Add latest changes from gitlab-org/gitlab@master

6 files changed, 988 insertions, 21 deletions

diff --git a/doc/development/database/add_foreign_key_to_existing_column.md b/doc/development/database/add_foreign_key_to_existing_column.md
index 0e1dd0f390e..d74f826cc14 100644
--- a/doc/development/database/add_foreign_key_to_existing_column.md
+++ b/doc/development/database/add_foreign_key_to_existing_column.md
@@ -4,11 +4,17 @@ group: Database
 info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
 ---
 
-# Adding foreign key constraint to an existing column
+# Add a foreign key constraint to an existing column
 
-Foreign keys help ensure consistency between related database tables. The current database review process **always** encourages you to add [foreign keys](../foreign_keys.md) when creating tables that reference records from other tables.
+Foreign keys ensure consistency between related database tables. The current database review process **always** encourages you to add [foreign keys](../foreign_keys.md) when creating tables that reference records from other tables.
 
-Starting with Rails version 4, Rails includes migration helpers to add foreign key constraints to database tables. Before Rails 4, the only way for ensuring some level of consistency was the [`dependent`](https://guides.rubyonrails.org/association_basics.html#options-for-belongs-to-dependent) option within the association definition. Ensuring data consistency on the application level could fail in some unfortunate cases, so we might end up with inconsistent data in the table. This is mostly affecting older tables, where we simply didn't have the framework support to ensure consistency on the database level. These data inconsistencies can easily cause unexpected application behavior or bugs.
+Starting with Rails version 4, Rails includes migration helpers to add foreign key constraints
+to database tables. Before Rails 4, the only way for ensuring some level of consistency was the
+[`dependent`](https://guides.rubyonrails.org/association_basics.html#options-for-belongs-to-dependent)
+option in the association definition. Ensuring data consistency on the application level could fail
+in some unfortunate cases, so we might end up with inconsistent data in the table. This mostly affects
+older tables, where we didn't have the framework support to ensure consistency on the database level.
+These data inconsistencies can cause unexpected application behavior or bugs.
 
 Adding a foreign key to an existing database column requires database structure changes and potential data changes. In case the table is in use, we should always assume that there is inconsistent data.
 
@@ -45,7 +51,7 @@ class Email < ActiveRecord::Base
 end
 ```
 
-Problem: when the user is removed, the email records related to the removed user will stay in the `emails` table:
+Problem: when the user is removed, the email records related to the removed user stays in the `emails` table:
 
 ```ruby
 user = User.find(1)
@@ -83,11 +89,13 @@ Avoid using the `add_foreign_key` constraint more than once per migration file,
 
 #### Data migration to fix existing records
 
-The approach here depends on the data volume and the cleanup strategy. If we can easily find "invalid" records by doing a simple database query and the record count is not that high, then the data migration can be executed within a Rails migration.
+The approach here depends on the data volume and the cleanup strategy. If we can find "invalid"
+records by doing a database query and the record count is not high, then the data migration can
+be executed in a Rails migration.
 
 In case the data volume is higher (>1000 records), it's better to create a background migration. If unsure, please contact the database team for advice.
 
