Oracle Tunning

The power of 64-bit computing can be imaged when you consider the thearetical limit for addressable memory. In unsigned 16-bit computing, we could directly address 64k of memory. For a standard oracle database, this allowed a tremendously increased system global area. The sga is where the most often used data can be stored and kept in memory for fast access.

Oracle offers a variety of indexing options. Knowing which options to use in a given situation can be crucial to an application's performance.

When accessing data from tables, oracle has two options: to read every row in the table or to access a single row at a time by ROWID. When accessing a small percentage of the rows of a large table, you would want to use an index.

The degree to which indexs help perfomance depends partly on the selectivity of the data and the way in which the data is distributed among the table's blocks.

Indexes will generally improve performance for queries. SELECT,the WHERE clauses of UPDATE commands and the WHERE clauses of DELETE statements can benefit form indexes.

When a single index has multiple columns that are indexed, it is called a concatenated or composite index. When oracle 9i's introduction of skip-scan index access has increased the optiomizer's options when using concatenated indexes. you should be careful when selecting the order of the columns in the index. In gerneral, the leading column of the index should be the one most likeing to be used in WHERE clauses and also the most selective column of the set.

Prior to the intorduction of skip-scan functionality, queries could only use the index if the leading column of the index was used in the WHERE clause.The two most common types of index scans are unique scans and range scans. In a unqiue scan, the database knows that the index contains a list of unique values. In a range scan, the database will be returning multiple values from the index according to the query criteria.

When you create a primary key or a unique constraint, oracle will automatically create a unique index based on the columns you specify. If you create a multicolumn primary key, oracle will create a concatenated index with the columns  in the same order in which you specified them when creating the primary key.

Indexes can only be used to find data that exists within a table. Unless you are using function-based indexes, using functions on indexed columns in WHERE clauses of a SQL statement will cause the optimizer to bypass indexes.

The clustering factor is a measure of the ordered-ness of an index in comparision to the table that it is based on. It is used to check the cost of a table lookup  following  an index access. The clustering factor records the number of blocks that will be read when scanning the index. If the index being used has a large clustering factor, then more table data blocks have to be vistited in order to get the rows in each index block. If the clustering factor is close to the number of blocks in the table, then the index is well ordered.

The Clustering_factor column in the user_indexes view gives an indication as to how organized the data is compared to the indexed column. The clustering factor can have an impact on SQL statements that perform range scans.