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.
|
See Virtual tables for SAI indexes and SSTables for details about the Storage Attached Indexing (SAI) virtual tables that reside in the |
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:
| Column name | CQL type | Column type |
|---|---|---|
keyspace_name |
text |
partition key |
| Column name | CQL type | Column type |
|---|---|---|
keyspace_name |
text |
partition key |
table_name |
text |
clustering key |
comment |
text |
regular |
| 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:
| 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 |
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.
|
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;Results
/*
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;Results
remaining
-----------
(0 rows)