Column is a data store object or an index object for fast search.
A column belongs to a table. Table has zero or more columns.
Both data store column and index column have type. Type of data store column specifies data range. In other words, it is "value type". Type of index column specifies set of documents to be indexed. A set of documents is a table in Groonga. In other words, type of index column must be a table.
Here are data store columns:
Here is an index column: