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Sunday, 7 July 2013


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NOTE: If data modification performance is poor, you may have too many indexes. While indexes favor “select operations”, they slow down “data modifications”.


Indexes are the most important physical design element in improving database performance:

• Indexes help prevent table scans. Instead of reading hundreds of data pages, a few index pages and data pages can satisfy many queries.

• For some queries, data can be retrieved from a nonclustered index without ever accessing the data rows.

• Clustered indexes can randomize data inserts, avoiding insert “hot spots” on the last page of a table.

• Indexes can help avoid sorts, if the index order matches the order of columns in an order by clause.

In addition to their performance benefits, indexes can enforce the uniqueness of data.

Indexes are database objects that can be created for a table to speed direct access to specific data rows. Indexes store the values of the key(s) that were named when the index was created, and logical pointers to the data pages or to other index pages.

Adaptive Server (SYBASE) provides two types of indexes:
• Clustered indexes, where the table data is physically stored in the order of the keys on the index:

• For allpages-locked tables, rows are stored in key order on pages, and pages are linked in key order.

• For data-only-locked tables, indexes are used to direct the storage of data on rows and pages, but strict key ordering is not maintained.

• Nonclustered indexes, where the storage order of data in the table is not related to index keys.

You can create only one clustered index on a table because there is only one possible physical ordering of the data rows. 

You can create up to 249 nonclustered indexes per table. A table that has no clustered index is called a “heap”.


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