IDENTITY Columns
An identity column is an integer or bigint column whose values are automatically generated from a system-defined sequence.
Note: Identity columns in Vector only work for INSERT…VALUE.
An identity column is a way to automatically generate a unique numeric value for each row in a table. A table can have only one column that is defined with the identity attribute.
The IDENTITY clause has the following format:
[GENERATED ALWAYS AS [seq_name] IDENTITY [(seq_options)]
| GENERATED BY DEFAULT AS [seq_name] IDENTITY [(seq_options)]]
where:
GENERATED ALWAYS AS [seq_name] IDENTITY [(seq_options)]
Indicates that the column value is determined by the corresponding sequence. The user cannot specify an explicit value for the column in an INSERT or UPDATE statement.
INSERT statements that contain ALWAYS identity columns in their column list must specify DEFAULT as the corresponding value. To override this behavior, use the OVERRIDING SYSTEM VALUE and OVERRIDING USER VALUE clauses of the INSERT statement.
The data type of the sequence matches the data type of the identity column.
GENERATED BY DEFAULT AS [seq_name] IDENTITY [(seq_options)]
Indicates that the user can optionally provide an explicit value for the column.
seq_name
Defines the name of the sequence.
seq_options
Control how the sequence supplies data when requested by an application. Sequence options can be specified in any order, and none are required.
Any of the following seq_options can be specified in a blank-space separated list:
START WITH number
Specifies the start of the sequence as an integer constant. The default value is 1 for positive sequences (positive increment) and -1 for negative sequences (negative increment). (This option is valid with the CREATE SEQUENCE statement only.)
RESTART WITH number
Specifies a new start value for the sequence. (This option is valid with the ALTER SEQUENCE statement only.)
INCREMENT BY number
Specifies the increment value (positive or negative) that produces successive values of the sequence.
Default: 1
MAXVALUE number
Specifies the maximum value allowed for the sequence.
Defaults:
For positive integer sequences: 2**31-1
For positive bigint sequences: 2**63-1
For positive decimal(n) sequences: 10**(n+1)-1
For negative sequences: -1
NO MAXVALUE / NOMAXVALUE
Specifies that sequences can generate values with an upper bound equivalent to that of the data type chosen to hold the sequence (for example, 2**31-1 for integers).
MINVALUE number
Specifies the minimum value allowed for the sequence.
Default:
For positive sequences: 1
For negative bigint sequences: -2**63
For negative integer sequences: -2**31
For negative decimal(n) sequences: -(10**(n+1)-1)
NO MINVALUE / NOMINVALUE
Specifies that sequences can generate values with a lower bound equivalent to that of the data type chosen to hold the sequence (for example, -2**31 for integers).
CACHE number
Specifies the number of sequence values held in server memory. When the supply of numbers is exhausted, Vector requires a catalog access to acquire the next set.
Default: 20
NO CACHE / NOCACHE
Specifies that sequence values are not to be cached by the server. When this option is selected, a catalog access is required for each request for a sequence value. This can severely degrade application performance.
Default: CACHE 20 (when neither CACHE nor NOCACHE are specified), which ensures low catalog overhead.
Note: Gaps between two sequence values may occur: If the DBMS is restarted between two requests of a sequence value, the cached but not used values are lost and the cached values are not written back when shutting down. After restart, the Sequence Cache is empty, so the next value is taken from iisequence.
Note: In a multiple DBMS shared cache environment, sequence values may not be in chronological order. Each DBMS maintains its own Sequence Cache. No value appears twice, but the order of the requested values depends on which DBMS is returning the sequence value.
CYCLE
Specifies that the sequence restarts at the beginning value once it reaches the minimum value (negative increment) or maximum value (positive increment).
Default: NO CYCLE
NO CYCLE / NOCYCLE
Specifies that the sequence is not cycled when the last valid value is generated. An error is issued to the requesting transaction.
ORDER
NO ORDER / NOORDER
These options are included solely for syntax compatibility with other DBMSes that implement sequences, and are not currently supported.
Default: NOORDER
SEQUENTIAL / UNORDERED
Specifies whether values are returned sequentially or in unordered sequence. RANDOM is a synonym for UNORDERED. This option is ignored for decimal-based sequences.
Default: SEQUENTIAL
The sequence created to manage identity column values is accessible by its generated name and can be used as any other sequence, with CURRENT VALUE and NEXT VALUE operators. The generated sequence, however, cannot be explicitly dropped; instead, the identity column or table must be dropped, or the ALTER TABLE … ALTER COLUMN … DROP IDENTITY statement must be used.
For example:
GENERATED ALWAYS AS IDENTITY (START WITH 1 INCREMENT BY 20)
Note: When using the COMBINE command on a table with an identity column unexpected results, such as duplicate values, may occur. When records are inserted using COMBINE rather than the SQL INSERT, the values in the identify column in the Vector table will not be synchronized with the identity values stored in the Ingres system catalogs.