Partition pruning is a crucial optimization technique in PostgreSQL that helps improve query performance by eliminating unnecessary partitions from being scanned. If a SQL statement isn't getting partition pruning, you can follow these steps to address the issue:

1. Check the Query Structure

Ensure that your query is written in a way that allows PostgreSQL to recognize the partitioning key. For example, if your table is partitioned by a date column, make sure your WHERE clause directly references this column with a constant or a parameter.

2. Verify Partition Constraints

Ensure that the partitions are correctly defined and that the constraints are properly set. Each partition should have a constraint that defines the range or list of values it contains. For example, if you have a table partitioned by month, each partition should have a constraint defining the specific month it contains.

3. Use Constants or Parameters

Partition pruning works best when the query uses constants or parameters that can be evaluated at the planning stage. For example, use:

SELECT * FROM partitioned_table WHERE partition_column = '2023-01-01';

instead of:

SELECT * FROM partitioned_table WHERE partition_column = CURRENT_DATE;

4. Check the Data Types

Ensure that the data types of the partitioning key in the query match the data types used in the partition constraints. Mismatched data types can prevent PostgreSQL from pruning partitions.

5. Enable Constraint Exclusion

Ensure that constraint exclusion is enabled. This can be done by setting the constraint_exclusion parameter to partition or on in your postgresql.conf file or by running:

SET constraint_exclusion = 'partition';

6. Analyze and Optimize Table Statistics

Ensure that PostgreSQL has up-to-date statistics for the table and partitions. Run the ANALYZE command on your partitioned table to collect statistics:

ANALYZE partitioned_table;

7. Use Explicit Casting

Sometimes explicit casting can help PostgreSQL recognize the partitioning key and apply pruning. For example:

SELECT * FROM partitioned_table WHERE partition_column = '2023-01-01'::date;

8. Rewrite the Query

If the above steps do not help, you might need to rewrite the query to make it more conducive to partition pruning. For example, break down complex queries into simpler subqueries that can better utilize the partitioning key.

Practical Example

Consider a table sales partitioned by the sale_date column:

CREATE TABLE sales (
    id serial PRIMARY KEY,
    sale_date date,
    amount numeric
) PARTITION BY RANGE (sale_date);

CREATE TABLE sales_2023 PARTITION OF sales FOR VALUES FROM ('2023-01-01') TO ('2024-01-01');

A query that benefits from partition pruning:

SELECT * FROM sales WHERE sale_date = '2023-07-15';

If partition pruning is not working, ensure that:

1. The query uses a constant value for sale_date.

2. The sale_date column is referenced directly in the WHERE clause.

3. The constraint_exclusion parameter is set appropriately.

4. The partition constraints are correctly defined and match the query's data type.

By following these steps, you can troubleshoot and fix issues related to partition pruning in PostgreSQL, improving query performance.