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SQL Anywhere 11.0.0 » SQL Remote » Replication Design for SQL Remote » SQL Remote installation design » Ensuring unique primary keys » Using global autoincrement default column values


How default values are chosen

The public option global_database_id in each database must be set to a unique, non-negative integer. The range of default values for a particular database is pn + 1 to p(n + 1), where p is the partition size and n is the value of the public option global_database_id. For example, if the partition size is 1000 and global_database_id is set to 3, then the range is from 3001 to 4000.

If global_database_id is set to a non-negative integer, SQL Anywhere chooses default values by applying the following rules:

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

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

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

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.

Should the values in a particular partition become exhausted, you can assign a new database ID to that database. You can assign new database ID numbers in any convenient manner. However, one possible technique is to maintain a pool of unused database ID values. This pool is maintained in the same manner as a pool of primary keys.

You can set an event handler to automatically notify the database administrator (or do some other action) when the partition is nearly exhausted. See Defining trigger conditions for events.

See global_database_id option [database].

For further information on pools, see Using primary key pools.