ClickHouse/docs/en/engines/table-engines/integrations/postgresql.md

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8	PostgreSQL

PosgtreSQL

The PostgreSQL engine allows you to perform SELECT queries on data that is stored on a remote PostgreSQL server.

Creating a Table

CREATE TABLE [IF NOT EXISTS] [db.]table_name [ON CLUSTER cluster]
(
    name1 [type1] [DEFAULT|MATERIALIZED|ALIAS expr1] [TTL expr1],
    name2 [type2] [DEFAULT|MATERIALIZED|ALIAS expr2] [TTL expr2],
    ...
) ENGINE = PostgreSQL('host:port', 'database', 'table', 'user', 'password');

See a detailed description of the CREATE TABLE query.

The table structure can differ from the original PostgreSQL table structure:

• Column names should be the same as in the original PostgreSQL table, but you can use just some of these columns and in any order.
• Column types may differ from those in the original PostgreSQL table. ClickHouse tries to cast values to the ClickHouse data types.
• Setting external_table_functions_use_nulls defines how to handle Nullable columns. Default is 1, if 0 - table function will not make nullable columns and will insert default values instead of nulls. This is also applicable for null values inside array data types.

Engine Parameters

• host:port — PostgreSQL server address.

• database — Remote database name.

• table — Remote table name.

• user — PostgreSQL user.

• password — User password.

SELECT Queries on PostgreSQL side run as COPY (SELECT ...) TO STDOUT inside read-only PostgreSQL transaction with commit after each SELECT query.

Simple WHERE clauses such as =, !=, >, >=, <, <=, IN are executed on the PostgreSQL server.

All joins, aggregations, sorting, IN [ array ] conditions and the LIMIT sampling constraint are executed in ClickHouse only after the query to PostgreSQL finishes.

INSERT Queries on PostgreSQL side run as COPY "table_name" (field1, field2, ... fieldN) FROM STDIN inside PostgreSQL transaction with auto-commit after each INSERT statement.

PostgreSQL Array types converts into ClickHouse arrays. Be careful in PostgreSQL an array data created like a type_name[] may contain multi-dimensional arrays of different dimensions in different table rows in same column, but in ClickHouse it is only allowed to have multidimensional arrays of the same count of dimensions in all table rows in same column.

Usage Example

Table in PostgreSQL:

postgres=# CREATE TABLE "public"."test" (
"int_id" SERIAL,
"int_nullable" INT NULL DEFAULT NULL,
"float" FLOAT NOT NULL,
"str" VARCHAR(100) NOT NULL DEFAULT '',
"float_nullable" FLOAT NULL DEFAULT NULL,
PRIMARY KEY (int_id));

CREATE TABLE

postgres=# insert into test (int_id, str, "float") VALUES (1,'test',2);
INSERT 0 1

postgresql> select * from test;
 int_id | int_nullable | float | str  | float_nullable
--------+--------------+-------+------+----------------
      1 |              |     2 | test |
(1 row)

Table in ClickHouse, retrieving data from the PostgreSQL table created above:

CREATE TABLE default.postgresql_table
(
    `float_nullable` Nullable(Float32),
    `str` String,
    `int_id` Int32
)
ENGINE = PostgreSQL('localhost:5432', 'public', 'test', 'postges_user', 'postgres_password');

SELECT * FROM postgresql_table WHERE str IN ('test') 

┌─float_nullable─┬─str──┬─int_id─┐
│           ᴺᵁᴸᴸ │ test │      1 │
└────────────────┴──────┴────────┘
1 rows in set. Elapsed: 0.019 sec.

See Also

• The ‘postgresql’ table function
• Using PostgreSQL as a source of external dictionary

Original article