21 lines
989 B
Markdown
21 lines
989 B
Markdown
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# Hash Indexes
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Both PostgreSQL and MySQL support hash indexes besides the regular btree
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indexes. Hash indexes however are to be avoided at all costs. While they may
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_sometimes_ provide better performance the cost of rehashing can be very high.
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More importantly: at least until PostgreSQL 10.0 hash indexes are not
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WAL-logged, meaning they are not replicated to any replicas. From the PostgreSQL
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documentation:
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> Hash index operations are not presently WAL-logged, so hash indexes might need
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> to be rebuilt with REINDEX after a database crash if there were unwritten
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> changes. Also, changes to hash indexes are not replicated over streaming or
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> file-based replication after the initial base backup, so they give wrong
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> answers to queries that subsequently use them. For these reasons, hash index
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> use is presently discouraged.
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RuboCop is configured to register an offence 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 btree indexes.
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