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SQL Anywhere 10.0.1 » SQL Anywhere Server - SQL Usage » Ensuring Data Integrity » Using column defaults

The AUTOINCREMENT default Next Page

The GLOBAL AUTOINCREMENT default


The GLOBAL AUTOINCREMENT default is intended for use when multiple databases are used in a SQL Remote replication or MobiLink synchronization environment. It ensures unique primary keys across multiple databases.

This option is similar to AUTOINCREMENT, except that the domain is partitioned. Each partition contains the same number of values. You assign each copy of the database a unique global database identification number. SQL Anywhere supplies default values in a database only from the partition uniquely identified by that database's number.

The partition size can be any positive integer, although the partition size is generally chosen so that the supply of numbers within any one partition will rarely, if ever, be exhausted.

If the column is of type BIGINT or UNSIGNED BIGINT, the default partition size is 232 = 4294967296; for columns of all other types, the default partition size is 216 = 65536. Since these defaults may be inappropriate, especially if your column is not of type INT or BIGINT, it is best to specify the partition size explicitly.

When using this option, the value of the public option global_database_id in each database must be set to a unique, non-negative integer. This value uniquely identifies the database and indicates from which partition default values are to be assigned. The range of allowed values is np + 1 to (n + 1) p, where n is the value of the public option global_database_id and p is the partition size. For example, if you define the partition size to be 1000 and set global_database_id to 3, then the range is from 3001 to 4000.

If the previous value is less than (n + 1) p, the next default value is one greater than the previous largest value in column. If the column contains no values, the first default value is np + 1. Default column values are not affected by values in the column outside of the current partition; that is, by numbers less than np + 1 or greater than p(n + 1). Such values may be present if they have been replicated from another database via MobiLink synchronization.

Because the public option global_database_id cannot be set to a negative value, the values chosen are always positive. The maximum identification number is restricted only by the column data type and the partition size.

If the public option global_database_id is set to the default value of 2147483647, a NULL value is inserted into the column. If NULL values are not permitted, attempting to insert the row causes an error. This situation arises, for example, if the column is contained in the table's primary key.

NULL default values are also generated when the supply of values within the partition has been exhausted. In this case, a new value of global_database_id should be assigned to the database to allow default values to be chosen from another partition. Attempting to insert the NULL value causes an error if the column does not permit NULLs. To detect that the supply of unused values is low and handle this condition, create an event of type GlobalAutoincrement. See Understanding events.

Global autoincrement columns are typically primary key columns or columns constrained to hold unique values (see Enforcing entity integrity).

While using the global autoincrement default in other cases is possible, doing so can adversely affect database performance. For example, in cases where the next value for each column is stored as a 64-bit signed integer, using values greater than 231 - 1 or large double or numeric values may cause wraparound to negative values.

You can retrieve the most recent value inserted into an autoincrement column using the @@identity global variable. For more information, see @@identity global variable.

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