SERIAL and BIGSERIAL data type


Informix supports the SERIAL, BIGSERIAL data types to produce automatic integer sequences:
  • SERIAL can produce 32 bit integers (INTEGER)
  • BIGSERIAL can produce 64 bit integers (BIGINT)
  • SERIAL8 is a synonym for BIGSERIAL
Steps to use serials with Informix:
  1. Create the table with a column using SERIAL, or BIGSERIAL.
  2. To generate a new serial, no value or a zero value is specified in the INSERT statement:
    INSERT INTO tab1 ( c ) VALUES ( 'aa' )
    INSERT INTO tab1 ( k, c ) VALUES ( 0, 'aa' )
  3. After INSERT, the new value of a SERIAL column is provided in sqlca.sqlerrd[2], while the new value of a BIGSERIAL value must be fetched with a SELECT dbinfo('bigserial') query.

Informix allows you to insert rows with a value different from zero for a serial column. Using an explicit value will automatically increment the internal serial counter, to avoid conflicts with future INSERT statements that are using a zero value:

SQL statement                                               Internal serial counter
CREATE TABLE tab ( pkey SERIAL, name VARCHAR(50) );                    0
INSERT INTO tab VALUES (  0, 'aaa' );                                  1
INSERT INTO tab VALUES ( 10, 'bbb' );                                 10
INSERT INTO tab VALUES (  0, 'ccc' );                                 11
DELETE FROM tab;                                                      11
INSERT INTO tab VALUES (  0, 'ddd' );                                 12

Oracle® MySQL and MariaDB

MySQL supports the AUTO_INCREMENT column definition option as well as the SERIAL keyword:

  • In CREATE TABLE, you specify a auto-incremented column with the AUTO_INCREMENT attribute

  • Auto-incremented columns have the same behavior as Informix SERIAL columns

  • A start value can be defined with ALTER TABLE tabname AUTO_INCREMENT = value

  • The column must be part of the primary key, or the first column of an index.

  • When using the InnoDB engine, with MySQL 5.7 and earlier, auto-incremented columns might reuse unused sequences after a server restart. For example, if you insert rows that generate the numbers 101, 102 and 103, then you delete rows 102 and 103; When the server is restarted, next generated number will be 101 + 1 = 102. Starting with MySQL 8.0, the last auto-incremented value is written to the disk and persists across server restarts.




For best SQL portability when using different types of databases, consider using sequences as described in Solution 3: Use native SEQUENCE database objects.

Note that MySQL (8.0) does not support CREATE SEQUENCE, while MariaDB (10.3) supports this feature. However, with MySQL it is possible to create a table dedicated to sequence generation, by using an AUTO_INCREMENT column.

The Informix SERIAL data type is emulated with MySQL AUTO_INCREMENT option.

The serial type emulation can be enabled or disabled with the following FGLPROFILE entries:
dbi.database.dbname.ifxemul.datatype.serial = {true|false}
dbi.database.dbname.ifxemul.datatype.serial8 = {true|false}
dbi.database.dbname.ifxemul.datatype.bigserial = {true|false}

Disabling automatic serial retrieval for sqlca.sqlerrd[2]

For Informix compatibility, when the SERIAL type emulation is active, the ODI drivers automatically execute another SQL query (or do a DB client API call when possible) after each INSERT statement, to get the last generated serial, and fill the sqlca.sqlerrd[2] register. This results in some overhead that can be avoided, if the sqlca.sqlerrd[2] register is not used by the program.

When serial emulation is required (to create temp tables with a serial column during program execution), and the sqlca.sqlerrd[2] register does not need to be filled, (typically because you use your own method to retrieve the last generated serial), you can set the ifxemul.datatype.serial.sqlerrd2 FGLPROFILE entry to false. This will avoid the automatic retrieval of last serial value to fill sqlca.sqlerrd[2]:

dbi.database.dbname.ifxemul.datatype.serial.sqlerrd2 = false
The above FGLPROFILE entry is useless, if Informix SERIAL type emulation is disabled with:
dbi.database.dbname.ifxemul.datatype.serial = false

See also db_get_last_serial().

Using the native serial emulation (only option)

After an insert, sqlca.sqlerrd[2] holds the last generated serial value. However, sqlca.sqlerrd[2] is defined as an INTEGER, it cannot hold values from BIGINT auto incremented columns. If you are using BIGINT auto incremented columns, you must use the LAST_INSERT_ID() SQL function.

AUTO_INCREMENT columns must be primary keys. This is handled automatically when you create a table in a BDL program.

Like Informix, MySQL allows you to specify a zero for auto-incremented columns. However, for SQL portability, it is recommended to review INSERT statements to remove the SERIAL column from the list.

For example, the following statement:
INSERT INTO tab (col1,col2) VALUES ( 0, p_value)
can be converted to:
INSERT INTO tab (col2) VALUES (p_value)

Static SQL INSERT using records defined from the schema file must also be reviewed:

DEFINE rec LIKE tab.*
INSERT INTO tab VALUES ( rec.* ) -- will use the serial column 

can be converted to:

INSERT INTO tab VALUES rec.* -- without parentheses, serial column is removed