Improving insert performance

Before data is inserted into a table, an insert search algorithm examines the free space control records (FSCRs) to find a page with enough space for the new data.

However, even when an FSCR indicates that a page has enough free space, that space might not be usable if it has been reserved by an uncommitted delete operation from another transaction.

The DB2MAXFSCRSEARCH registry variable specifies the number of FSCRs to search when adding a record to a table. The default is to search five FSCRs. Modifying this value enables you to balance insert speed with space reuse. Use large values to optimize for space reuse. Use small values to optimize for insert speed. Setting the value to -1 forces the database manager to search all FSCRs. If sufficient space is not found while searching FSCRs, the data is appended to the end of the table.

The APPEND ON option on the ALTER TABLE statement specifies that table data will be appended and that information about free space on pages will not be kept. Such tables must not have a clustering index. This option can improve performance for tables that only grow.

If a clustering index is defined on the table, the database manager attempts to insert records on the same page as other records with similar index key values. If there is no space on that page, surrounding pages are considered. If those pages are unsuitable, the FSCRs are searched, as described previously. In this case, however, a worst-fit approach is used instead of a first-fit approach. The worst-fit approach tends to choose pages with more free space. This method establishes a new clustering area for rows with similar key values.

If you have defined a clustering index on a table, use the PCTFREE clause on the ALTER TABLE statement before loading or reorganizing the table. The PCTFREE clause specifies the percentage of free space that should remain on a data page after a load or reorg operation. This increases the probability that the cluster index operation will find free space on the appropriate page.