Surrogate key


A surrogate key in a database is a unique identifier for either an entity in the modeled world or an object in the database. The surrogate key is not derived from application data, unlike a natural key which is derived from application data.

Definition

There are at least two definitions of a surrogate:
; Surrogate - Hall, Owlett and Todd : A surrogate represents an entity in the outside world. The surrogate is internally generated by the system but is nevertheless visible to the user or application.
; Surrogate - Wieringa and De Jonge : A surrogate represents an object in the database itself. The surrogate is internally generated by the system and is invisible to the user or application.
The Surrogate definition relates to a data model rather than a storage model and is used throughout this article. See Date.
An important distinction between a surrogate and a primary key depends on whether the database is a current database or a temporal database. Since a current database stores only currently valid data, there is a one-to-one correspondence between a surrogate in the modeled world and the primary key of the database. In this case the surrogate may be used as a primary key, resulting in the term surrogate key. In a temporal database, however, there is a many-to-one relationship between primary keys and the surrogate. Since there may be several objects in the database corresponding to a single surrogate, we cannot use the surrogate as a primary key; another attribute is required, in addition to the surrogate, to uniquely identify each object.
Although Hall et al. say nothing about this, others have argued that a surrogate should have the following characteristics:
In a current database, the surrogate key can be the primary key, generated by the database management system and not derived from any application data in the database. The only significance of the surrogate key is to act as the primary key. It is also possible that the surrogate key exists in addition to the database-generated UUID.
A surrogate key is frequently a sequential number. Some databases provide UUID/GUID as a possible data type for surrogate keys.
Having the key independent of all other columns insulates the database relationships from changes in data values or database design and guarantees uniqueness.
In a temporal database, it is necessary to distinguish between the surrogate key and the business key. Every row would have both a business key and a surrogate key. The surrogate key identifies one unique row in the database, the business key identifies one unique entity of the modeled world. One table row represents a slice of time holding all the entity's attributes for a defined timespan. Those slices depict the whole lifespan of one business entity. For example, a table EmployeeContracts may hold temporal information to keep track of contracted working hours. The business key for one contract will be identical in both rows however the surrogate key for each row is unique.
SurrogateKeyBusinessKeyEmployeeNameWorkingHoursPerWeekRowValidFromRowValidTo
1BOS0120John Smith402000-01-012000-12-31
56P0000123Bob Brown251999-01-012011-12-31
234BOS0120John Smith352001-01-012009-12-31

Some database designers use surrogate keys systematically regardless of the suitability of other candidate keys, while others will use a key already present in the data, if there is one.
Some of the alternate names describe the way of generating new surrogate values rather than the nature of the surrogate concept.
Approaches to generating surrogates include:

Stability

Surrogate keys typically do not change while the row exists. This has the following advantages:
Attributes that uniquely identify an entity might change, which might invalidate the suitability of natural keys. Consider the following example:
In these cases, generally a new attribute must be added to the natural key.
With a surrogate key, only the table that defines the surrogate key must be changed. With natural keys, all tables that use the natural key will have to change.
Some problem domains do not clearly identify a suitable natural key. Surrogate keys avoid choosing a natural key that might be incorrect.

Performance

Surrogate keys tend to be a compact data type, such as a four-byte integer. This allows the database to query the single key column faster than it could multiple columns. Furthermore, a non-redundant distribution of keys causes the resulting b-tree index to be completely balanced. Surrogate keys are also less expensive to join than compound keys.

Compatibility

While using several database application development systems, drivers, and object-relational mapping systems, such as Ruby on Rails or Hibernate, it is much easier to use an integer or GUID surrogate keys for every table instead of natural keys in order to support database-system-agnostic operations and object-to-row mapping.

Uniformity

When every table has a uniform surrogate key, some tasks can be easily automated by writing the code in a table-independent way.

Validation

It is possible to design key-values that follow a well-known pattern or structure which can be automatically verified. For instance, the keys that are intended to be used in some column of some table might be designed to "look differently from" those that are intended to be used in another column or table, thereby simplifying the detection of application errors in which the keys have been misplaced. However, this characteristic of the surrogate keys should never be used to drive any of the logic of the applications themselves, as this would violate the principles of Database normalization.

Disadvantages

Disassociation

The values of generated surrogate keys have no relationship to the real-world meaning of the data held in a row. When inspecting a row holding a foreign key reference to another table using a surrogate key, the meaning of the surrogate key's row cannot be discerned from the key itself. Every foreign key must be joined to see the related data item. If appropriate database constraints have not been set, or data imported from a legacy system where referential integrity was not employed, it is possible to have a foreign-key value that does not correspond to a primary-key value and is therefore invalid.
To discover such errors, one must perform a query that uses a left outer join between the table with the foreign key and the table with the primary key, showing both key fields in addition to any fields required to distinguish the record; all invalid foreign-key values will have the primary-key column as NULL. The need to perform such a check is so common that Microsoft Access actually provides a "Find Unmatched Query" wizard that generates the appropriate SQL after walking the user through a dialog. "Find Unmatched" queries are typically employed as part of a data cleansing process when inheriting legacy data.
Surrogate keys are unnatural for data that is exported and shared. A particular difficulty is that tables from two otherwise identical schemas can hold records that are equivalent in a business sense, but have different keys. This can be mitigated by NOT exporting surrogate keys, except as transient data.
When surrogate keys supplant natural keys, then domain specific referential integrity will be compromised. For example, in a customer master table, the same customer may have multiple records under separate customer IDs, even though the natural key would be unique. To prevent compromise, the natural key of the table must NOT be supplanted: it must be preserved as a unique constraint, which is implemented as a unique index on the combination of natural-key fields.

Query optimization

Relational databases assume a unique index is applied to a table's primary key. The unique index serves two purposes: to enforce entity integrity, since primary key data must be unique across rows and to quickly search for rows when queried. Since surrogate keys replace a table's identifying attributes—the natural key—and since the identifying attributes are likely to be those queried, then the query optimizer is forced to perform a full table scan when fulfilling likely queries. The remedy to the full table scan is to apply indexes on the identifying attributes, or sets of them. Where such sets are themselves a candidate key, the index can be a unique index.
These additional indexes, however, will take up disk space and slow down inserts and deletes.

Normalization

Surrogate keys can result in duplicate values in any natural keys. To prevent duplication, one must preserve the role of the natural keys as unique constraints when defining the table using either SQL's CREATE TABLE statement or ALTER TABLE...ADD CONSTRAINT statement, if the constraints are added as an afterthought.

Business process modeling

Because surrogate keys are unnatural, flaws can appear when modeling the business requirements. Business requirements, relying on the natural key, then need to be translated to the surrogate key. A strategy is to draw a clear distinction between the logical model and the physical implementation of that model, to ensure that the logical model is correct and reasonably well normalised, and to ensure that the physical model is a correct implementation of the logical model.

Inadvertent disclosure

Proprietary information can be leaked if sequential key generators are used. By subtracting a previously generated sequential key from a recently generated sequential key, one could learn the number of rows inserted during that time period. This could expose, for example, the number of transactions or new accounts per period. There are a few ways to overcome this problem:
Sequentially generated surrogate keys can imply that events with a higher key value occurred after events with a lower value. This is not necessarily true, because such values do not guarantee time sequence as it is possible for inserts to fail and leave gaps which may be filled at a later time. If chronology is important then date and time must be separately recorded.

Citations