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+# SQL Query Guidelines
+
+This document describes various guidelines to follow when writing SQL queries,
+either using ActiveRecord/Arel or raw SQL queries.
+
+## Using LIKE Statements
+
+The most common way to search for data is using the `LIKE` statement. For
+example, to get all issues with a title starting with "WIP:" you'd write the
+following query:
+
+```sql
+SELECT *
+FROM issues
+WHERE title LIKE 'WIP:%';
+```
+
+On PostgreSQL the `LIKE` statement is case-sensitive. On MySQL this depends on
+the case-sensitivity of the collation, which is usually case-insensitive. To
+perform a case-insensitive `LIKE` on PostgreSQL you have to use `ILIKE` instead.
+This statement in turn isn't supported on MySQL.
+
+To work around this problem you should write `LIKE` queries using Arel instead
+of raw SQL fragments as Arel automatically uses `ILIKE` on PostgreSQL and `LIKE`
+on MySQL. This means that instead of this:
+
+```ruby
+Issue.where('title LIKE ?', 'WIP:%')
+```
+
+You'd write this instead:
+
+```ruby
+Issue.where(Issue.arel_table[:title].matches('WIP:%'))
+```
+
+Here `matches` generates the correct `LIKE` / `ILIKE` statement depending on the
+database being used.
+
+If you need to chain multiple `OR` conditions you can also do this using Arel:
+
+```ruby
+table = Issue.arel_table
+
+Issue.where(table[:title].matches('WIP:%').or(table[:foo].matches('WIP:%')))
+```
+
+For PostgreSQL this produces:
+
+```sql
+SELECT *
+FROM issues
+WHERE (title ILIKE 'WIP:%' OR foo ILIKE 'WIP:%')
+```
+
+In turn for MySQL this produces:
+
+```sql
+SELECT *
+FROM issues
+WHERE (title LIKE 'WIP:%' OR foo LIKE 'WIP:%')
+```
+
+## LIKE & Indexes
+
+Neither PostgreSQL nor MySQL use any indexes when using `LIKE` / `ILIKE` with a
+wildcard at the start. For example, this will not use any indexes:
+
+```sql
+SELECT *
+FROM issues
+WHERE title ILIKE '%WIP:%';
+```
+
+Because the value for `ILIKE` starts with a wildcard the database is not able to
+use an index as it doesn't know where to start scanning the indexes.
+
+MySQL provides no known solution to this problem. Luckily PostgreSQL _does_
+provide a solution: trigram GIN indexes. These indexes can be created as
+follows:
+
+```sql
+CREATE INDEX [CONCURRENTLY] index_name_here
+ON table_name
+USING GIN(column_name gin_trgm_ops);
+```
+
+The key here is the `GIN(column_name gin_trgm_ops)` part. This creates a [GIN
+index][gin-index] with the operator class set to `gin_trgm_ops`. These indexes
+_can_ be used by `ILIKE` / `LIKE` and can lead to greatly improved performance.
+One downside of these indexes is that they can easily get quite large (depending
+on the amount of data indexed).
+
+To keep naming of these indexes consistent please use the following naming
+pattern:
+
+    index_TABLE_on_COLUMN_trigram
+
+For example, a GIN/trigram index for `issues.title` would be called
+`index_issues_on_title_trigram`.
+
+Due to these indexes taking quite some time to be built they should be built
+concurrently. This can be done by using `CREATE INDEX CONCURRENTLY` instead of
+just `CREATE INDEX`. Concurrent indexes can _not_ be created inside a
+transaction. Transactions for migrations can be disabled using the following
+pattern:
+
+```ruby
+class MigrationName < ActiveRecord::Migration
+  disable_ddl_transaction!
+end
+```
+
+For example:
+
+```ruby
+class AddUsersLowerUsernameEmailIndexes < ActiveRecord::Migration
+  disable_ddl_transaction!
+
+  def up
+    return unless Gitlab::Database.postgresql?
+
+    execute 'CREATE INDEX CONCURRENTLY index_on_users_lower_username ON users (LOWER(username));'
+    execute 'CREATE INDEX CONCURRENTLY index_on_users_lower_email ON users (LOWER(email));'
+  end
+
+  def down
+    return unless Gitlab::Database.postgresql?
+
+    remove_index :users, :index_on_users_lower_username
+    remove_index :users, :index_on_users_lower_email
+  end
+end
+```
+
+## Plucking IDs
+
+This can't be stressed enough: **never** use ActiveRecord's `pluck` to pluck a
+set of values into memory only to use them as an argument for another query. For
+example, this will make the database **very** sad:
+
+```ruby
+projects = Project.all.pluck(:id)
+
+MergeRequest.where(source_project_id: projects)
+```
+
+Instead you can just use sub-queries which perform far better:
+
+```ruby
+MergeRequest.where(source_project_id: Project.all.select(:id))
+```
+
+The _only_ time you should use `pluck` is when you actually need to operate on
+the values in Ruby itself (e.g. write them to a file). In almost all other cases
+you should ask yourself "Can I not just use a sub-query?".
+
+## Use UNIONs
+
+UNIONs aren't very commonly used in most Rails applications but they're very
+powerful and useful. In most applications queries tend to use a lot of JOINs to
+get related data or data based on certain criteria, but JOIN performance can
+quickly deteriorate as the data involved grows.
+
+For example, if you want to get a list of projects where the name contains a
+value _or_ the name of the namespace contains a value most people would write
+the following query:
+
+```sql
+SELECT *
+FROM projects
+JOIN namespaces ON namespaces.id = projects.namespace_id
+WHERE projects.name ILIKE '%gitlab%'
+OR namespaces.name ILIKE '%gitlab%';
+```
+
+Using a large database this query can easily take around 800 milliseconds to
+run. Using a UNION we'd write the following instead:
+
+```sql
+SELECT projects.*
+FROM projects
+WHERE projects.name ILIKE '%gitlab%'
+
+UNION
+
+SELECT projects.*
+FROM projects
+JOIN namespaces ON namespaces.id = projects.namespace_id
+WHERE namespaces.name ILIKE '%gitlab%';
+```
+
+This query in turn only takes around 15 milliseconds to complete while returning
+the exact same records.
+
+This doesn't mean you should start using UNIONs everywhere, but it's something
+to keep in mind when using lots of JOINs in a query and filtering out records
+based on the joined data.
+
+GitLab comes with a `Gitlab::SQL::Union` class that can be used to build a UNION
+of multiple `ActiveRecord::Relation` objects. You can use this class as
+follows:
+
+```ruby
+union = Gitlab::SQL::Union.new([projects, more_projects, ...])
+
+Project.from("(#{union.to_sql}) projects")
+```
+
+## Ordering by Creation Date
+
+When ordering records based on the time they were created you can simply order
+by the `id` column instead of ordering by `created_at`. Because IDs are always
+unique and incremented in the order that rows are created this will produce the
+exact same results. This also means there's no need to add an index on
+`created_at` to ensure consistent performance as `id` is already indexed by
+default.
+
+[gin-index]: http://www.postgresql.org/docs/current/static/gin.html