An OCCURS clause is a DMSII construct that describes the number of times an item is present or repeats within a data set. Because relational databases do not support the OCCURS construct, these clauses generate additional tables, which can degrade the performance of update processing.

You can control how items with an OCCURS clause are mapped on an item by item basis by flattening OCCURS. See Flattening OCCURS Clauses.

Default OCCURS Handling

If you don't flatten OCCURS, DATABridge Client creates a new table for each data item that contains an OCCURS clause. The keys from the data item’s parent data set are used as keys in the new table. In addition, a new key (named index1) is created to establish a unique composite key for each recurring data item.

For example, a DMSII data set has a data item with an OCCURS clause will result in two relational database tables:

  • The first table (called the primary table) is named the same as the DMSII data set and contains the key as well as all non-OCCURS data items.
  • The second table (called the secondary table) is named with the data set name plus the data item name. This table contains all of the OCCURS items; however, each table has a unique key created by index1.

Handling OCCURS items this way can significantly degrade the performance of update processing if the number of occurrences is large. The storage required to hold the keys of the secondary table items can also be substantial. For example, an OCCURS 100 TIMES clause can turn a single DMSII update into 101 relational database updates. See DMSII DASDL with OCCURS for an example of a DMSII data set that has a data item with an OCCURS clause.