Chapter 3. Data Types

Describes the built-in data types available in Postgres.

Postgres has a rich set of native data types available to users. Users may add new types to Postgres using the CREATE TYPE command.

In the context of data types, the following sections will discuss SQL standards compliance, porting issues, and usage. Some Postgres types correspond directly to SQL92-compatible types. In other cases, data types defined by SQL92 syntax are mapped directly into native Postgres types. Many of the built-in types have obvious external formats. However, several types are either unique to Postgres, such as open and closed paths, or have several possibilities for formats, such as the date and time types.

Note: The cidr and inet types are designed to handle any IP type but only ipv4 is handled in the current implementation. Everything here that talks about ipv4 will apply to ipv6 in a future release.

Postgres has features at the forefront of ORDBMS development. In addition to SQL3 conformance, substantial portions of SQL92 are also supported. Although we strive for SQL92 compliance, there are some aspects of the standard which are ill considered and which should not live through subsequent standards. Postgres will not make great efforts to conform to these features; however, these tend to apply in little-used or obsure cases, and a typical user is not likely to run into them.

Most of the input and output functions corresponding to the base types (e.g., integers and floating point numbers) do some error-checking. Some of the operators and functions (e.g., addition and multiplication) do not perform run-time error-checking in the interests of improving execution speed. On some systems, for example, the numeric operators for some data types may silently underflow or overflow.

Some of the input and output functions are not invertible. That is, the result of an output function may lose precision when compared to the original input.

Note: Floating point numbers are allowed to retain most of the intrinsic precision of the type (typically 15 digits for doubles, 6 digits for 4-byte floats). Other types with underlying floating point fields (e.g. geometric types) carry similar precision.

Numeric Types

Numeric types consist of two- and four-byte integers, four- and eight-byte floating point numbers and fixed-precision decimals.

The numeric types have a full set of corresponding arithmetic operators and functions. Refer to Numerical Operators and Mathematical Functions for more information.

The int8 type may not be available on all platforms since it relies on compiler support for this.

The Serial Type

The serial type is a special-case type constructed by Postgres from other existing components. It is typically used to create unique identifiers for table entries. In the current implementation, specifying

CREATE TABLE tablename (colname SERIAL);
     

CREATE SEQUENCE tablename_colname_seq;
CREATE TABLE tablename
    (colname INT4 DEFAULT nextval('tablename_colname_seq');
CREATE UNIQUE INDEX tablename_colname_key on tablename (colname);
     

CREATE TABLE tablename (colname SERIAL);
DROP TABLE tablename;
CREATE TABLE tablename (colname SERIAL);