IndexesdebuggingIntermediate

The index exists but the planner ignores it

Symptoms

There is a composite index that "covers" the query, but EXPLAIN shows a Seq Scan.
A similar query on the same table does use an index.

-- index present:
"users_city_country_idx" btree (city, country)

EXPLAIN SELECT id FROM users WHERE country = 'DE';
  Seq Scan on users   (rows=900)
    Filter: (country = 'DE')

Investigate

Inspect areas in any order (0/4 inspected). When you think you know the root cause, commit to it.

Column order in the index

Statistics freshness

Selectivity of country

The query that does use an index