Sybase 12.4.2 manual Indexing criteria disk space usage, Data types in the index, 143

CHAPTER 4 Adaptive Server IQ Indexes

These estimates are generally valid; however, other factors can take precedence:

•For range predicates, the number of unique values is a more important factor.

•With the set functions COUNT, COUNT DISTINCT, SUM, MIN, MAX, and AVG, in order to use any index other than the default, the entire query must be resolvable using a single table or join index.

•BIT data, and VARCHAR data greater than 255 bytes, can only be used in the default index.

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 4-3: Index disk space usage
	Estimated space
Type of index	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	As the number of distinct values
	times larger	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	Same as High_Group
	times larger

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.

Data types in the index

Only the default index supports the following data types:

•BIT data

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