What is an index field in salesforce?
I'm trying to find a proper list of what standard fields are indexed by default in salesforce, as well as under what conditions custom fields are indexed.
To start things off I know fields are indexed:
When salesforce has added a custom index at your request (but you can't rely on this in other orgs)
Any fields marked as external IDs
Master-Detail fields
Lookup fields
System audit fields (CreatedDate, SystemModStamp)
I've asked salesforce support and there is no authoritative documentation on this outside of this page: http://www.salesforce.com/us/developer/docs/apexcode/Content/langCon_apex_SOQL_VLSQ.htm
Just found that salesforce has posted a cheat sheet with much more data about index fields in salesforce than I've ever seen before. Take a look at the Query & Search Optimization Fields Cheat Sheet on the developer force wiki's cheat sheet area.
One big takeaway is that there are some substantial differences between performance of standard and custom indexes.
Fields that have standard index applied:
Custom lookup fields
Custom master-detail fields
Id
Name
Owner
Audit dates
On standard objects there are some others that aren't documented anywhere
Custom indexes:
Can be added by salesforce support
External Id fields
Unique fields