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System schema keyspace layout

This section describes layouts and usage of system_schema.* tables.

system_schema.computed_columns

Computed columns are a special kind of columns. Rather than having their value provided directly by the user, they are computed - possibly from other column values. Examples of such computed columns could be:

  • token column generated from the base partition key for secondary indexes

  • map value column, generated as the extraction of a single value from a map stored in a different column

Computed columns in many ways act as regular columns, so they are also present in the system_schema.columns table - system_schema.computed_columns is an additional mapping that marks the column as computed and provides its computation.

Schema:

CREATE TABLE system_schema.computed_columns (
    keyspace_name text,
    table_name text,
    column_name text,
    computation blob,
    PRIMARY KEY (keyspace_name, table_name, column_name)
) WITH CLUSTERING ORDER BY (table_name ASC, column_name ASC);

computation is stored as a blob and its contents are assumed to be a JSON representation of computation’s type and any custom fields needed.

Example representations:

{'type':'token'}

{'type':'map_value','map':'my_map_column_name','key':'AF$^GESHHgge6yhf'}

The token computation does not need additional arguments, as it returns the token of base’s partition key. In order to compute a map value, what’s additionally needed is the column name that stores the map and the key at which the value is expected.