Sybase Adaptive Server IQ 12.4.2 Administration And Performance Manual page 167

Advantages and disadvantages of High_Group
Comparison with other indexes
Additional indexes
Automatic creation of High_Group index
The following table lists advantages and disadvantages of High_Group
indexes.
Table 4-6: HG advantages/disadvantages
Advantages
Quickly processes queries with
.
GROUP BY
This index facilitates join index
processing. It is one of indexes
recommended for columns used in join
relationships. is the other.
LF
LF The determining factor is the number of unique values. Use High_Group
if the number of unique values for the column is high. Use Low_Fast if the
number of unique values is low.
The determining factor is whether the column is a join column, and/or
HNG
whether may be processed on the column. If either of these is true,
GROUP BY
use High_Group, either alone or in combination with High_Non_Group.
Otherwise, use High_Non_Group to save disk space.
In some cases, a column that meets the criteria for a High_Group index may be
used in queries where a different type of index may be faster. If this is the case,
create additional indexes for that column.
Adaptive Server IQ creates a High_Group index by default whenever you issue
a statement without specifying an index type.
CREATE INDEX
Adaptive Server IQ automatically creates a High_Group index for any column
with a or
UNIQUE PRIMARY KEY
CHAPTER 4 Adaptive Server IQ Indexes
Disadvantages
This index needs additional disk space
compared to the
HNG
as much as three times more space than raw
data).
This index type takes the longest time to
populate with data, and to delete.
Cannot use this index if data in your
columns is BIT , or VARCHAR
constraint.
index (it can take up
> 255 bytes.
147