Alias table engine

The Alias table engine creates a transparent proxy to another table. All operations are forwarded to the target table while the alias itself stores no data.

The Alias engine creates a proxy to another table. All read and write operations are forwarded to the target table, while the alias itself stores no data and only maintains a reference to the target table.

Creating a Table

CREATE TABLE [db_name.]alias_name
ENGINE = Alias(target_table)

Or with explicit database name:

CREATE TABLE [db_name.]alias_name
ENGINE = Alias(target_db, target_table)

The Alias table does not support explicit column definitions. Columns are automatically inherited from the target table. This ensures that the alias always matches the target table’s schema.

Engine Parameters

• target_db (optional) — Name of the database containing the target table.
• target_table — Name of the target table.

When target_db is omitted and target_table is not fully qualified (e.g., Alias('my_table')), the target is resolved to the same database as the alias itself, not the session’s current database.

Supported Operations

The Alias table engine supports all major operations.

Operations on Target Table

These operations are proxied to the target table:

Operation	Support	Description
SELECT	✅	Read data from target table
INSERT	✅	Write data to target table
INSERT SELECT	✅	Batch insert into target table
ALTER TABLE ADD COLUMN	✅	Add columns to target table
ALTER TABLE MODIFY SETTING	✅	Modify target table settings
ALTER TABLE PARTITION	✅	Partition operations (DETACH/ATTACH/DROP) on target
ALTER TABLE UPDATE	✅	Update rows in target table (mutation)
ALTER TABLE DELETE	✅	Delete rows from target table (mutation)
OPTIMIZE TABLE	✅	Optimize target table (merge parts)
TRUNCATE TABLE	✅	Truncate target table

Operations on Alias Itself

These operations only affect the alias, not the target table:

Operation	Support	Description
DROP TABLE	✅	Drop the alias only, target table remains unchanged
RENAME TABLE	✅	Rename the alias only, target table remains unchanged

Usage Examples

Basic Alias Creation

Create a simple alias in the same database:

-- Create source table
CREATE TABLE source_data (
    id UInt32,
    name String,
    value Float64
) ENGINE = MergeTree
ORDER BY id;

-- Insert some data
INSERT INTO source_data VALUES (1, 'one', 10.1), (2, 'two', 20.2);

-- Create alias
CREATE TABLE data_alias ENGINE = Alias('source_data');

-- Query through alias
SELECT * FROM data_alias;

┌─id─┬─name─┬─value─┐
│  1 │ one  │  10.1 │
│  2 │ two  │  20.2 │
└────┴──────┴───────┘

Cross-Database Alias

Create an alias pointing to a table in a different database:

-- Create databases
CREATE DATABASE db1;
CREATE DATABASE db2;

-- Create source table in db1
CREATE TABLE db1.events (
    timestamp DateTime,
    event_type String,
    user_id UInt32
) ENGINE = MergeTree
ORDER BY timestamp;

-- Create alias in db2 pointing to db1.events
CREATE TABLE db2.events_alias ENGINE = Alias('db1', 'events');

-- Or using database.table format
CREATE TABLE db2.events_alias2 ENGINE = Alias('db1.events');

-- Both aliases work identically
INSERT INTO db2.events_alias VALUES (now(), 'click', 100);
SELECT * FROM db2.events_alias2;

Write Operations Through Alias

All write operations are forwarded to the target table:

CREATE TABLE metrics (
    ts DateTime,
    metric_name String,
    value Float64
) ENGINE = MergeTree
ORDER BY ts;

CREATE TABLE metrics_alias ENGINE = Alias('metrics');

-- Insert through alias
INSERT INTO metrics_alias VALUES 
    (now(), 'cpu_usage', 45.2),
    (now(), 'memory_usage', 78.5);

-- Insert with SELECT
INSERT INTO metrics_alias 
SELECT now(), 'disk_usage', number * 10 
FROM system.numbers 
LIMIT 5;

-- Verify data is in the target table
SELECT count() FROM metrics;  -- Returns 7
SELECT count() FROM metrics_alias;  -- Returns 7

