Question: How can PostgreSQL automatically create partitions?

Answer

Partitioning in PostgreSQL is a technique used to divide large tables into smaller, more manageable pieces, called partitions. While PostgreSQL supports table partitioning, automatically creating new partitions as data grows can simplify management and improve performance. PostgreSQL does not directly support automatic creation of partitions for you; it requires some manual setup or the use of additional tools/scripts.

However, PostgreSQL introduced declarative partitioning since version 10, which simplifies the management of partitioned tables but still requires partitions to be created manually or through a custom mechanism. To implement auto-creation of partitions, you can use a combination of triggers and functions or rely on external tools and extensions that facilitate this process.

Using Triggers and Functions

You can use a trigger on the parent table that checks if the appropriate partition exists when inserting a new row. If the partition does not exist, the function creates it. Here's a simplified example:

1. Create the parent table:

CREATE TABLE measurement (
    city_id int not null,
    logdate date not null,
    peaktemp int,
    unitsales int
) PARTITION BY RANGE (logdate);

2. Create a function to create partitions dynamically:

CREATE OR REPLACE FUNCTION create_partition_and_insert()
RETURNS TRIGGER AS $$
BEGIN
    IF NEW.logdate >= '2024-01-01' AND NEW.logdate < '2024-02-01' THEN
        CREATE TABLE IF NOT EXISTS measurement_y2024m01 PARTITION OF measurement FOR VALUES FROM ('2024-01-01') TO ('2024-02-01');
    ELSIF NEW.logdate >= '2024-02-01' AND NEW.logdate < '2024-03-01' THEN
        CREATE TABLE IF NOT EXISTS measurement_y2024m02 PARTITION OF measurement FOR VALUES FROM ('2024-02-01') TO ('2024-03-01');
    -- Add more conditions for other months/years as needed
    ELSE
        RAISE EXCEPTION 'Date out of range. Partition does not exist.';
    END IF;
    RETURN NULL; -- Result is ignored since this is an AFTER trigger
END;
$$ LANGUAGE plpgsql;

3. Attach the trigger to the parent table:

CREATE TRIGGER trigger_measurement_insert
BEFORE INSERT ON measurement
FOR EACH ROW EXECUTE FUNCTION create_partition_and_insert();

This method, while functional, can lead to performance issues due to the overhead of checking conditions and possibly creating a new table on each insert. It's crucial to test thoroughly and consider the partitioning strategy carefully.

External Tools and Extensions

There are also PostgreSQL extensions like pg_partman, which can automate partition management, including creation, maintenance, and retention of partitions. Using such tools can significantly simplify the implementation and ensure better performance and reliability.

In conclusion, while PostgreSQL does not natively support the automatic creation of partitions upon insert, it can be achieved with custom scripts or by leveraging third-party tools designed for partition management.