Creating a Column Family

Abstract

Columns, including the column values, are grouped into column families for performance reasons. A column family is both the logical and physical grouping of columns. A column consists of a column family and a column qualifier. A fully qualified column name consists of a prefix, which is the column family name, followed by a : (colon) and the column qualifier. For example, if a table has a single column family cf1, which has column qualifiers col1, col2, and col3, the columns in the table would be cf1:col1, cf1:col2, and cf1:col3. The column family name must be composed of printable characters while the column qualifier can be any bytes. Column families must be declared when a table is created, but the column qualifiers may be created on an as-needed basis dynamically. Each row in a table has the same column families even though a column family may not store any data. A table can be defined as a sparse set of rows stored in column families. The maximum number of row versions is configured per column family. All column family members are stored together on disk. Empty cells in a table are not stored at all, not even as null values. As a store is defined per column family, each StoreFile (HFile) stored on disk is per column family, which implies that the data for a column family is stored separately on disk. The storage characteristics of a column family include the following: