48 KiB
| stage | group | info |
|---|---|---|
| Enablement | Database | 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. 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:
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.
The execution of the query can be largely broken down into three steps:
- The database accesses both
namespacesandprojectstables to find all projects from all groups in the group hierarchy. - The database retrieves
issuesrecords for each project causing heavy disk I/O. Ideally, an appropriate index configuration should optimize this process. - The database sorts the
issuesrows in memory bycreated_atand returnsLIMIT 20rows to the end-user. For large groups, this final step requires both large memory and CPU resources.
Expand this sentence to see the execution plan for this DB query.
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)
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",
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:
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:
LIMITis present, which usually means that the query is paginated (offset or keyset pagination).- The column used with the
INquery and the columns in theORDER BYclause 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, andorder by column 2. - The columns in the
ORDER BYclause 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 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,
use the utility class QueryBuilder from the module.
NOTE:
The generic keyset pagination module introduced in the merge request
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 that we examined earlier:
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:
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)
scoperepresents the originalActiveRecord::Relationobject without theINquery. The relation should define an order which must be supported by the keyset pagination library.array_scopecontains theActiveRecord::Relationobject, which represents the originalIN (subquery). The select values must contain the columns by which the subquery is "connected" to the main query: theidof the project record.array_mapping_scopedefines a lambda returning anActiveRecord::Relationobject. The lambda matches (=) single select values from thearray_scope. The lambda yields as many arguments as the select values defined in thearray_scope. The arguments are Arel SQL expressions.finder_queryloads 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 arecreated_atandidSQL expressions. Finding a record is very fast via the primary key, so we don't use thecreated_atvalue.
The following database index on the issues table must be present
to make the query execute efficiently:
"idx_issues_on_project_id_and_created_at_and_id" btree (project_id, created_at, id)
Expand this sentence to see the SQL query.
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
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:
"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):
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:
"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:
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:
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:
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:
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:
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)
Expand this sentence to see the SQL query.
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
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.
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.
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
queries.
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.
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:
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.
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:
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):
- Sort the initial resultset according to the
ORDER BYclause. - Pick the top cursor to fetch the record, this is our first record. In the example,
this cursor would be (
2020-01-05,3) forproject_id=9. - We can use (
2020-01-05,3) to fetch the next issue respecting theORDER BYclauseproject_id=9filter. 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 |
- Repeat 1 to 3 with the updated resultset until we have fetched
N=20records.
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
recordscolumn contains our sorted database records, and the initializer query sets the first value toNULL, which is filtered out later. - The
countcolumn 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:
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:
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:
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 = 2020-01-05andid > 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
recordscolumns. (result_collector_columnsmethod) - 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:
(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:
-- 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:
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:
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.