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Once you create the hash cluster, you create the cluster tables just as you would in a regular cluster. The HASHKEYS value specifies the number of unique hash values that can be generated by the hash function.

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Oracle indexes provide speedy access to table rows by storing sorted values of specified columns, and using those sorted values to easily look up the associated table rows, much the same way you use a book s index to quickly find a particular item you re interested in Indexes enable you to find a row with a certain column value without your having to look at more than a small fraction of the total rows in the table Thus, the proper use of indexes will reduce your expensive disk I/Os to a bare minimum Indexes are purely optional database structures, and they re maintained completely by Oracle Using an index involves a tradeoff between speedy retrieval of query results and slower updates and insertions.

The first part of the tradeoff, the speedy execution of queries, is quite apparent: if you look up a sorted index rather than performing a full table scan, your queries will be faster But every time you update, insert, or delete a row in a table with indexes, the indexes have to be updated, inserted, or deleted as well This makes these processes more time consuming on a table with indexes In addition, don t forget that large tables will have large indexes, and you need a large disk to accommodate these indexes in addition to the table data In general, if your tables are mostly used for reading (selecting) data, as in a data warehouse, you are better off with more indexes If your database is more of an OLTP type, with heavy inserts, updates, and deletes, you are better off with fewer indexes.

Unless you need to access most of the rows of a table, indexed queries often provide results much more quickly than queries that do not use indexes There is no limit to the number of indexes you can have on a single Oracle table but, as mentioned previously, there are performance implications An index is completely transparent to the user that is, the user s SQL statement does not have to be changed when you create indexes However, it is incumbent upon application developers to be well versed in the subject of indexes and how they work, so that they can build efficient queries..

For illustrative purposes, our script doesn t do the obvious and use the Use% value from the df output; it instead calculates disk consumption directly from the 1K-blocks and Used values. So now you have a script that can display the percent utilization of the / file system. Let s call this root_check.

For managed objects, object destruction and memory reclamation are decoupled. This supports reliable code in two ways. First, accessing managed objects that are already destroyed cannot corrupt the state of some other random object; instead, it often ends up in a welldefined reaction the ObjectDisposedException. The second benefit is that the runtime can

Oracle indexes can be of several types, the most important of which are listed here: Unique and nonunique indexes: Unique indexes are those based on a unique column, usually something like the social security number of an employee. Although you can explicitly create unique indexes, Oracle recommends that you not do so. Oracle advises you to use unique constraints instead. When you place a unique constraint on a table s column, Oracle will automatically create unique indexes on those columns. Primary and secondary indexes: Primary indexes are the unique indexes in a table that must always possess a value; they can t be null. Secondary indexes are other indexes in the same table that may not be unique. Composite indexes: Composite indexes are indexes that contain two or more columns from the same table. They re also known as concatenated indexes. Composite indexes are especially useful for enforcing uniqueness in a table s columns in cases where there s no single column that can uniquely identify a row.