-Example for cleaning up records in the `emails` table within a database migration:
+Example for cleaning up records in the `emails` table in a database migration:
 
 ```ruby
 class RemoveRecordsWithoutUserFromEmailsTable < Gitlab::Database::Migration[1.0]
@@ -112,7 +120,7 @@ end
 
 ### Validate the foreign key
 
-Validating the foreign key will scan the whole table and make sure that each relation is correct.
+Validating the foreign key scans the whole table and makes sure that each relation is correct.
 
 NOTE:
 When using [background migrations](../background_migrations.md), foreign key validation should happen in the next GitLab release.
diff --git a/doc/development/database/constraint_naming_convention.md b/doc/development/database/constraint_naming_convention.md
index 3faef8aee09..a22ddc1551c 100644
--- a/doc/development/database/constraint_naming_convention.md
+++ b/doc/development/database/constraint_naming_convention.md
@@ -6,7 +6,10 @@ info: To determine the technical writer assigned to the Stage/Group associated w
 
 # Constraints naming conventions
 
-The most common option is to let Rails pick the name for database constraints and indexes or let PostgreSQL use the defaults (when applicable). However, when needing to define custom names in Rails or working in Go applications where no ORM is used, it is important to follow strict naming conventions to improve consistency and discoverability.
+The most common option is to let Rails pick the name for database constraints and indexes or let
+PostgreSQL use the defaults (when applicable). However, when defining custom names in Rails, or
+working in Go applications where no ORM is used, it is important to follow strict naming conventions
+to improve consistency and discoverability.
 
 The table below describes the naming conventions for custom PostgreSQL constraints.
 The intent is not to retroactively change names in existing databases but rather ensure consistency of future changes.
diff --git a/doc/development/database/database_reviewer_guidelines.md b/doc/development/database/database_reviewer_guidelines.md
index 7a9c08d9d49..59653c6dde3 100644
--- a/doc/development/database/database_reviewer_guidelines.md
+++ b/doc/development/database/database_reviewer_guidelines.md
@@ -19,7 +19,7 @@ Database reviewers are domain experts who have substantial experience with datab
 A database review is required whenever an application update [touches the database](../database_review.md#general-process).
 
 The database reviewer is tasked with reviewing the database specific updates and
-making sure that any queries or modifications will perform without issues
+making sure that any queries or modifications perform without issues
 at the scale of GitLab.com.
 
 For more information on the database review process, check the [database review guidelines](../database_review.md).
@@ -72,7 +72,7 @@ topics and use cases. The most frequently required during database reviewing are
 - [Avoiding downtime in migrations](../avoiding_downtime_in_migrations.md).
 - [SQL guidelines](../sql.md) for working with SQL queries.
 
-## How to apply for becoming a database maintainer
+## How to apply to become a database maintainer
 
 Once a database reviewer feels confident on switching to a database maintainer,
 they can update their [team profile](https://gitlab.com/gitlab-com/www-gitlab-com/-/blob/master/data/team.yml)
diff --git a/doc/development/database/efficient_in_operator_queries.md b/doc/development/database/efficient_in_operator_queries.md
new file mode 100644
index 00000000000..207fa6c3832
--- /dev/null
+++ b/doc/development/database/efficient_in_operator_queries.md
@@ -0,0 +1,948 @@
+---
+stage: Enablement
+group: Database
+info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
+---
+
+# Efficient `IN` operator queries
+
+This document describes a technique for building efficient ordered database queries with the `IN`
+SQL operator and the usage of a GitLab utility module to help apply the technique.
+
+NOTE:
+The described technique makes heavy use of
+[keyset pagination](pagination_guidelines.md#keyset-pagination).
+It's advised to get familiar with the topic first.
+
+## Motivation
+
+In GitLab, many domain objects like `Issue` live under nested hierarchies of projects and groups.
+To fetch nested database records for domain objects at the group-level,
+we often perform queries with the `IN` SQL operator.
+We are usually interested in ordering the records by some attributes
+and limiting the number of records using `ORDER BY` and `LIMIT` clauses for performance.
+Pagination may be used to fetch subsequent records.
+
+Example tasks requiring querying nested domain objects from the group level:
+
+- Show first 20 issues by creation date or due date from the group `gitlab-org`.
+- Show first 20 merge_requests by merged at date from the group `gitlab-com`.
+
+Unfortunately, ordered group-level queries typically perform badly
+as their executions require heavy I/O, memory, and computations.
+Let's do an in-depth examination of executing one such query.
+
+### Performance problems with `IN` queries
+
+Consider the task of fetching the twenty oldest created issues
+from the group `gitlab-org` with the following query:
+
+```sql
+SELECT "issues".*
+FROM "issues"
+WHERE "issues"."project_id" IN
+    (SELECT "projects"."id"
+     FROM "projects"
+     WHERE "projects"."namespace_id" IN
+         (SELECT traversal_ids[array_length(traversal_ids, 1)] AS id
+          FROM "namespaces"
+          WHERE (traversal_ids @> ('{9970}'))))
+ORDER BY "issues"."created_at" ASC,
+         "issues"."id" ASC
+LIMIT 20
+```
+
+NOTE:
+For pagination, ordering by the `created_at` column is not enough,
+we must add the `id` column as a
+[tie-breaker](pagination_performance_guidelines.md#tie-breaker-column).
+
+The execution of the query can be largely broken down into three steps:
+
+1. The database accesses both `namespaces` and `projects` tables
+   to find all projects from all groups in the group hierarchy.
+1. The database retrieves `issues` records for each project causing heavy disk I/O.
+   Ideally, an appropriate index configuration should optimize this process.
+1. The database sorts the `issues` rows in memory by `created_at` and returns `LIMIT 20` rows to
+   the end-user. For large groups, this final step requires both large memory and CPU resources.
+
+<details>
+<summary>Expand this sentence to see the execution plan for this DB query.</summary>
+<pre><code>
+ Limit  (cost=90170.07..90170.12 rows=20 width=1329) (actual time=967.597..967.607 rows=20 loops=1)
+   Buffers: shared hit=239127 read=3060
+   I/O Timings: read=336.879
+   ->  Sort  (cost=90170.07..90224.02 rows=21578 width=1329) (actual time=967.596..967.603 rows=20 loops=1)
+         Sort Key: issues.created_at, issues.id
+         Sort Method: top-N heapsort  Memory: 74kB
+         Buffers: shared hit=239127 read=3060
+         I/O Timings: read=336.879
+         ->  Nested Loop  (cost=1305.66..89595.89 rows=21578 width=1329) (actual time=4.709..797.659 rows=241534 loops=1)
+               Buffers: shared hit=239121 read=3060
+               I/O Timings: read=336.879
+               ->  HashAggregate  (cost=1305.10..1360.22 rows=5512 width=4) (actual time=4.657..5.370 rows=1528 loops=1)
+                     Group Key: projects.id
+                     Buffers: shared hit=2597
+                     ->  Nested Loop  (cost=576.76..1291.32 rows=5512 width=4) (actual time=2.427..4.244 rows=1528 loops=1)
+                           Buffers: shared hit=2597
+                           ->  HashAggregate  (cost=576.32..579.06 rows=274 width=25) (actual time=2.406..2.447 rows=265 loops=1)
+                                 Group Key: namespaces.traversal_ids[array_length(namespaces.traversal_ids, 1)]
+                                 Buffers: shared hit=334
+                                 ->  Bitmap Heap Scan on namespaces  (cost=141.62..575.63 rows=274 width=25) (actual time=1.933..2.330 rows=265 loops=1)
+                                       Recheck Cond: (traversal_ids @> '{9970}'::integer[])
+                                       Heap Blocks: exact=243
+                                       Buffers: shared hit=334
+                                       ->  Bitmap Index Scan on index_namespaces_on_traversal_ids  (cost=0.00..141.55 rows=274 width=0) (actual time=1.897..1.898 rows=265 loops=1)
+                                             Index Cond: (traversal_ids @> '{9970}'::integer[])
+                                             Buffers: shared hit=91
+                           ->  Index Only Scan using index_projects_on_namespace_id_and_id on projects  (cost=0.44..2.40 rows=20 width=8) (actual time=0.004..0.006 rows=6 loops=265)
+                                 Index Cond: (namespace_id = (namespaces.traversal_ids)[array_length(namespaces.traversal_ids, 1)])
+                                 Heap Fetches: 51
+                                 Buffers: shared hit=2263
+               ->  Index Scan using index_issues_on_project_id_and_iid on issues  (cost=0.57..10.57 rows=544 width=1329) (actual time=0.114..0.484 rows=158 loops=1528)
