Situations hashing is not advantageous - Oracle Clusters

Discussion

• Nirja Shah   -Posted on 02 Sep 15
  - We need to decide when can we use hash clusters by contrasting situations where hashing is most useful against situations where there is no advantage.
  
  - If you find your decision is to use indexing rather than hashing, then you should consider whether to store a table individually or as part of a cluster.
  
  - Hashing is not advantageous in the following situations:
  
  1. Most queries on the table retrieve rows over a range of cluster key values. For example, in full table scans or queries such as the following, a hash function cannot be used to determine the location of specific hash keys. Instead, the equivalent of a full table scan must be done to fetch the rows for the query.
  SELECT . . . WHERE cluster_key < . . . ;
  
  2. With an index, key values are ordered in the index, so cluster key values that satisfy the WHERE clause of a query can be found with relatively few I/Os.
  
  3. The table is not static, but instead is continually growing. If a table grows without limit, the space required over the life of the table (its cluster) cannot be predetermined.
  
  4. Applications frequently perform full-table scans on the table and the table is sparsely populated. A full-table scan in this situation takes longer under hashing.
  
  5. You cannot afford to preallocate the space that the hash cluster will eventually need.

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