Venice
What is an "Index", a "Key" and a "Key Segment"?
 
An "Index" is an associative key-value array (or dictionary) that is physically stored either in or alongside a database table and it allows for a performant way to retrieve one or more specific records in a database. The index is sorted by key and thus also defines an ordering of records retrieved via this index.
 
The "Key" is the sequence of bytes that is used to locate an item in the index, and the "Value" is a reference to the actual record.
A database table typically has multiple indexes allowing to find the required records for different usage cases.
 
A key is composed of one or more "Key segments". In the case of Venice the value in a key segment will always be equal to a field in the record. Each key segment has its own type, collation, and ordering.