gitlab-org--gitlab-foss/doc/development/hash_indexes.md
Nick Thomas 5a574883f9
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 !
2019-08-16 14:05:08 +01:00

20 lines
975 B
Markdown

# 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.