scylladb/docs/dev/system_schema_keyspace.md

System schema keyspace layout

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

system_schema.keyspaces

This table contains one row per keyspaces.

Schema:

CREATE TABLE system_schema.keyspaces (
    keyspace_name text PRIMARY KEY,
    durable_writes boolean,
    replication frozen<map<text, text>>
)

Columns:

• keyspace_name - name of the keyspace

• durable_writes - whether writes to the keyspace are using commitlog.

• replication - replication settings for the keyspace. The value for the "class" key determines replication strategy name. The structure of other options depends on the replication strategy.

  For NetworkTopologyStrategy the other options specify replication factors for datacenters, stored as a flattened map of the extended options map (see below).

  For SimpleStrategy there is a single option "replication_factor" specifying the replication factor.

Extended options map used by NetworkTopologyStrategy is a map where values can be either strings or lists of strings.

For example:

   {
      'dc1': '3',
      'dc2': ['rack1', 'rack2'],
      'dc3': []
   }

The options above mean that the replication factor for datacenter dc1 is 3, for datacenter dc2 it is 2, with replicas placed on racks rack1 and rack2. For 'dc3' the replication factor is 0, expressed as an empty list of racks.

The extended map is stored in the "replication" column in a flattened form, where values which are lists are represented as multiple entries in the map with the list index appended to the key, with : as the separator. The index can be negative, which is used to indicate that the list is empty.

The example extended options map from above has a flattened representation of:

  {
    'dc1': '3',
    'dc2:0': 'rack1',
    'dc2:1': 'rack2',
    'dc3:-1': ''
  }

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.