Column Store (Architecture Guide)

	Data Structures	Source Location
	WT_BTREE	src/include/btree.h

Column Stores are Btrees stored in WiredTiger that have as their key a record id, that is, a 64 bit unsigned integer. Thus, they implement a specialized version of a Btree, where the key is a predictable length.

A particular kind of column store is the fixed length column store. As its name implies, the value is fixed length, and furthermore the value is restricted to 1 and 8 bits in length. The bit length is specified when the column store is created. The fixed length column store has specialized use cases like bitmaps.

The more general case is the variable length column store which allows for values that have any length, and may have arbitrary types, including aggregates of various types.

Internally, row stores and both kinds of column stores all use a common WT_BTREE structure. Column stores are distinguished in that WT_BTREE->type == BTREE_COL_VAR for variable length column stores and WT_BTREE->type == BTREE_COL_FIX for fixed length column stores. Internal functions that navigate, access and manipulate Btrees have code sprinkled throughout that is conditional on WT_BTREE->type.