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Indexing criteria: disk space usage
Data types in the index
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
, in order to use any index other than the default, the entire query must
AVG
be resolvable using a single table or join index.
•
data, and
BIT
VARCHAR
the default index.
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
Default
Smaller than or equal to
High_Group
Smaller than up to 2
times larger
High_Non_Group
Smaller than or equal to
Low_Fast
Smaller than up to 2
times larger
For
and
indexes, the index size depends on the number of unique values.
LF
HG
The more unique values, the more space the index takes.
Only the default index supports the following data types:
•
data
BIT
CHAPTER 4
Adaptive Server IQ Indexes
,
COUNT
COUNT DISTINCT
data greater than 255 bytes, can only be used in
Comments
If the number of distinct values is
less than 255, this index uses
significantly less space than the raw
data
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
Smaller than the raw data in most
cases
Same as High_Group
,
,
,
, and
SUM
MIN
MAX
143
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