gitlab-org--gitlab-foss/doc/development/hash_indexes.md

# Hash Indexes

PostgreSQL supports hash indexes besides the regular btree
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 offence when it detects the use of a hash
index.

Instead of using hash indexes you should use regular btree indexes.
Document not using database hash indexes 2017-08-11 12:16:08 +00:00			`# Hash Indexes`

Remove MySQL references from development docs I noticed the doc/development/testing_guide/best_practices.md still referenced the `[run mysql]` tags, etc. They no longer work, so I removed them, then realised I had better clean up the rest of doc/development ! 2019-08-12 10:29:10 +00:00			`PostgreSQL supports hash indexes besides the regular btree`
Document not using database hash indexes 2017-08-11 12:16:08 +00:00			`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 offence when it detects the use of a hash`
			`index.`

			`Instead of using hash indexes you should use regular btree indexes.`