MaterializedPostgreSQL 

Creates ClickHouse table with an initial data dump of PostgreSQL table and starts replication process, i.e. executes background job to apply new changes as they happen on PostgreSQL table in the remote PostgreSQL database.

If more than one table is required, it is highly recommended to use the MaterializedPostgreSQL database engine instead of the table engine and use the materialized_postgresql_tables_list setting, which specifies the tables to be replicated. It will be much better in terms of CPU, fewer connections and fewer replication slots inside the remote PostgreSQL database.

Creating a Table 

CREATE TABLE postgresql_db.postgresql_replica (key UInt64, value UInt64)
ENGINE = MaterializedPostgreSQL('postgres1:5432', 'postgres_database', 'postgresql_replica', 'postgres_user', 'postgres_password')
PRIMARY KEY key;

Engine Parameters

  • host:port — PostgreSQL server address.
  • database — Remote database name.
  • table — Remote table name.
  • user — PostgreSQL user.
  • password — User password.

Requirements 

  1. The wal_level setting must have a value logical and max_replication_slots parameter must have a value at least 2 in the PostgreSQL config file.

  2. A table with MaterializedPostgreSQL engine must have a primary key — the same as a replica identity index (by default: primary key) of a PostgreSQL table (see details on replica identity index).

  3. Only database Atomic is allowed.

Virtual columns 

  • _version — Transaction counter. Type: UInt64.

  • _sign — Deletion mark. Type: Int8. Possible values:

    • 1 — Row is not deleted,
    • -1 — Row is deleted.

These columns do not need to be added when a table is created. They are always accessible in SELECT query.
_version column equals LSN position in WAL, so it might be used to check how up-to-date replication is.

CREATE TABLE postgresql_db.postgresql_replica (key UInt64, value UInt64)
ENGINE = MaterializedPostgreSQL('postgres1:5432', 'postgres_database', 'postgresql_replica', 'postgres_user', 'postgres_password')
PRIMARY KEY key;

SELECT key, value, _version FROM postgresql_db.postgresql_replica;

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