Virtually Hashed Tables

Create virtually hashed tables to efficiently organize tables.

Note: Virtually hashed tables are available only on Linux pSeries.

You can perform hash-based index scans using nonclustered indexes or clustered indexes on data-only-locked tables. During this scan, each worker process navigates the higher levels of the index and reads the leaf-level pages of the index. Each worker process then hashes on either the data page ID or the key value in a separate hash table to determine which data pages or data rows to process.

A virtually hashed table can be a more efficient way to organize a table because it does not require a separate hash table. Instead, it stores the rows so that, using the hash key, the query processor can determine the row ID (based on the row’s ordinal number) and the location of the data. Because it does not use a separate hash table to hold the information, it is called a “virtually” hashed table.

For systems that must make more efficient use of their central-processing unit (CPU), the virtually hashed table is a good option.

To create a virtually hashed table, specify the maximum value for the hash region using the create table command.