5.14. SEQUENCE (GENERATOR)

A sequence  — or generator — is a database object used to get unique number values to fill a series. Sequence is the SQL-compliant term for the same thing which — in Firebird — has traditionally been known as generator. Firebird has syntax for both terms.

Sequences are stored as 64-bit integers, regardless of the SQL dialect of the database.

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Caution

If a client is connected using Dialect 1, the server handles sequence values as 32-bit integers. Passing a sequence value to a 32-bit field or variable will not cause errors as long as the current value of the sequence does not exceed the limits of a 32-bit number. However, as soon as the sequence value exceeds this limit, a database in Dialect 3 will produce an error. A database in Dialect 1 will truncate (overflow) the value, which could compromise the uniqueness of the series.

This section describes how to create, alter, set and drop sequences.

5.14.1. CREATE SEQUENCE

Used forCreating a new SEQUENCE (GENERATOR)

Available inDSQL, ESQL

Syntax 

  |CREATE {SEQUENCE | GENERATOR} seq_name
  |  [START WITH start_value]
  |  [INCREMENT [BY] increment]

Table 5.36. CREATE SEQUENCE Statement Parameters
ParameterDescription

seq_name

Sequence name. The maximum length is 63 characters

start_value

First value produced by NEXT VALUE FOR seq_name. A 64-bit integer from -2-63 to 263-1. Default is 1.

increment

Increment of the sequence when using NEXT VALUE FOR seq_name; cannot be 0. Default is 1.

When a sequence is created, its current value is set so that the next value produced by NEXT VALUE FOR seq_name is equal to start_value. In other words, the current value of the sequence is set to (start_value - increment).

The optional INCREMENT [BY] clause allows you to specify a non-zero increment for the NEXT VALUE FOR seq_name expression.

The GEN_ID(seq_name, step) function can be called instead, to step the sequence by a different increment. The increment specified through INCREMENT [BY] is not used by GEN_ID. Using both NEXT VALUE FOR and GEN_ID, especially when the sequence has an increment other than 1, may result in values you did not expect. For example, if you execute CREATE SEQUENCE x START WITH 10 INCREMENT BY 10, and then use GEN_ID(x, 1), the value returned is 1, and if you then call NEXT VALUE FOR x, you get 11.

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Non-standard behaviour for negative increments

The SQL standard specifies that sequences with a negative increment should start at the maximum value of the sequence (263 - 1) and count down. Firebird does not do that, and instead starts at 1 unless you specify a START WITH value.

This may change in a future Firebird version.

The statements CREATE SEQUENCE and CREATE GENERATOR are synonymous — both create a new sequence. Either can be used, but CREATE SEQUENCE is recommended as that is the syntax defined in the SQL standard.

5.14.1.1. Who Can Create a Sequence?

The CREATE SEQUENCE (CREATE GENERATOR) statement can be executed by:

  • Administrators

  • Users with the CREATE SEQUENCE (CREATE GENERATOR) privilege

The user executing CREATE SEQUENCE (CREATE GENERATOR) becomes its owner.

5.14.1.2. Examples of CREATE SEQUENCE

  1. Creating the EMP_NO_GEN sequence using CREATE SEQUENCE.

      |CREATE SEQUENCE EMP_NO_GEN;

  2. Creating the EMP_NO_GEN sequence using CREATE GENERATOR.

      |CREATE GENERATOR EMP_NO_GEN;

  3. Creating the EMP_NO_GEN sequence with an initial value of 5 and an increment of 1.

      |CREATE SEQUENCE EMP_NO_GEN START WITH 5;

  4. Creating the EMP_NO_GEN sequence with an initial value of 1 and an increment of 10.

      |CREATE SEQUENCE EMP_NO_GEN INCREMENT BY 10;

  5. Creating the EMP_NO_GEN sequence with an initial value of 5 and an increment of 10.

      |CREATE SEQUENCE EMP_NO_GEN START WITH 5 INCREMENT BY 10;

See alsoSection 5.14.2, “ALTER SEQUENCE”, Section 5.14.3, “CREATE OR ALTER SEQUENCE”, Section 5.14.4, “DROP SEQUENCE”, Section 5.14.5, “RECREATE SEQUENCE”, Section 5.14.6, “SET GENERATOR”, NEXT VALUE FOR, GEN_ID() function

5.14.2. ALTER SEQUENCE

Used forSetting the next value of a sequence or changing its increment

Available inDSQL

Syntax 

  |ALTER {SEQUENCE | GENERATOR} seq_name
  |  [RESTART [WITH start_value]]
  |  [INCREMENT [BY] increment]

Table 5.37. ALTER SEQUENCE Statement Parameters
ParameterDescription

seq_name

Sequence name

start_value

Next value produced by NEXT VALUE FOR seq_name. A 64-bit integer from -2-63 to 263-1. Default is the start value in the metadata.

increment

Increment of the sequence (when using NEXT VALUE FOR seq_name); cannot be 0.

