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---
stage: Enablement
group: Database
info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#designated-technical-writers
---
# Hash Indexes
PostgreSQL supports hash indexes besides the regular B-tree
indexes. Hash indexes however are to be avoided at all costs. While they may
_sometimes_ provide better performance the cost of rehashing can be very high.
More importantly: at least until PostgreSQL 10.0 hash indexes are not
WAL-logged, meaning they are not replicated to any replicas. From the PostgreSQL
documentation:
> Hash index operations are not presently WAL-logged, so hash indexes might need
> to be rebuilt with REINDEX after a database crash if there were unwritten
> changes. Also, changes to hash indexes are not replicated over streaming or
> file-based replication after the initial base backup, so they give wrong
> answers to queries that subsequently use them. For these reasons, hash index
> use is presently discouraged.
RuboCop is configured to register an offense when it detects the use of a hash
index.
Instead of using hash indexes you should use regular B-tree indexes.
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