identity burning set factor

identity burning set factor changes the percentage of potential column values that is made available in a block of column values.

IDENTITY columns are of type numeric and scale zero whose values are generated by SAP ASE. Column values can range from a low of 1 to a high determined by the column precision.

For each table with an IDENTITY column, SAP ASE divides the set of possible column values into blocks of consecutive numbers, and makes one block at a time available in memory. Each time you insert a row into a table, SAP ASE assigns the IDENTITY column the next available value from the block. When all the numbers in a block have been used, the next block becomes available.

This method of choosing IDENTITY column values improves server performance. When SAP ASE assigns a new column value, it reads the current maximum value from memory and adds 1. Disk access becomes necessary only after all values within the block have been used. Because all remaining numbers in a block are discarded in the event of server failure (or shutdown with nowait), this method can lead to gaps in IDENTITY column values.

The value for identity burning set factor should be high enough for good performance, but not so high that gaps in column values are unacceptably large. The default value, 5000, releases .05 percent of the potential IDENTITY column values for use at one time.

To get the correct value for sp_configure, express the percentage in decimal form, and then multiply it by 10 ⁷ (10,000,000). For example, to release 15 percent (.15) of the potential IDENTITY column values at a time, specify a value of .15 times 10⁷ (or 1,500,000) in sp_configure.