The ALTER SEQUENCE statement sets the next value of the sequence, and/or changes the increment of the sequence.

The RESTART WITH start_value clause sets the current value of the sequence so that the next value obtained from NEXT VALUE FOR seq_name is equal to start_value. To achieve this, the current value of the sequence is set to (start_value - increment) with increment either as specified in the statement, or from the metadata of the sequence. The RESTART clause without WITH start_value behaves as if WITH start_value is specified with the start value from the metadata of the sequence.

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Note

Contrary to Firebird 3.0, since Firebird 4.0 RESTART WITH start_value only restarts the sequence with the specified value, and does not store start_value as the new start value of the sequence. A subsequent ALTER SEQUENCE RESTART will use the start value specified when the sequence was created, and not the start_value of this statement. This behaviour is specified in the SQL standard.

It is currently not possible to change the start value stored in the metadata.

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Warning

Incorrect use of ALTER SEQUENCE — changing the current value of the sequence or generator — is likely to break the logical integrity of data, or result in primary key or unique constraint violations.

INCREMENT [BY] allows you to change the sequence increment for the NEXT VALUE FOR expression.

Changing the increment value takes effect for all queries that run after the transaction commits. Procedures that are called for the first time after changing the commit, will use the new value if they use NEXT VALUE FOR. Procedures that were already cached in the metadata cache will continue to use the old increment. You may need to close all connections to the database for the metadata cache to clear, and the new increment to be used. Procedures using NEXT VALUE FOR do not need to be recompiled to see the new increment. Procedures using GEN_ID(gen, expression) are not affected when the increment is changed.

5.14.2.1. Who Can Alter a Sequence?

The ALTER SEQUENCE (ALTER GENERATOR) statement can be executed by:

  • Administrators

  • The owner of the sequence

  • Users with the ALTER ANY SEQUENCE (ALTER ANY GENERATOR) privilege

5.14.2.2. Examples of ALTER SEQUENCE

  1. Setting the value of the EMP_NO_GEN sequence so the next value is 145.

      |ALTER SEQUENCE EMP_NO_GEN RESTART WITH 145;

  2. Resetting the sequence EMP_NO_GEN to the start value stored in the metadata

      |ALTER SEQUENCE EMP_NO_GEN RESTART;

  3. Changing the increment of sequence EMP_NO_GEN to 10

      |ALTER SEQUENCE EMP_NO_GEN INCREMENT BY 10;

See alsoSection 5.14.6, “SET GENERATOR”, Section 5.14.1, “CREATE SEQUENCE”, Section 5.14.3, “CREATE OR ALTER SEQUENCE”, Section 5.14.4, “DROP SEQUENCE”, Section 5.14.5, “RECREATE SEQUENCE”, NEXT VALUE FOR, GEN_ID() function

5.14.3. CREATE OR ALTER SEQUENCE

Used forCreating a new or modifying an existing sequence

Available inDSQL, ESQL

Syntax 

  |CREATE OR ALTER {SEQUENCE | GENERATOR} seq_name
  |  {RESTART | START WITH start_value}
  |  [INCREMENT [BY] increment]

Table 5.38. CREATE OR ALTER SEQUENCE Statement Parameters
ParameterDescription

seq_name

Sequence name. The maximum length is 63 characters

start_value

First or next value produced by NEXT VALUE FOR seq_name. A 64-bit integer from -2-63 to 263-1. Default is 1.

increment

Increment of the sequence when using NEXT VALUE FOR seq_name; cannot be 0. Default is 1.

If the sequence does not exist, it will be created as documented under Section 5.14.1, “CREATE SEQUENCE”. An existing sequence will be changed:

  • If RESTART is specified, the sequence is restarted with the start value stored in the metadata

  • If the START WITH clause is specified, the sequence is restarted with start_value, but the start_value is not stored. In other words, it behaves as RESTART WITH in Section 5.14.2, “ALTER SEQUENCE”.

