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Named sequences are sequences that are defined by CREATE SEQUENCE. Unlike IDENTITY sequences, which are defined in a table's DDL, you create a named sequence as an independent object, and then set it as the default value of a table column.
Named sequences are used most often when an application requires a unique identifier in a table or an expression. After a named sequence returns a value, it never returns the same value again in the same session.
You create a named sequence with
CREATE SEQUENCE. The statement requires only a sequence name; all other parameters are optional. To create a sequence, a user must have CREATE privileges on a schema that contains the sequence.
The following example creates an ascending named sequence, my_seq, starting at the value 100:
=> CREATE SEQUENCE my_seq START 100;
CREATE SEQUENCE
When you create a named sequence object, you can also specify its increment or decrement value by setting its INCREMENT parameter. If you omit this parameter, as in the previous example, the default is set to 1.
You increment or decrement a sequence by calling the function
NEXTVAL on it—either directly on the sequence itself, or indirectly by adding new rows to a table that references the sequence. When called for the first time on a new sequence, NEXTVAL initializes the sequence to its start value. Vertica also creates a cache for the sequence. Subsequent NEXTVAL calls on the sequence increment its value.
The following call to NEXTVAL initializes the new my_seq sequence to 100:
=> SELECT NEXTVAL('my_seq');
nextval
---------
100
(1 row)
You can obtain the current value of a sequence by calling
CURRVAL on it. For example:
=> SELECT CURRVAL('my_seq');
CURRVAL
---------
100
(1 row)
CURRVAL returns an error if you call it on a new sequence that has not yet been initialized by NEXTVAL, or an existing sequence that has not yet been accessed in a new session. For example:
=> CREATE SEQUENCE seq2;
CREATE SEQUENCE
=> SELECT currval('seq2');
ERROR 4700: Sequence seq2 has not been accessed in the session
A table can set the default values of any column to a named sequence. The table creator must have the following privileges: SELECT on the sequence, and USAGE on its schema.
In the following example, column id gets its default values from named sequence my_seq:
=> CREATE TABLE customer(id INTEGER DEFAULT my_seq.NEXTVAL,
lname VARCHAR(25),
fname VARCHAR(25),
membership_card INTEGER
);
For each row that you insert into table customer, the sequence invokes the NEXTVAL function to set the value of the id column. For example:
=> INSERT INTO customer VALUES (default, 'Carr', 'Mary', 87432);
=> INSERT INTO customer VALUES (default, 'Diem', 'Nga', 87433);
=> COMMIT;
For each row, the insert operation invokes NEXTVAL on the sequence my_seq, which increments the sequence to 101 and 102, and sets the id column to those values:
=> SELECT * FROM customer;
id | lname | fname | membership_card
-----+-------+-------+-----------------
101 | Carr | Mary | 87432
102 | Diem | Nga | 87433
(1 row)
When you create a sequence, its CACHE parameter determines the number of sequence values each node maintains during a session. The default cache value is 250K, so each node reserves 250,000 values per session for each sequence. The default cache size provides an efficient means for large insert or copy operations.
If sequence caching is set to a low number, nodes are liable to request a new set of cache values more frequently. While it supplies a new cache, Vertica must lock the catalog. Until Vertica releases the lock, other database activities such as table inserts are blocked, which can adversely affect overall performance.
When a new session starts, node caches are initially empty. By default, the initiator node requests and reserves cache for all nodes in a cluster. You can change this default so each node requests its own cache, by setting configuration parameter ClusterSequenceCacheMode to 0.
For information on how Vertica requests and distributes cache among all nodes in a cluster, refer to Sequence caching.
Vertica distributes a session across all nodes. The first time a cluster node calls the function NEXTVAL on a sequence to increment (or decrement) its value, the node requests its own cache of sequence values. The node then maintains that cache for the current session. As other nodes call NEXTVAL, they too create and maintain their own cache of sequence values.
