Sybase 12.4.2 - Manual (Page 167)

CHAPTER 4 Adaptive Server IQ Indexes
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Advantages and disadvantages of High_Group
The following table lists advantages and disadvantages of High_Group
indexes.
Table 4-6: HG advantages/disadvantages
Comparison with other indexes
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.
HNG The determining factor is whether the column is a join column, and/or
whether GROUP BY may be processed on the column. If either of these is true,
use High_Group, either alone or in combination with High_Non_Group.
Otherwise, use High_Non_Group to save disk space.
Additional indexes
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.
Automatic creation of High_Group index
Adaptive Server IQ creates a High_Group index by default whenever you issue
a
CREATE INDEX statement without specifying an index type.
Adaptive Server IQ automatically creates a High_Group index for any column
with a
UNIQUE or PRIMARY KEY constraint.
Advantages Disadvantages
Quickly processes queries with
GROUP BY .
This index needs additional disk space
compared to the
HNG index (it can take up
as much as three times more space than raw
data).
This index facilitates join index
processing. It is one of indexes
recommended for columns used in join
relationships.
LF is the other.
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 > 255 bytes.