What data structure does a default PostgreSQL index use?

Explanation

B-trees keep keys sorted, so they support equality, ranges (<, BETWEEN), sorting and prefix LIKE 'abc%' (with the right collation or operator class). Hash, GIN, GiST and BRIN are for special cases.

Further reading

• PostgreSQL: Index types (B-tree, Hash, GIN, GiST, BRIN) ↗
• Use The Index, Luke: SQL indexing and tuning ↗