+                     Index Cond: (project_id = projects.id)
+                     Buffers: shared hit=236524 read=3060
+                     I/O Timings: read=336.879
+ Planning Time: 7.750 ms
+ Execution Time: 967.973 ms
+(36 rows)
+</code></pre>
+</details>
+
+The performance of the query depends on the number of rows in the database.
+On average, we can say the following:
+
+- Number of groups in the group-hierarchy: less than 1 000
+- Number of projects: less than 5 000
+- Number of issues: less than 100 000
+
+From the list, it's apparent that the number of `issues` records has
+the largest impact on the performance.
+As per normal usage, we can say that the number of issue records grows
+at a faster rate than the `namespaces` and the `projects` records.
+
+This problem affects most of our group-level features where records are listed
+in a specific order, such as group-level issues, merge requests pages, and APIs.
+For very large groups the database queries can easily time out, causing HTTP 500 errors.
+
+## Optimizing ordered `IN` queries
+
+In the talk
+["How to teach an elephant to dance rock'n'roll"](https://www.youtube.com/watch?v=Ha38lcjVyhQ),
+Maxim Boguk demonstrated a technique to optimize a special class of ordered `IN` queries,
+such as our ordered group-level queries.
+
+A typical ordered `IN` query may look like this:
+
+```sql
+SELECT t.* FROM t
+WHERE t.fkey IN (value_set)
+ORDER BY t.pkey
+LIMIT N;
+```
+
+Here's the key insight used in the technique: we need at most `|value_set| + N` record lookups,
+rather than retrieving all records satisfying the condition `t.fkey IN value_set` (`|value_set|`
+is the number of values in `value_set`).
+
+We adopted and generalized the technique for use in GitLab by implementing utilities in the
+`Gitlab::Pagination::Keyset::InOperatorOptimization` class to facilitate building efficient `IN`
+queries.
+
+### Requirements
+
+The technique is not a drop-in replacement for the existing group-level queries using `IN` operator.
+The technique can only optimize `IN` queries that satisfy the following requirements:
+
+- `LIMIT` is present, which usually means that the query is paginated
+  (offset or keyset pagination).
+- The column used with the `IN` query and the columns in the `ORDER BY`
+  clause are covered with a database index. The columns in the index must be
+  in the following order: `column_for_the_in_query`, `order by column 1`, and
+  `order by column 2`.
+- The columns in the `ORDER BY` clause are distinct
+  (the combination of the columns uniquely identifies one particular column in the table).
+
+WARNING:
+This technique will not improve the performance of the `COUNT(*)` queries.
+
+## The `InOperatorOptimization` module
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/67352) in GitLab 14.3.
+
+The `Gitlab::Pagination::Keyset::InOperatorOptimization` module implements utilities for applying a generalized version of
+the efficient `IN` query technique described in the previous section.
+
+To build optimized, ordered `IN` queries that meet [the requirements](#requirements),
+use the utility class `QueryBuilder` from the module.
+
+NOTE:
+The generic keyset pagination module introduced in the merge request
+[51481](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/51481)
+plays a fundamental role in the generalized implementation of the technique
+in `Gitlab::Pagination::Keyset::InOperatorOptimization`.
+
+### Basic usage of `QueryBuilder`
+
+To illustrate a basic usage, we will build a query that
+fetches 20 issues with the oldest `created_at` from the group `gitlab-org`.
+
+The following ActiveRecord query would produce a query similar to
+[the unoptimized query](#performance-problems-with-in-queries) that we examined earlier:
+
+```ruby
+scope = Issue
+  .where(project_id: Group.find(9970).all_projects.select(:id)) # `gitlab-org` group and its subgroups
+  .order(:created_at, :id)
+  .limit(20)
+```
+
+Instead, use the query builder `InOperatorOptimization::QueryBuilder` to produce an optimized
+version:
+
+```ruby
+scope = Issue.order(:created_at, :id)
+array_scope = Group.find(9970).all_projects.select(:id)
+array_mapping_scope = -> (id_expression) { Issue.where(Issue.arel_table[:project_id].eq(id_expression)) }
+finder_query = -> (created_at_expression, id_expression) { Issue.where(Issue.arel_table[:id].eq(id_expression)) }
+
+Gitlab::Pagination::Keyset::InOperatorOptimization::QueryBuilder.new(
+  scope: scope,
+  array_scope: array_scope,
+  array_mapping_scope: array_mapping_scope,
+  finder_query: finder_query
+).execute.limit(20)
+```
+
+- `scope` represents the original `ActiveRecord::Relation` object without the `IN` query. The
+  relation should define an order which must be supported by the
+  [keyset pagination library](keyset_pagination.md#usage).
+- `array_scope` contains the `ActiveRecord::Relation` object, which represents the original
+  `IN (subquery)`. The select values must contain the columns by which the subquery is "connected"
+  to the main query: the `id` of the project record.
+- `array_mapping_scope` defines a lambda returning an `ActiveRecord::Relation` object. The lambda
+  matches (`=`) single select values from the `array_scope`. The lambda yields as many
+  arguments as the select values defined in the `array_scope`. The arguments are Arel SQL expressions.
+- `finder_query` loads the actual record row from the database. It must also be a lambda, where
+  the order by column expressions is available for locating the record. In this example, the
+  yielded values are `created_at` and `id` SQL expressions. Finding a record is very fast via the
+  primary key, so we don't use the `created_at` value.
+
+The following database index on the `issues` table must be present
+to make the query execute efficiently:
+
+```sql
+"idx_issues_on_project_id_and_created_at_and_id" btree (project_id, created_at, id)
+```
+
+<details>
+<summary>Expand this sentence to see the SQL query.</summary>
+<pre><code>
+SELECT "issues".*
+FROM
+  (WITH RECURSIVE "array_cte" AS MATERIALIZED
+     (SELECT "projects"."id"
+ FROM "projects"
+ WHERE "projects"."namespace_id" IN
+     (SELECT traversal_ids[array_length(traversal_ids, 1)] AS id
+      FROM "namespaces"
+      WHERE (traversal_ids @> ('{9970}')))),
+                  "recursive_keyset_cte" AS (  -- initializer row start
+                                               (SELECT NULL::issues AS records,
+                                                       array_cte_id_array,
+                                                       issues_created_at_array,
+                                                       issues_id_array,
+                                                       0::bigint AS COUNT
+                                                FROM
+                                                  (SELECT ARRAY_AGG("array_cte"."id") AS array_cte_id_array,
+                                                          ARRAY_AGG("issues"."created_at") AS issues_created_at_array,
+                                                          ARRAY_AGG("issues"."id") AS issues_id_array
+                                                   FROM
+                                                     (SELECT "array_cte"."id"
+                                                      FROM array_cte) array_cte
+                                                   LEFT JOIN LATERAL
+                                                     (SELECT "issues"."created_at",
+                                                             "issues"."id"
+                                                      FROM "issues"
+                                                      WHERE "issues"."project_id" = "array_cte"."id"
+                                                      ORDER BY "issues"."created_at" ASC, "issues"."id" ASC
+                                                      LIMIT 1) issues ON TRUE
+                                                   WHERE "issues"."created_at" IS NOT NULL
+                                                     AND "issues"."id" IS NOT NULL) array_scope_lateral_query
+                                                LIMIT 1)
+                                                -- initializer row finished
+                                             UNION ALL
+                                               (SELECT
+                                                  -- result row start
+                                                  (SELECT issues -- record finder query as the first column
+                                                   FROM "issues"
+                                                   WHERE "issues"."id" = recursive_keyset_cte.issues_id_array[position]
+                                                   LIMIT 1),
+                                                   array_cte_id_array,
+                                                   recursive_keyset_cte.issues_created_at_array[:position_query.position-1]||next_cursor_values.created_at||recursive_keyset_cte.issues_created_at_array[position_query.position+1:],
+                                                   recursive_keyset_cte.issues_id_array[:position_query.position-1]||next_cursor_values.id||recursive_keyset_cte.issues_id_array[position_query.position+1:],
+                                                   recursive_keyset_cte.count + 1
+                                                -- result row finished
+                                                FROM recursive_keyset_cte,
+                                                     LATERAL
+                                                  -- finding the cursor values of the next record start
+                                                  (SELECT created_at,
+                                                          id,
+                                                          position
+                                                   FROM UNNEST(issues_created_at_array, issues_id_array) WITH
+                                                   ORDINALITY AS u(created_at, id, position)
+                                                   WHERE created_at IS NOT NULL
+                                                     AND id IS NOT NULL
+                                                   ORDER BY "created_at" ASC, "id" ASC
+                                                   LIMIT 1) AS position_query,
+                                                  -- finding the cursor values of the next record end
+                                                  -- finding the next cursor values (next_cursor_values_query) start
+                                                             LATERAL
+                                                  (SELECT "record"."created_at",
+                                                          "record"."id"
+                                                   FROM (
+                                                         VALUES (NULL,
+                                                                 NULL)) AS nulls
+                                                   LEFT JOIN
+                                                     (SELECT "issues"."created_at",
+                                                             "issues"."id"
+                                                      FROM (
+                                                              (SELECT "issues"."created_at",
+                                                                      "issues"."id"
+                                                               FROM "issues"
+                                                               WHERE "issues"."project_id" = recursive_keyset_cte.array_cte_id_array[position]
+                                                                 AND recursive_keyset_cte.issues_created_at_array[position] IS NULL
+                                                                 AND "issues"."created_at" IS NULL
+                                                                 AND "issues"."id" > recursive_keyset_cte.issues_id_array[position]
+                                                               ORDER BY "issues"."created_at" ASC, "issues"."id" ASC)
+                                                            UNION ALL
+                                                              (SELECT "issues"."created_at",
+                                                                      "issues"."id"
+                                                               FROM "issues"
+                                                               WHERE "issues"."project_id" = recursive_keyset_cte.array_cte_id_array[position]
+                                                                 AND recursive_keyset_cte.issues_created_at_array[position] IS NOT NULL
+                                                                 AND "issues"."created_at" IS NULL
+                                                               ORDER BY "issues"."created_at" ASC, "issues"."id" ASC)
+                                                            UNION ALL
+                                                              (SELECT "issues"."created_at",
+                                                                      "issues"."id"
+                                                               FROM "issues"
+                                                               WHERE "issues"."project_id" = recursive_keyset_cte.array_cte_id_array[position]
+                                                                 AND recursive_keyset_cte.issues_created_at_array[position] IS NOT NULL
+                                                                 AND "issues"."created_at" > recursive_keyset_cte.issues_created_at_array[position]
+                                                               ORDER BY "issues"."created_at" ASC, "issues"."id" ASC)
+                                                            UNION ALL
+                                                              (SELECT "issues"."created_at",
+                                                                      "issues"."id"
+                                                               FROM "issues"
+                                                               WHERE "issues"."project_id" = recursive_keyset_cte.array_cte_id_array[position]
+                                                                 AND recursive_keyset_cte.issues_created_at_array[position] IS NOT NULL
+                                                                 AND "issues"."created_at" = recursive_keyset_cte.issues_created_at_array[position]
+                                                                 AND "issues"."id" > recursive_keyset_cte.issues_id_array[position]
+                                                               ORDER BY "issues"."created_at" ASC, "issues"."id" ASC)) issues
+                                                      ORDER BY "issues"."created_at" ASC, "issues"."id" ASC
+                                                      LIMIT 1) record ON TRUE
+                                                   LIMIT 1) AS next_cursor_values))
+                                                  -- finding the next cursor values (next_cursor_values_query) END
+SELECT (records).*
+   FROM "recursive_keyset_cte" AS "issues"
+   WHERE (COUNT <> 0)) issues -- filtering out the initializer row
+LIMIT 20
+</code></pre>
+</details>
+
+### Using the `IN` query optimization
+
+#### Adding more filters
+
+In this example, let's add an extra filter by `milestone_id`.
+
+Be careful when adding extra filters to the query. If the column is not covered by the same index,
+then the query might perform worse than the non-optimized query. The `milestone_id` column in the
+`issues` table is currently covered by a different index:
+
+```sql
+"index_issues_on_milestone_id" btree (milestone_id)
+```
+
+Adding the `miletone_id = X` filter to the `scope` argument or to the optimized scope causes bad performance.
+
+Example (bad):
+
+```ruby
+Gitlab::Pagination::Keyset::InOperatorOptimization::QueryBuilder.new(
+  scope: scope,
+  array_scope: array_scope,
+  array_mapping_scope: array_mapping_scope,
+  finder_query: finder_query
+).execute
+  .where(milestone_id: 5)
+  .limit(20)
+```
+
+To address this concern, we could define another index:
+
+```sql
+"idx_issues_on_project_id_and_milestone_id_and_created_at_and_id" btree (project_id, milestone_id, created_at, id)
+```
+
+Adding more indexes to the `issues` table could significantly affect the performance of
+the `UPDATE` queries. In this case, it's better to rely on the original query. It means that if we
+want to use the optimization for the unfiltered page we need to add extra logic in the application code:
+
+```ruby
+if optimization_possible? # no extra params or params covered with the same index as the ORDER BY clause
+  run_optimized_query
+else
+  run_normal_in_query
+end
+```
+
+#### Multiple `IN` queries
+
+Let's assume that we want to extend the group-level queries to include only incident and test case
+issue types.
+
+The original ActiveRecord query would look like this:
+
+```ruby
+scope = Issue
+  .where(project_id: Group.find(9970).all_projects.select(:id)) # `gitlab-org` group and its subgroups
+  .where(issue_type: [:incident, :test_case]) # 1, 2
+  .order(:created_at, :id)
+  .limit(20)
+```
+
+To construct the array scope, we'll need to take the Cartesian product of the `project_id IN` and
+the `issue_type IN` queries. `issue_type` is an ActiveRecord enum, so we need to
+construct the following table:
+
+| `project_id` | `issue_type_value` |
+| ------------ | ------------------ |
+| 2            | 1                  |
+| 2            | 2                  |
+| 5            | 1                  |
+| 5            | 2                  |
+| 10           | 1                  |
+| 10           | 2                  |
+| 9            | 1                  |
+| 9            | 2                  |
+
