARY KEY (employee_id)

The database automatically creates an index for the primary key. That means there is an index on the EMPLOYEE_ID column, even though there is no create index statement.


Appendix C, “Example Schema contains scripts to populate the EMPLOYEES table with sample data. You can use it to test the examples in your own environment.

To follow the text, it is enough to know that the table contains 1000 rows.

The following query uses the primary key to retrieve an employee’s name:

SELECT first_name, last_name
  FROM employees
 WHERE employee_id = 123

The where clause cannot match multiple rows because the primary key constraint ensures uniqueness of the EMPLOYEE_ID values. The database does not need to follow the index leaf nodes—it is enough to traverse the index tree. We can use the so-called execution plan for verification:


The following execution plan was gathered with the last_explained view available from the appendix.

Explain Plan
ID | Operation             |                Rows | Cost
 1 | RETURN                |                     |   13
 2 |  FETCH EMPLOYEES      |    1 of 1 (100.00%) |   13
 3 |   IXSCAN EMPLOYEES_PK | 1 of 1000 (   .10%) |    6

Predicate Information
 3 - START (Q1.EMPLOYEE_ID = +00123.)
      STOP (Q1.EMPLOYEE_ID = +00123.)

The Operation IXSCAN is similar to Oracle’s INDEX [RANGE|UNIQUE] SCAN. From this output, we cannot decided if it is a unique or range scan. The FETCH operation corresponds to Oracle’s TABLE ACCESS BY INDEX ROWID.


| id | table     | type  | key     | key_len | rows | Extra |
|  1 | employees | const | PRIMARY | 5       |    1 |       |

Type const is MySQL’s equivalent of Oracle’s INDEX UNIQUE SCAN.


|Id |Operation                   | Name         | Rows | Cost |
| 0 |SELECT STATEMENT            |              |    1 |    2 |
|*2 |  INDEX UNIQUE SCAN         | EMPLOYEES_PK |    1 |    1 |

Predicate Information (identified by operation id):
   2 - access("EMPLOYEE_ID"=123)


                QUERY PLAN
 Index Scan using employees_pk on employees 
   (cost=0.00..8.27 rows=1 width=14)
   Index Cond: (employee_id = 123::numeric)

The PostgreSQL operation Index Scan combines the INDEX [UNIQUE/RANGE] SCAN and TABLE ACCES BY INDEX ROWID operations from the Oracle Database. It is not visible from the execution plan if the index access might potentially return more than one row.

SQL Server

|--Nested Loops(Inner Join)
   |--Index Seek(OBJECT:employees_pk,
   |               SEEK:employees.employee_id=@1
   |            ORDERED FORWARD)
   |--RID Lookup(OBJECT:employees,
                 LOOKUP ORDERED FORWARD)

The SQL Server operation INDEX SEEK and RID Lookup correspond to Oracle’s INDEX RANGE SCAN and TABLE ACCESS BY ROWID respectively. Unlike the Oracle Database, SQL Server explicitly shows the Nested Loops join to combine the index and table data.

The Oracle execution plan shows an INDEX UNIQUE SCAN—the operation that only traverses the index tree. It fully utilizes the logarithmic scalability of the index to find the entry very quickly—almost independent of the table size.


The execution plan (sometimes explain plan or query plan) shows the steps the database takes to execute an SQL statement. Appendix A explains how to retrieve and read execution plans with other databases.

After accessing the index, the database must do one more step to fetch the queried data (FIRST_NAME, LAST_NAME) from the table storage: the TABLE ACCESS BY INDEX ROWID operation. This operation can become a performance bottleneck—as explained in Slow Indexes, Part I—but there is no such risk in connection with an INDEX UNIQUE SCAN. This operation cannot deliver more than one entry so it cannot trigger more than one table access. That means that the ingredients of a slow query are not present with an INDEX UNIQUE SCAN.

Primary Keys without Unique Index

A primary key does not necessarily need a unique index—you can use a non-unique index as well. In that case the Oracle database does not use an INDEX UNIQUE SCAN but instead the INDEX RANGE SCAN operation. Nonetheless, the constraint still maintains the uniqueness of keys so that the index lookup delivers at most one entry.

One of the reasons for using non-unique indexes for a primary keys are deferrable constraints. As opposed to regular constraints, which are validated during statement execution, the database postpones the validation of deferrable constraints until the transaction is committed. Deferred constraints are required for inserting data into tables with circular dependencies.

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