Indexing criteria: disk space usage

The following table provides estimates of the amount of space each index uses compared to the amount of column data from the source database or flat file.

**Table 6-3: Index disk space usage**
Type of index	Estimated space versus raw data	Comments
Default	Smaller than or equal to	If the number of distinct values is less than 255, this index uses significantly less space than the raw data
High_Group	Smaller than up to 2 times larger	As the number of distinct values decreases (that is, the number of entries per group increases), the space used decreases in proportion to the size of the raw data
High_Non_Group	Smaller than or equal to	Smaller than the raw data in most cases
Low_Fast	Smaller than up to 2 times larger	Same as High_Group
Date	Smaller than or equal to	Larger than High_Non_Group
Time	Smaller than or equal to	Larger than High_Non_Group
Datetime	Smaller than or equal to	Larger than High_Non_Group

For LF and HG indexes, the index size depends on the number of unique values. The more unique values, the more space the index takes.

Because CMP indexes are always an additional index, they do not save disk space.