+For the `issue_types` query we can construct a value list without querying a table:
+
+```ruby
+value_list = Arel::Nodes::ValuesList.new([[Issue.issue_types[:incident]],[Issue.issue_types[:test_case]]])
+issue_type_values = Arel::Nodes::Grouping.new(value_list).as('issue_type_values (value)').to_sql
+
+array_scope = Group
+  .find(9970)
+  .all_projects
+  .from("#{Project.table_name}, #{issue_type_values}")
+  .select(:id, :value)
+```
+
+Building the `array_mapping_scope` query requires two arguments: `id` and `issue_type_value`:
+
+```ruby
+array_mapping_scope = -> (id_expression, issue_type_value_expression) { Issue.where(Issue.arel_table[:project_id].eq(id_expression)).where(Issue.arel_table[:issue_type].eq(issue_type_value_expression)) }
+```
+
+The `scope` and the `finder` queries don't change:
+
+```ruby
+scope = Issue.order(:created_at, :id)
+finder_query = -> (created_at_expression, id_expression) { Issue.where(Issue.arel_table[:id].eq(id_expression)) }
+
+Gitlab::Pagination::Keyset::InOperatorOptimization::QueryBuilder.new(
+  scope: scope,
+  array_scope: array_scope,
+  array_mapping_scope: array_mapping_scope,
+  finder_query: finder_query
+).execute.limit(20)
+```
+
+<details>
+<summary>Expand this sentence to see the SQL query.</summary>
+<pre><code lang='sql'>
+SELECT "issues".*
+FROM
+  (WITH RECURSIVE "array_cte" AS MATERIALIZED
+     (SELECT "projects"."id", "value"
+      FROM projects, (
+                      VALUES (1), (2)) AS issue_type_values (value)
+      WHERE "projects"."namespace_id" IN
+          (WITH RECURSIVE "base_and_descendants" AS (
+                                                       (SELECT "namespaces".*
+                                                        FROM "namespaces"
+                                                        WHERE "namespaces"."type" = 'Group'
+                                                          AND "namespaces"."id" = 9970)
+                                                     UNION
+                                                       (SELECT "namespaces".*
+                                                        FROM "namespaces", "base_and_descendants"
+                                                        WHERE "namespaces"."type" = 'Group'
+                                                          AND "namespaces"."parent_id" = "base_and_descendants"."id")) SELECT "id"
+           FROM "base_and_descendants" AS "namespaces")),
+                  "recursive_keyset_cte" AS (
+                                               (SELECT NULL::issues AS records,
+                                                       array_cte_id_array,
+                                                       array_cte_value_array,
+                                                       issues_created_at_array,
+                                                       issues_id_array,
+                                                       0::bigint AS COUNT
+                                                FROM
+                                                  (SELECT ARRAY_AGG("array_cte"."id") AS array_cte_id_array,
+                                                          ARRAY_AGG("array_cte"."value") AS array_cte_value_array,
+                                                          ARRAY_AGG("issues"."created_at") AS issues_created_at_array,
+                                                          ARRAY_AGG("issues"."id") AS issues_id_array
+                                                   FROM
+                                                     (SELECT "array_cte"."id",
+                                                             "array_cte"."value"
+                                                      FROM array_cte) array_cte
+                                                   LEFT JOIN LATERAL
+                                                     (SELECT "issues"."created_at",
+                                                             "issues"."id"
+                                                      FROM "issues"
+                                                      WHERE "issues"."project_id" = "array_cte"."id"
+                                                        AND "issues"."issue_type" = "array_cte"."value"
+                                                      ORDER BY "issues"."created_at" ASC, "issues"."id" ASC
+                                                      LIMIT 1) issues ON TRUE
+                                                   WHERE "issues"."created_at" IS NOT NULL
+                                                     AND "issues"."id" IS NOT NULL) array_scope_lateral_query
+                                                LIMIT 1)
+                                             UNION ALL
+                                               (SELECT
+                                                  (SELECT issues
+                                                   FROM "issues"
+                                                   WHERE "issues"."id" = recursive_keyset_cte.issues_id_array[POSITION]
+                                                   LIMIT 1), array_cte_id_array,
+                                                             array_cte_value_array,
+                                                             recursive_keyset_cte.issues_created_at_array[:position_query.position-1]||next_cursor_values.created_at||recursive_keyset_cte.issues_created_at_array[position_query.position+1:], recursive_keyset_cte.issues_id_array[:position_query.position-1]||next_cursor_values.id||recursive_keyset_cte.issues_id_array[position_query.position+1:], recursive_keyset_cte.count + 1
+                                                FROM recursive_keyset_cte,
+                                                     LATERAL
+                                                  (SELECT created_at,
+                                                          id,
+                                                          POSITION
+                                                   FROM UNNEST(issues_created_at_array, issues_id_array) WITH
+                                                   ORDINALITY AS u(created_at, id, POSITION)
+                                                   WHERE created_at IS NOT NULL
+                                                     AND id IS NOT NULL
+                                                   ORDER BY "created_at" ASC, "id" ASC
+                                                   LIMIT 1) AS position_query,
+                                                             LATERAL
+                                                  (SELECT "record"."created_at",
+                                                          "record"."id"
+                                                   FROM (
+                                                         VALUES (NULL,
+                                                                 NULL)) AS nulls
+                                                   LEFT JOIN
+                                                     (SELECT "issues"."created_at",
+                                                             "issues"."id"
+                                                      FROM (
+                                                              (SELECT "issues"."created_at",
+                                                                      "issues"."id"
+                                                               FROM "issues"
+                                                               WHERE "issues"."project_id" = recursive_keyset_cte.array_cte_id_array[POSITION]
+                                                                 AND "issues"."issue_type" = recursive_keyset_cte.array_cte_value_array[POSITION]
+                                                                 AND recursive_keyset_cte.issues_created_at_array[POSITION] IS NULL
+                                                                 AND "issues"."created_at" IS NULL
+                                                                 AND "issues"."id" > recursive_keyset_cte.issues_id_array[POSITION]
+                                                               ORDER BY "issues"."created_at" ASC, "issues"."id" ASC)
+                                                            UNION ALL
+                                                              (SELECT "issues"."created_at",
+                                                                      "issues"."id"
+                                                               FROM "issues"
+                                                               WHERE "issues"."project_id" = recursive_keyset_cte.array_cte_id_array[POSITION]
+                                                                 AND "issues"."issue_type" = recursive_keyset_cte.array_cte_value_array[POSITION]
+                                                                 AND recursive_keyset_cte.issues_created_at_array[POSITION] IS NOT NULL
+                                                                 AND "issues"."created_at" IS NULL