During a session, nodes call NEXTVAL independently and at different frequencies. Each node uses its own cache to populate the sequence. All sequence values are guaranteed to be unique, but can be out of order with a NEXTVAL statement executed on another node. As a result, sequence values are often non-contiguous.
In all cases, increments a sequence only once per row. Thus, if the same sequence is referenced by multiple columns, NEXTVAL sets all columns in that row to the same value. This applies to rows of joined tables.
Vertica calculates the current value of a sequence as follows:
At the end of every statement, the state of all sequences used in the session is returned to the initiator node.
The initiator node calculates the maximum
CURRVAL of each sequence across all states on all nodes.
This maximum value is used as CURRVAL in subsequent statements until another NEXTVAL is invoked.
Sequence values in cache can be lost in the following situations:
If a statement fails after NEXTVAL is called (thereby consuming a sequence value from the cache), the value is lost.
If a disconnect occurs (for example, dropped session), any remaining values in cache that have not been returned through NEXTVAL are lost.
When the initiator node distributes a new block of cache to each node where one or more nodes has not used up its current cache allotment. For information on this scenario, refer to Sequence caching.
You can recover lost sequence values by using ALTER SEQUENCE...RESTART, which resets the sequence to the specified value in the next session.
ALTER SEQUENCE can change sequences in two ways:
ALTER SEQUENCE can change one or more sequence attributes through the following parameters:
| These parameters... | Control... |
|---|---|
INCREMENT |
How much to increment or decrement the sequence on each call to NEXTVAL. |
MINVALUE/MAXVALUE |
Range of valid integers. |
RESTART |
Sequence value on its next call to NEXTVAL. |
CACHE/NO CACHE |
How many sequence numbers are pre-allocated and stored in memory for faster access. |
CYCLE/NO CYCLE |
Whether the sequence wraps when its minimum or maximum values are reached. |
These changes take effect only when you start a new database session. For example, if you create a named sequence my_sequence that starts at 10 and increments by 1 (the default), each sequence call to NEXTVAL increments its value by 1:
=> CREATE SEQUENCE my_sequence START 10;
=> SELECT NEXTVAL('my_sequence');
nextval
---------
10
(1 row)
=> SELECT NEXTVAL('my_sequence');
nextval
---------
11
(1 row)
The following ALTER SEQUENCE statement specifies to restart the sequence at 50:
=>ALTER SEQUENCE my_sequence RESTART WITH 50;
However, this change has no effect in the current session. The next call to NEXTVAL increments the sequence to 12:
=> SELECT NEXTVAL('my_sequence');
NEXTVAL
---------
12
(1 row)
The sequence restarts at 50 only after you start a new database session:
=> \q
$ vsql
Welcome to vsql, the Vertica Analytic Database interactive terminal.
=> SELECT NEXTVAL('my_sequence');
NEXTVAL
---------
50
(1 row)
You can use ALTER SEQUENCE to make the following changes to a sequence:
Rename it (supported only for named sequences).
Move it to another schema (supported only for named sequences).
Reassign ownership.
Each of these changes requires separate ALTER SEQUENCE statements. These changes take effect immediately.
For example, the following statement renames a sequence from my_seq to serial:
=> ALTER SEQUENCE s1.my_seq RENAME TO s1.serial;
This statement moves sequence s1.serial to schema s2:
=> ALTER SEQUENCE s1.my_seq SET SCHEMA TO s2;
The following statement reassigns ownership of s2.serial to another user:
=> ALTER SEQUENCE s2.serial OWNER TO bertie;
Use
DROP SEQUENCE to remove a named sequence. For example:
=> DROP SEQUENCE my_sequence;
You cannot drop a sequence if one of the following conditions is true:
Other objects depend on the sequence. DROP SEQUENCE does not support cascade operations.
A column's DEFAULT expression references the sequence. Before dropping the sequence, you must remove all column references to it.