What is the Indexed check box when adding or editing certain types of custom fields?
Custom Fields, Field Indexing
When creating or editing a custom field, the Indexed check-box specifies whether an index for the custom field should be created in the appropriate database table. That is, indexed incident custom fields have indexes on the incidents table; indexed contacts custom fields have indexes on the contacts table, and indexed answer custom fields have indexes on the answers table.
Note: You can also specify whether your answer variables should be indexed. This option is not available with the Opt-in and Yes/No data types.
The purpose of an index is to speed up record searches when searching by the indexed field. For example, if you have an incident custom field called language, and you frequently search your knowledge base for all incidents related to a specific language, you would want this custom field indexed to improve search performance.
There are also drawbacks to having custom fields indexed. Specifying too many indexes on a specific table can degrade search performance rather than improve it. In general, it is best to have fewer than 15 total fields indexed in any given table.
The incidents table is most likely to be adversely affected by too many indexes. However, the answers, organizations, and contacts tables can also experience poor performance due to too many indexes. Each table has some fields indexed by default. In addition, our Cloud Operations group may add custom indexes to specific fields to improve site performance. The best general rule of thumb when indexing custom fields is to have fewer than 10 additional custom fields indexed for a given type of custom field (answer, incident, customer, or company).
In addition, too many table indexes can disrupt the site update process. If your site has too many indexes on any one table, a variety of failures may occur--including problems when updating.
For additional information on when fields may benefit from indexing please review Answer ID 4494: When to index a custom field