Schema Modification

Alter operations modify the target table schema:

CREATE TABLE users (
    id UInt32,
    name String
) ENGINE = MergeTree
ORDER BY id;

CREATE TABLE users_alias ENGINE = Alias('users');

-- Add column through alias
ALTER TABLE users_alias ADD COLUMN email String DEFAULT '';

-- Column is added to target table
DESCRIBE users;

┌─name──┬─type───┬─default_type─┬─default_expression─┐
│ id    │ UInt32 │              │                    │
│ name  │ String │              │                    │
│ email │ String │ DEFAULT      │ ''                 │
└───────┴────────┴──────────────┴────────────────────┘

Data Mutations

UPDATE and DELETE operations are supported:

CREATE TABLE products (
    id UInt32,
    name String,
    price Float64,
    status String DEFAULT 'active'
) ENGINE = MergeTree
ORDER BY id;

CREATE TABLE products_alias ENGINE = Alias('products');

INSERT INTO products_alias VALUES 
    (1, 'item_one', 100.0, 'active'),
    (2, 'item_two', 200.0, 'active'),
    (3, 'item_three', 300.0, 'inactive');

-- Update through alias
ALTER TABLE products_alias UPDATE price = price * 1.1 WHERE status = 'active';

-- Delete through alias
ALTER TABLE products_alias DELETE WHERE status = 'inactive';

-- Changes are applied to target table
SELECT * FROM products ORDER BY id;

┌─id─┬─name─────┬─price─┬─status─┐
│  1 │ item_one │ 110.0 │ active │
│  2 │ item_two │ 220.0 │ active │
└────┴──────────┴───────┴────────┘

Partition Operations

For partitioned tables, partition operations are forwarded:

CREATE TABLE logs (
    date Date,
    level String,
    message String
) ENGINE = MergeTree
PARTITION BY toYYYYMM(date)
ORDER BY date;

CREATE TABLE logs_alias ENGINE = Alias('logs');

INSERT INTO logs_alias VALUES 
    ('2024-01-15', 'INFO', 'message1'),
    ('2024-02-15', 'ERROR', 'message2'),
    ('2024-03-15', 'INFO', 'message3');

-- Detach partition through alias
ALTER TABLE logs_alias DETACH PARTITION '202402';

SELECT count() FROM logs_alias;  -- Returns 2 (partition 202402 detached)

-- Attach partition back
ALTER TABLE logs_alias ATTACH PARTITION '202402';

SELECT count() FROM logs_alias;  -- Returns 3

Table Optimization

Optimize operations merge parts in the target table:

CREATE TABLE events (
    id UInt32,
    data String
) ENGINE = MergeTree
ORDER BY id;

CREATE TABLE events_alias ENGINE = Alias('events');

-- Multiple inserts create multiple parts
INSERT INTO events_alias VALUES (1, 'data1');
INSERT INTO events_alias VALUES (2, 'data2');
INSERT INTO events_alias VALUES (3, 'data3');

-- Check parts count
SELECT count() FROM system.parts 
WHERE database = currentDatabase() 
  AND table = 'events' 
  AND active;

-- Optimize through alias
OPTIMIZE TABLE events_alias FINAL;

-- Parts are merged in target table
SELECT count() FROM system.parts 
WHERE database = currentDatabase() 
  AND table = 'events' 
  AND active;  -- Returns 1

Alias Management

Aliases can be renamed or dropped independently:

CREATE TABLE important_data (
    id UInt32,
    value String
) ENGINE = MergeTree
ORDER BY id;

INSERT INTO important_data VALUES (1, 'critical'), (2, 'important');

CREATE TABLE old_alias ENGINE = Alias('important_data');

-- Rename alias (target table unchanged)
RENAME TABLE old_alias TO new_alias;

-- Create another alias to same table
CREATE TABLE another_alias ENGINE = Alias('important_data');

-- Drop one alias (target table and other aliases unchanged)
DROP TABLE new_alias;

SELECT * FROM another_alias;  -- Still works
SELECT count() FROM important_data;  -- Data intact, returns 2