system_virtual_schema

The system_virtual_schema keyspace contains metadata about virtual keyspaces and tables. It is a meta-keyspace that describes the virtual table system itself.

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Overview

This keyspace provides introspection capabilities for virtual tables, similar to how system_schema describes regular tables.

-- List all virtual keyspaces
SELECT * FROM system_virtual_schema.keyspaces;

-- List all virtual tables
SELECT keyspace_name, table_name, comment
FROM system_virtual_schema.tables;

-- Describe a specific virtual table
SELECT column_name, type, kind, position
FROM system_virtual_schema.columns
WHERE keyspace_name = 'system_views'
  AND table_name = 'thread_pools';

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Tables

keyspaces

Lists available virtual keyspaces.

VIRTUAL TABLE system_virtual_schema.keyspaces (
    keyspace_name text PRIMARY KEY
)

Column	Type	Description
keyspace_name	text	Virtual keyspace name

Example:

SELECT * FROM system_virtual_schema.keyspaces;

 keyspace_name
-----------------------
 system_virtual_schema
 system_views

tables

Lists all virtual tables with descriptions.

VIRTUAL TABLE system_virtual_schema.tables (
    keyspace_name text,
    table_name text,
    comment text,
    PRIMARY KEY (keyspace_name, table_name)
) WITH CLUSTERING ORDER BY (table_name ASC)

Column	Type	Description
keyspace_name	text	Parent keyspace
table_name	text	Table name
comment	text	Description of the table’s purpose

Example:

-- List all tables in system_views
SELECT table_name, comment
FROM system_virtual_schema.tables
WHERE keyspace_name = 'system_views';

 table_name                  | comment
-----------------------------+------------------------------------------
 batch_metrics               | Metrics specific to batch statements
 caches                      | system caches
 clients                     | currently connected clients
 coordinator_read_latency    |
 coordinator_scan_latency    |
 coordinator_write_latency   |
 ...

columns

Describes columns in each virtual table.

VIRTUAL TABLE system_virtual_schema.columns (
    keyspace_name text,
    table_name text,
    column_name text,
    clustering_order text,
    column_name_bytes blob,
    kind text,
    position int,
    type text,
    PRIMARY KEY (keyspace_name, table_name, column_name)
) WITH CLUSTERING ORDER BY (table_name ASC, column_name ASC)

Column	Type	Description
keyspace_name	text	Parent keyspace
table_name	text	Parent table
column_name	text	Column name
type	text	CQL data type
kind	text	partition_key, clustering, or regular
position	int	Position in primary key (0-indexed)
clustering_order	text	asc or desc for clustering columns
column_name_bytes	blob	Binary representation of column name

Example:

-- Get schema for thread_pools table
SELECT column_name, type, kind, position
FROM system_virtual_schema.columns
WHERE keyspace_name = 'system_views'
  AND table_name = 'thread_pools';

Sort by position in application.

 column_name          | type   | kind          | position
----------------------+--------+---------------+----------
 name                 | text   | partition_key |        0
 active_tasks         | int    | regular       |       -1
 active_tasks_limit   | int    | regular       |       -1
 blocked_tasks        | bigint | regular       |       -1
 blocked_tasks_all_time | bigint | regular     |       -1
 completed_tasks      | bigint | regular       |       -1
 pending_tasks        | int    | regular       |       -1

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Use Cases

Discovering Available Virtual Tables

-- All virtual tables in system_views
SELECT table_name, comment
FROM system_virtual_schema.tables
WHERE keyspace_name = 'system_views';

Filter in application by table_name pattern (e.g., tables containing ‘repair’).

Programmatic Schema Discovery

from cassandra.cluster import Cluster

cluster = Cluster()
session = cluster.connect()

# Get all virtual tables
tables = session.execute("""
    SELECT keyspace_name, table_name
    FROM system_virtual_schema.tables
""")

for table in tables:
    # Get columns for each table
    columns = session.execute("""
        SELECT column_name, type
        FROM system_virtual_schema.columns
        WHERE keyspace_name = %s AND table_name = %s
    """, (table.keyspace_name, table.table_name))

    print(f"\n{table.keyspace_name}.{table.table_name}")
    for col in columns:
        print(f"  {col.column_name}: {col.type}")

Validating Virtual Table Availability

-- Check if a specific virtual table exists (version compatibility)
SELECT table_name
FROM system_virtual_schema.tables
WHERE keyspace_name = 'system_views'
  AND table_name = 'sai_column_indexes';

-- If empty result, SAI tables not available (pre-5.0)

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Related Documentation

• Virtual Tables Overview - Introduction to virtual tables
• Metrics Tables - Latency and performance metrics
• Configuration Tables - Runtime settings