+                                                               ORDER BY "issues"."created_at" ASC, "issues"."id" ASC)
+                                                            UNION ALL
+                                                              (SELECT "issues"."created_at",
+                                                                      "issues"."id"
+                                                               FROM "issues"
+                                                               WHERE "issues"."project_id" = recursive_keyset_cte.array_cte_id_array[POSITION]
+                                                                 AND "issues"."issue_type" = recursive_keyset_cte.array_cte_value_array[POSITION]
+                                                                 AND recursive_keyset_cte.issues_created_at_array[POSITION] IS NOT NULL
+                                                                 AND "issues"."created_at" > recursive_keyset_cte.issues_created_at_array[POSITION]
+                                                               ORDER BY "issues"."created_at" ASC, "issues"."id" ASC)
+                                                            UNION ALL
+                                                              (SELECT "issues"."created_at",
+                                                                      "issues"."id"
+                                                               FROM "issues"
+                                                               WHERE "issues"."project_id" = recursive_keyset_cte.array_cte_id_array[POSITION]
+                                                                 AND "issues"."issue_type" = recursive_keyset_cte.array_cte_value_array[POSITION]
+                                                                 AND recursive_keyset_cte.issues_created_at_array[POSITION] IS NOT NULL
+                                                                 AND "issues"."created_at" = recursive_keyset_cte.issues_created_at_array[POSITION]
+                                                                 AND "issues"."id" > recursive_keyset_cte.issues_id_array[POSITION]
+                                                               ORDER BY "issues"."created_at" ASC, "issues"."id" ASC)) issues
+                                                      ORDER BY "issues"."created_at" ASC, "issues"."id" ASC
+                                                      LIMIT 1) record ON TRUE
+                                                   LIMIT 1) AS next_cursor_values)) SELECT (records).*
+   FROM "recursive_keyset_cte" AS "issues"
+   WHERE (COUNT <> 0)) issues
+LIMIT 20
+</code>
+</details>
+
+NOTE:
+To make the query efficient, the following columns need to be covered with an index: `project_id`, `issue_type`, `created_at`, and `id`.
+
+#### Batch iteration
+
+Batch iteration over the records is possible via the keyset `Iterator` class.
+
+```ruby
+scope = Issue.order(:created_at, :id)
+array_scope = Group.find(9970).all_projects.select(:id)
+array_mapping_scope = -> (id_expression) { Issue.where(Issue.arel_table[:project_id].eq(id_expression)) }
+finder_query = -> (created_at_expression, id_expression) { Issue.where(Issue.arel_table[:id].eq(id_expression)) }
+
+opts = {
+  in_operator_optimization_options: {
+    array_scope: array_scope,
+    array_mapping_scope: array_mapping_scope,
+    finder_query: finder_query
+  }
+}
+
+Gitlab::Pagination::Keyset::Iterator.new(scope: scope, **opts).each_batch(of: 100) do |records|
+  puts records.select(:id).map { |r| [r.id] }
+end
+```
+
+#### Keyset pagination
+
+The optimization works out of the box with GraphQL and the `keyset_paginate` helper method.
+Read more about [keyset pagination](database/keyset_pagination.md).
+
+```ruby
+array_scope = Group.find(9970).all_projects.select(:id)
+array_mapping_scope = -> (id_expression) { Issue.where(Issue.arel_table[:project_id].eq(id_expression)) }
+finder_query = -> (created_at_expression, id_expression) { Issue.where(Issue.arel_table[:id].eq(id_expression)) }
+
+opts = {
+  in_operator_optimization_options: {
+    array_scope: array_scope,
+    array_mapping_scope: array_mapping_scope,
+    finder_query: finder_query
+  }
+}
+
+issues = Issue
+  .order(:created_at, :id)
+  .keyset_paginate(per_page: 20, keyset_order_options: opts)
+  .records
+```
+
+#### Offset pagination with Kaminari
+
+The `ActiveRecord` scope produced by the `InOperatorOptimization` class can be used in
+[offset-paginated](database/pagination_guidelines.md#offset-pagination)
+queries.
+
+```ruby
+Gitlab::Pagination::Keyset::InOperatorOptimization::QueryBuilder
+  .new(...)
+  .execute
+  .page(1)
+  .per(20)
+  .without_count
+```
+
+## Generalized `IN` optimization technique
+
+Let's dive into how `QueryBuilder` builds the optimized query
+to fetch the twenty oldest created issues from the group `gitlab-org`
+using the generalized `IN` optimization technique.
+
+### Array CTE
+
+As the first step, we use a common table expression (CTE) for collecting the `projects.id` values.
+This is done by wrapping the incoming `array_scope` ActiveRecord relation parameter with a CTE.
+
+```sql
+WITH array_cte AS MATERIALIZED (
+  SELECT "projects"."id"
+   FROM "projects"
+   WHERE "projects"."namespace_id" IN
+       (SELECT traversal_ids[array_length(traversal_ids, 1)] AS id
+        FROM "namespaces"
+        WHERE (traversal_ids @> ('{9970}')))
+)
+```
+
+This query produces the following result set with only one column (`projects.id`):
+
+| ID  |
+| --- |
+| 9   |
+| 2   |
+| 5   |
+| 10  |
+
+### Array mapping
+
+For each project (that is, each record storing a project ID in `array_cte`),
+we will fetch the cursor value identifying the first issue respecting the `ORDER BY` clause.
+
+As an example, let's pick the first record `ID=9` from `array_cte`.
+The following query should fetch the cursor value `(created_at, id)` identifying
+the first issue record respecting the `ORDER BY` clause for the project with `ID=9`:
+
+```sql
+SELECT "issues"."created_at", "issues"."id"
+FROM "issues"."project_id"=9
+ORDER BY "issues"."created_at" ASC, "issues"."id" ASC
+LIMIT 1;
+```
+
+We will use `LATERAL JOIN` to loop over the records in the `array_cte` and find the
+cursor value for each project. The query would be built using the `array_mapping_scope` lambda
+function.
+
+```sql
+SELECT ARRAY_AGG("array_cte"."id") AS array_cte_id_array,
+  ARRAY_AGG("issues"."created_at") AS issues_created_at_array,
+  ARRAY_AGG("issues"."id") AS issues_id_array
+FROM (
+  SELECT "array_cte"."id" FROM array_cte
+) array_cte
+LEFT JOIN LATERAL
+(
+  SELECT "issues"."created_at", "issues"."id"
+  FROM "issues"
+  WHERE "issues"."project_id" = "array_cte"."id"
+  ORDER BY "issues"."created_at" ASC, "issues"."id" ASC
+  LIMIT 1
+) issues ON TRUE
+```
+
+Since we have an index on `project_id`, `created_at`, and `id`,
+index-only scans should quickly locate all the cursor values.
+
+This is how the query could be translated to Ruby:
+
+```ruby
+created_at_values = []
+id_values = []
+project_ids.map do |project_id|
+  created_at, id = Issue.select(:created_at, :id).where(project_id: project_id).order(:created_at, :id).limit(1).first # N+1 but it's fast
+  created_at_values << created_at
+  id_values << id
+end
+```
+
+This is what the result set would look like:
+
+| `project_ids` | `created_at_values` | `id_values` |
+| ------------- | ------------------- | ----------- |
+| 2             | 2020-01-10          | 5           |
+| 5             | 2020-01-05          | 4           |
+| 10            | 2020-01-15          | 7           |
+| 9             | 2020-01-05          | 3           |
+
+The table shows the cursor values (`created_at, id`) of the first record for each project
+respecting the `ORDER BY` clause.
+
+At this point, we have the initial data. To start collecting the actual records from the database,
+we'll use a recursive CTE query where each recursion locates one row until
+the `LIMIT` is reached or no more data can be found.
+
+Here's an outline of the steps we will take in the recursive CTE query
+(expressing the steps in SQL is non-trivial but will be explained next):
+
+1. Sort the initial resultset according to the `ORDER BY` clause.
+1. Pick the top cursor to fetch the record, this is our first record. In the example,
+this cursor would be (`2020-01-05`, `3`) for `project_id=9`.
+1. We can use (`2020-01-05`, `3`) to fetch the next issue respecting the `ORDER BY` clause
+`project_id=9` filter. This produces an updated resultset.
+
+  | `project_ids` | `created_at_values` | `id_values` |
+  | ------------- | ------------------- | ----------- |
+  | 2             | 2020-01-10          | 5           |
+  | 5             | 2020-01-05          | 4           |
+  | 10            | 2020-01-15          | 7           |
+  | **9**         | **2020-01-06**      | **6**       |
+
+1. Repeat 1 to 3 with the updated resultset until we have fetched `N=20` records.
+
+### Initializing the recursive CTE query
