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Random freezes, missing sound and display glitches usually trace back to one bad driver. Find and replace yours safely.Free scan · under a minuteIn PostgreSQL 17, both B-tree and hash indexes can support equality lookups, but only B-trees support range predicates and sorted output. Hash indexes are limited to equality comparisons, so the right choice depends on the query, schema requirements, and database system—not simply on whether a query uses =. Exact behavior varies by database and version.
PostgreSQL 17: which queries can each index support?
PostgreSQL 17 lists B-tree as its default index type for common situations. It supports equality and range searches when the indexed data has a sortable ordering. A hash index supports equality checks only, using the = operator.
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| Query need | B-tree (PostgreSQL 17) | Hash (PostgreSQL 17) |
|---|---|---|
Equality, such as column = value |
Supported | Supported for = |
Range predicates: <, <=, >=, > |
Supported | Not supported |
BETWEEN or IN |
Can be implemented with B-tree searches | Not supported as range or set searches |
| Sorted output by the indexed key | Can return rows in sorted order | Cannot provide ordering by the indexed key |
These are capabilities, not guarantees that PostgreSQL will choose an index for every query. The planner decides whether an available index is useful for the particular query and data.
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PostgreSQL hash indexes are persistent, on-disk indexes, but they store a hash value rather than the original column value. Each index tuple stores a four-byte hash value. Because different values can produce the same hash, a hash-index scan is lossy: PostgreSQL may need to visit matching table rows and recheck the original condition.
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- One column: PostgreSQL 17 hash indexes are single-column.
- No uniqueness enforcement: They cannot perform uniqueness checking. If the schema requires a unique constraint, a hash index is not a substitute.
- Potentially smaller for long keys: Storing hash values may make an index smaller than a B-tree for longer data such as UUIDs or URLs. This is a possibility, not a universal size or speed advantage.
When a PostgreSQL 17 hash index is worth evaluating
PostgreSQL documentation describes hash indexes as best optimized for equality scans on larger tables in SELECT- and UPDATE-heavy workloads. The rationale is that a B-tree search descends to a leaf, while a hash index accesses the relevant bucket page. Actual benefit depends on the workload and data: overflow pages can chain off a bucket and require additional scanning, and an unbalanced hash index can take more block accesses than a B-tree for some data.
Use a hash index only when equality lookups dominate and the index’s constraints fit the schema. Compare it with a B-tree using the actual query plans and measurements for your application; the documentation does not promise a performance win, and no general speedup follows from choosing hash.
How to choose for a query or schema
- Check the predicate. If queries need only equality comparisons, either index type may be eligible in PostgreSQL 17. If they need ranges,
BETWEEN, orIN, choose B-tree among these two. - Check ordering needs. If the query needs rows ordered by the indexed key, use B-tree; hash cannot supply that ordering.
- Check schema requirements. If uniqueness must be enforced or the index must cover multiple columns, PostgreSQL 17 hash indexes do not meet that requirement.
- Evaluate the workload. For long keys and large tables with equality-heavy SELECT and UPDATE activity, test whether hash’s possible space or access tradeoff helps. Include rechecks and any overflow-page work in the evaluation.
- Verify planner behavior. Inspect plans and measure representative queries; support for a predicate does not mean the planner will always choose that index.
MySQL behavior is engine-specific
Oracle’s MySQL 26.7 documentation discusses hash indexes in the context of the MEMORY storage engine. There, hash indexes support equality comparisons with = or the null-safe equality operator <=>, and cannot speed up ORDER BY. Treat this as guidance for that documented engine, not a universal description of every MySQL index configuration.
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Official documentation
- PostgreSQL 17: Index Types
- PostgreSQL 17: Hash Indexes
- MySQL 26.7: Comparison of B-Tree and Hash Indexes
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