Indexes are used by Ability to speed up sort and search operations on table fields. They are also used to determine the type of link that can be made from one table to another in a relation (See Joining tables).
Each record in the field is given its own index position, so to speak, by which it can be ordered. You should set an index for a field if you think you may need to search or sort on that field frequently. Indexes can also be used to prevent certain fields holding duplicate data.
The only disadvantage in using indexes is that they may sometimes slow down various table operations, such as query operations, especially when there are many records in the table.
You set an index for a field when you set the data type. The fields that can be indexed are those that are capable of being ordered, either numerically or alphabetically, such as numeric, character, date and time fields (See Creating indexes and primary keys).
There are three types of index: primary keys, unique indexes, and non-unique indexes. In addition, you can create multiple-field indexes.
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