Foreign key columns without indexes cause slow JOINs and cascade operations.
Missing indexes on foreign key columns force sequential scans on JOIN operations and slow down ON DELETE CASCADE chains, especially as tables grow.
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Foreign key constraints enforce referential integrity but PostgreSQL does NOT automatically create indexes on the referencing columns. Without indexes, JOINs, WHERE clauses, and CASCADE deletes require full table scans.
Run splinter lint 0001 (unindexed_foreign_keys) or query directly:
SELECT
c.conrelid::regclass AS table_name,
a.attname AS fk_column
FROM pg_constraint c
JOIN pg_attribute a ON a.attrelid = c.conrelid AND a.attnum = ANY(c.conkey)
WHERE c.contype = 'f'
AND NOT EXISTS (
SELECT 1 FROM pg_index i
WHERE i.indrelid = c.conrelid
AND a.attnum = ANY(i.indkey)
);CREATE INDEX CONCURRENTLY idx_your_table_fk_col
ON your_table (foreign_key_column);0001 (unindexed_foreign_keys)require-concurrent-index-creation