+
+For the initial recursive query, we'll need to produce exactly one row, we call this the
+initializer query (`initializer_query`).
+
+Use `ARRAY_AGG` function to compact the initial result set into a single row
+and use the row as the initial value for the recursive CTE query:
+
+Example initializer row:
+
+| `records`      | `project_ids`   | `created_at_values` | `id_values` | `Count` | `Position` |
+| -------------- | --------------- | ------------------- | ----------- | ------- | ---------- |
+| `NULL::issues` | `[9, 2, 5, 10]` | `[...]`             | `[...]`     | `0`     | `NULL`     |
+
+- The `records` column contains our sorted database records, and the initializer query sets the
+first value to `NULL`, which is filtered out later.
+- The `count` column tracks the number of records found. We use this column to filter out the
+initializer row from the result set.
+
+### Recursive portion of the CTE query
+
+The result row is produced with the following steps:
+
+1. [Order the keyset arrays.](#order-the-keyset-arrays)
+1. [Find the next cursor.](#find-the-next-cursor)
+1. [Produce a new row.](#produce-a-new-row)
+
+#### Order the keyset arrays
+
+Order the keyset arrays according to the original `ORDER BY` clause with `LIMIT 1` using the
+`UNNEST [] WITH ORDINALITY` table function. The function locates the "lowest" keyset cursor
+values and gives us the array position. These cursor values are used to locate the record.
+
+NOTE:
+At this point, we haven't read anything from the database tables, because we relied on
+fast index-only scans.
+
+| `project_ids` | `created_at_values` | `id_values` |
+| ------------- | ------------------- | ----------- |
+| 2             | 2020-01-10          | 5           |
+| 5             | 2020-01-05          | 4           |
+| 10            | 2020-01-15          | 7           |
+| 9             | 2020-01-05          | 3           |
+
+The first row is the 4th one (`position = 4`), because it has the lowest `created_at` and
+`id` values. The `UNNEST` function also exposes the position using an extra column (note:
+PostgreSQL uses 1-based index).
+
+Demonstration of the `UNNEST [] WITH ORDINALITY` table function:
+
+```sql
+SELECT position FROM unnest('{2020-01-10, 2020-01-05, 2020-01-15, 2020-01-05}'::timestamp[], '{5, 4, 7, 3}'::int[])
+  WITH ORDINALITY AS t(created_at, id, position) ORDER BY created_at ASC, id ASC LIMIT 1;
+```
+
+Result:
+
+```sql
+position
+----------
+         4
+(1 row)
+```
+
+#### Find the next cursor
+
+Now, let's find the next cursor values (`next_cursor_values_query`) for the project with `id = 9`.
+To do that, we build a keyset pagination SQL query. Find the next row after
+`created_at = 2020-01-05` and `id = 3`. Because we order by two database columns, there can be two
+cases:
+
+- There are rows with `created_at = 2020-01-05` and `id > 3`.
+- There are rows with `created_at > 2020-01-05`.
+
+Generating this query is done by the generic keyset pagination library. After the query is done,
+we have a temporary table with the next cursor values:
+
+| `created_at` | ID  |
+| ------------ | --- |
+| 2020-01-06   | 6   |
+
+#### Produce a new row
+
+As the final step, we need to produce a new row by manipulating the initializer row
+(`data_collector_query` method). Two things happen here:
+
+- Read the full row from the DB and return it in the `records` columns. (`result_collector_columns`
+method)
+- Replace the cursor values at the current position with the results from the keyset query.
+
+Reading the full row from the database is only one index scan by the primary key. We use the
+ActiveRecord query passed as the `finder_query`:
+
+```sql
+(SELECT "issues".* FROM issues WHERE id = id_values[position] LIMIT 1)
+```
+
+By adding parentheses, the result row can be put into the `records` column.
+
+Replacing the cursor values at `position` can be done via standard PostgreSQL array operators:
+
+```sql
+-- created_at_values column value
+created_at_values[:position-1]||next_cursor_values.created_at||created_at_values[position+1:]
+
+-- id_values column value
+id_values[:position-1]||next_cursor_values.id||id_values[position+1:]
+```
+
+The Ruby equivalent would be the following:
+
+```ruby
+id_values[0..(position - 1)] + [next_cursor_values.id] + id_values[(position + 1)..-1]
+```
+
+After this, the recursion starts again by finding the next lowest cursor value.
+
+### Finalizing the query
+
+For producing the final `issues` rows, we're going to wrap the query with another `SELECT` statement:
+
+```sql
+SELECT "issues".*
+FROM (
+  SELECT (records).* -- similar to ruby splat operator
+  FROM recursive_keyset_cte
+  WHERE recursive_keyset_cte.count <> 0 -- filter out the initializer row
+) AS issues
+```
+
+### Performance comparison
+
+Assuming that we have the correct database index in place, we can compare the query performance by
+looking at the number of database rows accessed by the query.
+
+- Number of groups: 100
+- Number of projects: 500
+- Number of issues (in the group hierarchy): 50 000
+
+Standard `IN` query:
+
+| Query                    | Entries read from index | Rows read from the table | Rows sorted in memory |
+| ------------------------ | ----------------------- | ------------------------ | --------------------- |
+| group hierarchy subquery | 100                     | 0                        | 0                     |
+| project lookup query     | 500                     | 0                        | 0                     |
+| issue lookup query       | 50 000                  | 20                       | 50 000                |
+
+Optimized `IN` query:
+
+| Query                    | Entries read from index | Rows read from the table | Rows sorted in memory |
+| ------------------------ | ----------------------- | ------------------------ | --------------------- |
+| group hierarchy subquery | 100                     | 0                        | 0                     |
+| project lookup query     | 500                     | 0                        | 0                     |
+| issue lookup query       | 519                     | 20                       | 10 000                |
+
+The group and project queries are not using sorting, the necessary columns are read from database
+indexes. These values are accessed frequently so it's very likely that most of the data will be
+in the PostgreSQL's buffer cache.
+
+The optimized `IN` query will read maximum 519 entries (cursor values) from the index:
+
+- 500 index-only scans for populating the arrays for each project. The cursor values of the first
+record will be here.
+- Maximum 19 additional index-only scans for the consecutive records.
+
+The optimized `IN` query will sort the array (cursor values per project array) 20 times, which
+means we'll sort 20 x 500 rows. However, this might be a less memory-intensive task than
+sorting 10 000 rows at once.
+
+Performance comparison for the `gitlab-org` group:
+
+| Query                | Number of 8K Buffers involved | Uncached execution time | Cached execution time |
+| -------------------- | ----------------------------- | ----------------------- | --------------------- |
+| `IN` query           | 240833                        | 1.2s                    | 660ms                 |
+| Optimized `IN` query | 9783                          | 450ms                   | 22ms                  |
+
+NOTE:
+Before taking measurements, the group lookup query was executed separately in order to make
+the group data available in the buffer cache. Since it's a frequently called query, it's going to
+hit many shared buffers during the query execution in the production environment.
diff --git a/doc/development/database/index.md b/doc/development/database/index.md
index b61a71ffb8e..a7b752e14ef 100644
--- a/doc/development/database/index.md
+++ b/doc/development/database/index.md
@@ -62,6 +62,7 @@ info: To determine the technical writer assigned to the Stage/Group associated w
 - [Query performance guidelines](../query_performance.md)
 - [Pagination guidelines](pagination_guidelines.md)
   - [Pagination performance guidelines](pagination_performance_guidelines.md)
+- [Efficient `IN` operator queries](efficient_in_operator_queries.md)
 
 ## Case studies
 
diff --git a/doc/development/database/keyset_pagination.md b/doc/development/database/keyset_pagination.md
index e30c3cc8832..fd62c36b753 100644
--- a/doc/development/database/keyset_pagination.md
+++ b/doc/development/database/keyset_pagination.md
@@ -36,7 +36,8 @@ Keyset pagination works without any configuration for simple ActiveRecord querie
 - Order by one column.
 - Order by two columns, where the last column is the primary key.
 
-The library can detect nullable and non-distinct columns and based on these, it will add extra ordering using the primary key. This is necessary because keyset pagination expects distinct order by values:
+The library detects nullable and non-distinct columns and based on these, adds extra ordering
+using the primary key. This is necessary because keyset pagination expects distinct order by values:
 
