1
0
Fork 0

updated hash index information

This commit is contained in:
Jan Steemann 2015-09-14 17:45:01 +02:00
parent a3f7002a41
commit 72cc037ef3
1 changed files with 5 additions and 11 deletions

View File

@ -21,18 +21,12 @@ different usage scenarios:
cannot be used for sorting.
The hash index is a good candidate if all or most queries on the indexed
attribute(s) are equality comparisons. It will be the most efficient index
type if the index is declared unique.
attribute(s) are equality comparisons. The unique hash index provides an
amortized complexity of O(1) for insert, update, remove and lookup operations.
The non-unique hash index provides O(1) inserts, updates and removes, and
will allow looking up documents by index value with amortized O(n) complexity,
with *n* being the number of documents with that index value.
Insertions into a non-unique hash index are also very efficient. Update and
removal performance in a non-unique hash index depend on the key selectivity.
If the selectivity is low and keys repeat a lot, update and removal performance
in a non-unique hash index will degrade.
A non-unique hash index should therefore not be used if duplicate index values
are allowed and it is known that there will be many duplicate values in the index
and there will be updates or removals.
A non-unique hash index on an optional document attribute should be declared
sparse so that it will not index documents for which the index attribute is
not set.