PostgreSQL
Allows to connect to databases on a remote PostgreSQL server. Supports read and write operations (SELECT and INSERT queries) to exchange data between ClickHouse and PostgreSQL.
Gives the real-time access to table list and table structure from remote PostgreSQL with the help of SHOW TABLES and DESCRIBE TABLE queries.
Supports table structure modifications (ALTER TABLE ... ADD|DROP COLUMN). If use_table_cache parameter (see the Engine Parameters below) it set to 1, the table structure is cached and not checked for being modified, but can be updated with DETACH and ATTACH queries.
Creating a Database
CREATE DATABASE test_database
ENGINE = PostgreSQL('host:port', 'database', 'user', 'password'[, `schema`, `use_table_cache`]);
Engine Parameters
host:port— PostgreSQL server address.database— Remote database name.user— PostgreSQL user.password— User password.schema— PostgreSQL schema.use_table_cache— Defines if the database table structure is cached or not. Optional. Default value:0.
Data Types Support
| PostgreSQL | ClickHouse |
|---|---|
| DATE | Date |
| TIMESTAMP | DateTime |
| REAL | Float32 |
| DOUBLE | Float64 |
| DECIMAL, NUMERIC | Decimal |
| SMALLINT | Int16 |
| INTEGER | Int32 |
| BIGINT | Int64 |
| SERIAL | UInt32 |
| BIGSERIAL | UInt64 |
| TEXT, CHAR | String |
| INTEGER | Nullable(Int32) |
| ARRAY | Array |
Examples of Use
Database in ClickHouse, exchanging data with the PostgreSQL server:
CREATE DATABASE test_database
ENGINE = PostgreSQL('postgres1:5432', 'test_database', 'postgres', 'mysecretpassword', 'schema_name',1);
SHOW DATABASES;
┌─name──────────┐
│ default │
│ test_database │
│ system │
└───────────────┘
SHOW TABLES FROM test_database;
┌─name───────┐
│ test_table │
└────────────┘
Reading data from the PostgreSQL table:
SELECT * FROM test_database.test_table;
┌─id─┬─value─┐
│ 1 │ 2 │
└────┴───────┘
Writing data to the PostgreSQL table:
INSERT INTO test_database.test_table VALUES (3,4);
SELECT * FROM test_database.test_table;
┌─int_id─┬─value─┐
│ 1 │ 2 │
│ 3 │ 4 │
└────────┴───────┘
Consider the table structure was modified in PostgreSQL:
postgre> ALTER TABLE test_table ADD COLUMN data Text
As the use_table_cache parameter was set to 1 when the database was created, the table structure in ClickHouse was cached and therefore not modified:
DESCRIBE TABLE test_database.test_table;
┌─name───┬─type──────────────┐
│ id │ Nullable(Integer) │
│ value │ Nullable(Integer) │
└────────┴───────────────────┘
After detaching the table and attaching it again, the structure was updated:
DETACH TABLE test_database.test_table;
ATTACH TABLE test_database.test_table;
DESCRIBE TABLE test_database.test_table;
┌─name───┬─type──────────────┐
│ id │ Nullable(Integer) │
│ value │ Nullable(Integer) │
│ data │ Nullable(String) │
└────────┴───────────────────┘