 ```ruby
 Project.order(:created_at).keyset_paginate.records # ORDER BY created_at, id
@@ -79,7 +80,7 @@ cursor = paginator.cursor_for_next_page # encoded column attributes for the next
 paginator = Project.order(:name).keyset_paginate(cursor: cursor).records # loading the next page
 ```
 
-Since keyset pagination does not support page numbers, we are restricted to go to the following pages:
+Because keyset pagination does not support page numbers, we are restricted to go to the following pages:
 
 - Next page
 - Previous page
@@ -111,7 +112,8 @@ In the HAML file, we can render the records:
 
 The performance of the keyset pagination depends on the database index configuration and the number of columns we use in the `ORDER BY` clause.
 
-In case we order by the primary key (`id`), then the generated queries will be efficient since the primary key is covered by a database index.
+In case we order by the primary key (`id`), then the generated queries are efficient because
+the primary key is covered by a database index.
 
 When two or more columns are used in the `ORDER BY` clause, it's advised to check the generated database query and make sure that the correct index configuration is used. More information can be found on the [pagination guideline page](pagination_guidelines.md#index-coverage).
 
@@ -149,7 +151,9 @@ puts paginator2.records.to_a # UNION query
 
 ## Complex order configuration
 
-Common `ORDER BY` configurations will be handled by the `keyset_paginate` method automatically so no manual configuration is needed. There are a few edge cases where order object configuration is necessary:
+Common `ORDER BY` configurations are handled by the `keyset_paginate` method automatically
+so no manual configuration is needed. There are a few edge cases where order object
+configuration is necessary:
 
 - `NULLS LAST` ordering.
 - Function-based ordering.
@@ -170,12 +174,13 @@ scope.keyset_paginate # raises: Gitlab::Pagination::Keyset::Paginator::Unsupport
 
 The `keyset_paginate` method raises an error because the order value on the query is a custom SQL string and not an [`Arel`](https://www.rubydoc.info/gems/arel) AST node. The keyset library cannot automatically infer configuration values from these kinds of queries.
 
-To make keyset pagination work, we need to configure custom order objects, to do so, we need to collect information about the order columns:
+To make keyset pagination work, we must configure custom order objects, to do so, we must
+collect information about the order columns:
 
-- `relative_position` can have duplicated values since no unique index is present.
-- `relative_position` can have null values because we don't have a not null constraint on the column. For this, we need to determine where will we see NULL values, at the beginning of the resultset or the end (`NULLS LAST`).
-- Keyset pagination requires distinct order columns, so we'll need to add the primary key (`id`) to make the order distinct.
-- Jumping to the last page and paginating backwards actually reverses the `ORDER BY` clause. For this, we'll need to provide the reversed `ORDER BY` clause.
+- `relative_position` can have duplicated values because no unique index is present.
+- `relative_position` can have null values because we don't have a not null constraint on the column. For this, we must determine where we see NULL values, at the beginning of the result set, or the end (`NULLS LAST`).
+- Keyset pagination requires distinct order columns, so we must add the primary key (`id`) to make the order distinct.
+- Jumping to the last page and paginating backwards actually reverses the `ORDER BY` clause. For this, we must provide the reversed `ORDER BY` clause.
 
 Example:
 
@@ -206,7 +211,8 @@ scope.keyset_paginate.records # works
 
 ### Function-based ordering
 
-In the following example, we multiply the `id` by 10 and ordering by that value. Since the `id` column is unique, we need to define only one column:
+In the following example, we multiply the `id` by 10 and order by that value. Because the `id`
+column is unique, we define only one column:
 
 ```ruby
 order = Gitlab::Pagination::Keyset::Order.build([
@@ -233,7 +239,8 @@ The `add_to_projections` flag tells the paginator to expose the column expressio
 
 ### `iid` based ordering
 
-When ordering issues, the database ensures that we'll have distinct `iid` values within a project. Ordering by one column is enough to make the pagination work if the `project_id` filter is present:
+When ordering issues, the database ensures that we have distinct `iid` values in a project.
+Ordering by one column is enough to make the pagination work if the `project_id` filter is present:
 
 ```ruby
 order = Gitlab::Pagination::Keyset::Order.build([