Question: How can you partition a table in PostgreSQL without using a primary key?

Answer

Partitioning in PostgreSQL is a technique that allows the division of one large table into smaller physical pieces called partitions, based on certain criteria such as ranges of values or list of values. This can significantly improve query performance on large datasets. Traditionally, partitioning requires defining a primary key that includes the partition key to ensure uniqueness across all partitions. However, there might be scenarios where a table needs to be partitioned without declaring a primary key.

In PostgreSQL, while it's common to include the partition key in a primary key or unique constraint, it's not strictly required for partitioning itself. Here’s how you can create a range-partitioned table without a primary key:

-- Create a parent table
CREATE TABLE measurement (
    city_id int not null,
    logdate date not null,
    peaktemp int,
    unitsales int
) PARTITION BY RANGE (logdate);

-- Create a partition of the parent table
CREATE TABLE measurement_y2020 PARTITION OF measurement
    FOR VALUES FROM ('2020-01-01') TO ('2021-01-01');

In this example, measurement is partitioned by range using the logdate column. Each partition, like measurement_y2020, inherits its structure from the parent table but contains data only for its specified range. Notice that we haven’t defined a primary key in either table definition.

Why omit a primary key?

1. Simplicity and Performance: If your application does not require the strict enforcement of uniqueness for each row across partitions, omitting the primary key can simplify the schema and potentially improve insertion performance.

2. Use case specific: Some use cases, like time-series data where rows are uniquely identified by a combination of columns (none of which alone are unique), might not fit well with the traditional primary key model.

Considerations

• Uniqueness: Without a primary key or unique constraints, you're responsible for ensuring data uniqueness if it is important for your application logic.
• Referential Integrity: Foreign keys referencing partitioned tables must include all columns used in the partitioning key.

Partitioning in PostgreSQL offers flexibility to optimize data management and querying, even without enforcing a primary key. Ensure that the decision to omit the primary key aligns with your data integrity requirements and application use case.