Hints and Costs in PostgreSQL

Hints and costs are two important concepts in PostgreSQL query optimization that can help improve query performance by providing the optimizer with additional information about how to execute a query.

Hints are comments or directives that are added to a query to guide the optimizer in choosing an execution plan. Hints are typically used when the optimizer is not choosing the optimal execution plan or when the query performance needs to be improved.

For example, consider a query that joins two large tables using a nested loop join. If the optimizer is not choosing a more efficient join algorithm, a hint can be added to the query to instruct the optimizer to use a different join algorithm, such as a hash join.

Costs are values that are used by the optimizer to estimate the cost of different execution plans. Costs are based on the statistics and metadata of the tables being queried and are used by the optimizer to choose the most efficient execution plan for a query.

For example, consider a query that filters a large table based on the values in another table. The optimizer can estimate the cost of different execution plans, such as using an index scan or a sequential scan, based on the statistics of the tables and the selectivity of the filter conditions.

To use hints and costs in PostgreSQL, you can use the SET command to specify the hint or cost value for a particular query. For example, the following query uses a hint to instruct the optimizer to use a hash join:

SELECT /*+ HASH JOIN */ *
FROM table1
JOIN table2 ON table1.id = table2.id;

Similarly, the following query uses a cost parameter to instruct the optimizer to use an index scan:

SET enable_seqscan = off;
SET enable_indexscan = on;
SET seq_page_cost = 1.0;
SET random_page_cost = 4.0;
SELECT *
FROM table1
WHERE indexed_column = ‘value’;

By using hints and costs, PostgreSQL users can optimize query performance and improve query execution times. However, it’s important to note that hints and costs should be used judiciously and only when necessary, as they can potentially interfere with the optimizer’s ability to choose the best execution plan.

In conclusion, hints and costs are important concepts in PostgreSQL query optimization that can help improve query performance by providing the optimizer with additional information about how to execute a query. By using hints and costs, PostgreSQL users can optimize query performance and improve query execution times, but they should be used judiciously and only when necessary.

About Shiv Iyer 422 Articles
Open Source Database Systems Engineer with a deep understanding of Optimizer Internals, Performance Engineering, Scalability and Data SRE. Shiv currently is the Founder, Investor, Board Member and CEO of multiple Database Systems Infrastructure Operations companies in the Transaction Processing Computing and ColumnStores ecosystem. He is also a frequent speaker in open source software conferences globally.