What is Secondary Index

While making the index, generally the index table is kept in the primary memory (RAM) and the main table, because of its size is keeps in the secondary memory (Hard Disk). Theoretically, a table may have millions of records (as the telephone directory of a large city), for which even a sparse index becomes so large in size that we cannot keep it in the primary memory. And if we cannot keep the index in the primary memory, then we lose the benefit of the speed of access. For very large table, it is better to classify the index in multiple levels.