Adaptive Server provides two types of indexes:
Clustered indexes, where the table data is physically stored in the order of the keys on the index:
For allpages-locked tables, rows are stored in key order on pages, and pages are linked in key order.
For data-only-locked tables, indexes are used to direct the storage of data on rows and pages, but strict key ordering is not maintained.
Nonclustered indexes, where the storage order of data in the table is not related to index keys
You can create only one clustered index on a table because there is only one possible physical ordering of the data rows. You can create up to 249 nonclustered indexes per table.
A table that has no clustered index is called a heap. The rows in the table are in no particular order, and all new rows are added to the end of the table. Chapter 7, “Data Storage,” discusses heaps and SQL operations on heaps.