Virtual keyspaces and tables

Provides details about virtual keyspaces and tables.

CASSANDRA-7622 introduced support for virtual keyspaces and virtual tables. Virtual tables are exposed by the server to the users as local tables. They are read-only, but can sometime can support updates. Virtual tables are always part of a virtual keyspaces. The two available virtual keyspaces described in this topic are system_virtual_schema and system_views. These virtual tables are not associated with physical files on disk, have no memtables or SSTables, and do not support secondary indexes or materialized views; they are designed to expose server data such as configuration or metrics in memory.

Tip: See Virtual tables for SAI indexes and SSTables for details about the Storage Attached Indexing (SAI) virtual tables that reside in the system_views keyspace.

system_virtual_schema keyspace

The system_virtual_schema contains the information about the existing virtual keyspaces and tables. It contains three virtual tables which expose the definitions of the named data object:

Table 1. keyspaces
Column name	CQL type	Column type
keyspace_name	text	partition key

Table 2. tables
Column name	CQL type	Column type
keyspace_name	text	partition key
table_name	text	clustering key
comment	text	regular

Table 3. columns
Column Name	CQL Type	Column type
keyspace_name	text	partition key
table_name	text	clustering key
column_name	text	clustering key
clustering_order	text	regular
column_name_bytes	blob	regular
position	int	regular
type	text	regular

system_views keyspace

System views expose information about the server internals. The sstable_tasks system view exposes the SSTable tasks that are currently running on the host:

Table 4. sstable_tasks
Column name	CQL type	Column type
keyspace_name	text	partition key
table_name	text	clustering key
task_id	uuid	clustering key
kind	text	regular
progress	bigint	regular
total	bigint	regular
unit	text	regular

Tip: The remaining amount of work can be computer using

total -
      progress AS remaining

in a SELECT clause.

Virtual tables and cqlsh

Virtual tables are exposed as local tables, so cqlsh will display data for the currently connected host. To displace schema for virtual tables, use DESCRIBE FULL SCHEMA to return schema for both the real keyspaces and the virtual ones.

Note: Users are not allowed to create virtual keyspaces or virtual tables.

To see the schema for the virtual table sstable_tasks, use:

DESCRIBE TABLE system_views.sstable_tasks;

/*
Warning: Table system_views.sstable_tasks is a virtual table and cannot be recreated with CQL.
Structure, for reference:
VIRTUAL TABLE system_views.sstable_tasks (
    keyspace_name text,
    table_name text,
    task_id uuid,
    kind text,
    progress bigint,
    total bigint,
    unit text,
    PRIMARY KEY (keyspace_name, table_name, task_id)
) WITH CLUSTERING ORDER BY (table_name ASC, task_id ASC)
    AND comment = 'current sstable tasks';
*/

To discover how much time is remaining for SSTable tasks, use the following query:

SELECT total - progress AS remaining
FROM system_views.sstable_tasks;