How to identify foreign keys without indexes

Douglas Ortiz
Douglas Ortiz

Foreign keys play a crucial role in maintaining data integrity in relational databases by enforcing referential constraints between tables. On the other hand, indexes are essential for optimizing query performance.

As most queries using joins are related with a foreign key and can benefit from having an index on the foreign key columns, this article discusses a technique to create indexes on all foreign keys as well as pros and cons of this technique.

Understanding Foreign Keys and Indexes

Foreign keys are relational database constraints that ensure the consistency and validity of data between related tables. They create a link between the primary key of one table and a corresponding column (or columns) in another table, establishing a parent-child relationship. On the other hand, indexes are data structures that provide quick access to specific columns, allowing the database engine to efficiently retrieve and process data.

Importance of Indexes on Foreign Keys

Having indexes on foreign keys is vital for optimal database performance. When performing queries that involve joins between tables through foreign keys, indexes help in avoiding full table scans, which can be resource-intensive and time-consuming. Instead, the database engine can use the indexes, leading to faster query execution times and improved overall performance.

Identifying Missing Indexes on Foreign Keys

Identifying missing indexes on foreign keys requires careful analysis of query execution plans and database performance metrics. You can use built-in tools to examine query plans and identify instances where foreign key join operations lack proper index support. Queries executed with high resource consumption can indicate possible missing indexes that need to be created.

Best Practices for Creating Indexes on Foreign Keys

Creating indexes on foreign keys should follow some best practices to ensure effective database optimization without causing unnecessary overhead. Consider the cardinality of the data and the specific queries that will benefit from the index. Choose the right columns for indexing. Regularly monitor and review index usage to ensure they continue to provide value as data evolves over time.

Here is an example on how to identify foreign keys missing an index. The query brings the CREATE INDEX command and how big is the table:

'CREATE INDEX CONCURRENTLY ixfk_' || tc.conrelid::regclass || '_' || string_agg(ta.attname, ', ' ORDER BY tx.n) || ' ON ' || tc.conrelid::regclass || 
'(' || string_agg(ta.attname, ', ' ORDER BY tx.n) || 
'); --' || pg_size_pretty(pg_catalog.pg_relation_size(tc.conrelid)) AS create_index_with_table_size
FROM pg_catalog.pg_constraint tc CROSS JOIN LATERAL unnest(tc.conkey) WITH ORDINALITY AS tx(attnum, n)
JOIN pg_catalog.pg_attribute ta ON ta.attnum = tx.attnum AND ta.attrelid = tc.conrelid
SELECT 1 FROM pg_catalog.pg_index i
WHERE i.indrelid = tc.conrelid and 
(i.indkey::smallint[])[0:cardinality(tc.conkey)-1] @> tc.conkey) AND 
tc.contype = 'f' and pg_catalog.pg_relation_size(tc.conrelid) > 409600 --bigger than 400kb but you can change
GROUP BY tc.conrelid, tc.conname, tc.confrelid
ORDER BY pg_catalog.pg_relation_size(tc.conrelid);

Potential Risks and Drawbacks

While adding indexes on foreign keys can significantly enhance query performance, there are potential risks to be aware of. Over-indexing, or creating indexes on columns that are rarely or never used in queries, can lead to unnecessary overhead and increased storage requirements. Additionally, indexes need to be maintained, which can result in additional system resources being consumed during data modification operations.


Missing indexes on foreign keys can severely impact database performance, leading to slow query execution. By identifying and creating appropriate indexes on foreign keys, database administrators can significantly enhance performance. Regular monitoring, maintenance, and following best practices will ensure that the indexes continue to offer value as the database grows and evolves.

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