Indexes

An Index is database structure that is used by the server to find a row in a table quickly. It is composed of a key value (the column in a row) and the rowid. Indexes are used to improve the performance of queries. It provides a faster access route to table data. Properly used, indexes are the primary means to reduce disk I/O.
• Index a column only if you frequently want to retrieve less than 15 percent of the rows in a large table.
• Do not index small tables with few rows.
• If the column data contains many nulls and you frequently search for these nulls, do not create an index on this column.
• Likewise if the column contains very similar data, don't create an index on that column.