  • If the INCREMENT [BY] clause is specified, increment is stored as the increment in the metadata, and used for subsequent calls to NEXT VALUE FOR

5.14.3.1. Example of CREATE OR ALTER SEQUENCE

Create a new or modify an existing sequence EMP_NO_GEN 

  |CREATE OR ALTER SEQUENCE EMP_NO_GEN
  |  START WITH 10
  |  INCREMENT BY 1

See alsoSection 5.14.1, “CREATE SEQUENCE”, Section 5.14.2, “ALTER SEQUENCE”, Section 5.14.4, “DROP SEQUENCE”, Section 5.14.5, “RECREATE SEQUENCE”, Section 5.14.6, “SET GENERATOR”, NEXT VALUE FOR, GEN_ID() function

5.14.4. DROP SEQUENCE

Used forDropping (deleting) a SEQUENCE (GENERATOR)

Available inDSQL, ESQL

Syntax 

  |DROP {SEQUENCE | GENERATOR} seq_name

Table 5.39. DROP SEQUENCE Statement Parameter
ParameterDescription

seq_name

Sequence name. The maximum length is 63 characters

The statements DROP SEQUENCE and DROP GENERATOR are equivalent: both drop (delete) an existing sequence. Either is valid but DROP SEQUENCE, being defined in the SQL standard, is recommended.

The statements will fail if the sequence has dependencies.

5.14.4.1. Who Can Drop a Sequence?

The DROP SEQUENCE (DROP GENERATOR) statement can be executed by:

  • Administrators

  • The owner of the sequence

  • Users with the DROP ANY SEQUENCE (DROP ANY GENERATOR) privilege

5.14.4.2. Example of DROP SEQUENCE

Dropping the EMP_NO_GEN series: 

  |DROP SEQUENCE EMP_NO_GEN;

See alsoSection 5.14.1, “CREATE SEQUENCE”, Section 5.14.3, “CREATE OR ALTER SEQUENCE”, Section 5.14.5, “RECREATE SEQUENCE”

5.14.5. RECREATE SEQUENCE

Used forCreating or recreating a sequence

Available inDSQL, ESQL

Syntax 

  |RECREATE {SEQUENCE | GENERATOR} seq_name
  |  [START WITH start_value]
  |  [INCREMENT [BY] increment]

Table 5.40. RECREATE SEQUENCE Statement Parameters
ParameterDescription

seq_name

Sequence name. The maximum length is 63 characters

start_value

First value produced by NEXT VALUE FOR seq_name. A 64-bit integer from -2-63 to 263-1. Default is 1.

increment

Increment of the sequence (when using NEXT VALUE FOR seq_name); cannot be 0. Default is 1.

See Section 5.14.1, “CREATE SEQUENCE” for the full syntax of CREATE SEQUENCE and descriptions of defining a sequences and its options.

RECREATE SEQUENCE creates or recreates a sequence. If a sequence with this name already exists, the RECREATE SEQUENCE statement will try to drop it and create a new one. Existing dependencies will prevent the statement from executing.

5.14.5.1. Example of RECREATE SEQUENCE

Recreating sequence EMP_NO_GEN 

  |RECREATE SEQUENCE EMP_NO_GEN
  |  START WITH 10
  |  INCREMENT BY 2;

See alsoSection 5.14.1, “CREATE SEQUENCE”, Section 5.14.2, “ALTER SEQUENCE”, Section 5.14.3, “CREATE OR ALTER SEQUENCE”, Section 5.14.4, “DROP SEQUENCE”, Section 5.14.6, “SET GENERATOR”, NEXT VALUE FOR, GEN_ID() function

5.14.6. SET GENERATOR

Used forSetting the current value of a sequence or generator to a specified value

Available inDSQL, ESQL

Syntax 

  |SET GENERATOR seq_name TO new_val

Table 5.41. SET GENERATOR Statement Parameters
ParameterDescription

seq_name

Sequence name

new_val

New sequence value. A 64-bit integer from -2-63 to 263-1.

The SET GENERATOR statement sets the current value of a sequence or generator to the specified value.

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Note

Although SET GENERATOR is considered outdated, it is retained for backward compatibility. Use of the standards-compliant ALTER SEQUENCE is recommended.

5.14.6.1. Who Can Use a SET GENERATOR?

The SET GENERATOR statement can be executed by:

  • Administrators

  • The owner of the sequence

  • Users with the ALTER ANY SEQUENCE (ALTER ANY GENERATOR) privilege

5.14.6.2. Example of SET GENERATOR

Setting the value of the EMP_NO_GEN sequence to 145: 

  |SET GENERATOR EMP_NO_GEN TO 145;

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Note

Similar effects can be achieved with Section 5.14.2, “ALTER SEQUENCE”:

  |ALTER SEQUENCE EMP_NO_GEN
  |  RESTART WITH 145 + increment;

Here, the value of increment is the current increment of the sequence. We need add it as ALTER SEQUENCE calculates the current value to set based on the next value it should produce.

See alsoSection 5.14.2, “ALTER SEQUENCE”, Section 5.14.1, “CREATE SEQUENCE”, Section 5.14.3, “CREATE OR ALTER SEQUENCE”, Section 5.14.4, “DROP SEQUENCE”, NEXT VALUE FOR, GEN_ID() function