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stage: Enablement
group: Database
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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/#assignments
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# Hash Indexes
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PostgreSQL supports hash indexes besides the regular B-tree
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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.
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RuboCop is configured to register an offense when it detects the use of a hash
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index.
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Instead of using hash indexes you should use regular B-tree indexes.
