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| author | Yorick Peterse <yorickpeterse@gmail.com> | 2017-08-11 14:16:08 +0200 |
|---|---|---|
| committer | Yorick Peterse <yorickpeterse@gmail.com> | 2017-08-16 16:39:37 +0200 |
| commit | a4a8cae7e1d4f5c72ddc0fce18d8530bf0e6c911 (patch) | |
| tree | 2b0c30ed99c17577caad3b81b09c64da19ab4185 | |
| parent | 6735e1dc9a3ae2e55c837f4ecea4bc3c6972a671 (diff) | |
| download | gitlab-ce-a4a8cae7e1d4f5c72ddc0fce18d8530bf0e6c911.tar.gz | |
Document not using database hash indexes
| -rw-r--r-- | doc/development/README.md | 1 | ||||
| -rw-r--r-- | doc/development/hash_indexes.md | 20 |
2 files changed, 21 insertions, 0 deletions
diff --git a/doc/development/README.md b/doc/development/README.md index 50b064ee960..dd150421b65 100644 --- a/doc/development/README.md +++ b/doc/development/README.md @@ -59,6 +59,7 @@ - [Iterating Tables In Batches](iterating_tables_in_batches.md) - [Ordering Table Columns](ordering_table_columns.md) - [Verifying Database Capabilities](verifying_database_capabilities.md) +- [Hash Indexes](hash_indexes.md) ## i18n diff --git a/doc/development/hash_indexes.md b/doc/development/hash_indexes.md new file mode 100644 index 00000000000..e6c1b3590b1 --- /dev/null +++ b/doc/development/hash_indexes.md @@ -0,0 +1,20 @@ +# Hash Indexes + +Both PostgreSQL and MySQL support 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. |