diff options
Diffstat (limited to 'doc/development/iterating_tables_in_batches.md')
| -rw-r--r-- | doc/development/iterating_tables_in_batches.md | 288 |
1 files changed, 234 insertions, 54 deletions
diff --git a/doc/development/iterating_tables_in_batches.md b/doc/development/iterating_tables_in_batches.md index bae55bccf7c..3f2b467fec9 100644 --- a/doc/development/iterating_tables_in_batches.md +++ b/doc/development/iterating_tables_in_batches.md @@ -4,7 +4,7 @@ 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 --- -# Iterating Tables In Batches +# Iterating tables in batches Rails provides a method called `in_batches` that can be used to iterate over rows in batches. For example: @@ -15,7 +15,7 @@ User.in_batches(of: 10) do |relation| end ``` -Unfortunately this method is implemented in a way that is not very efficient, +Unfortunately, this method is implemented in a way that is not very efficient, both query and memory usage wise. To work around this you can include the `EachBatch` module into your models, @@ -44,17 +44,18 @@ all of the arguments that `in_batches` supports. You should always use ## Avoid iterating over non-unique columns -One should proceed with extra caution, and possibly avoid iterating over a column that can contain duplicate values. -When you iterate over an attribute that is not unique, even with the applied max batch size, there is no guarantee that the resulting batches will not surpass it. -The following snippet demonstrates this situation, when one attempt to select `Ci::Build` entries for users with `id` between `1` and `10,s000`, database returns `1 215 178` -matching rows +One should proceed with extra caution, and possibly avoid iterating over a column that can contain +duplicate values. When you iterate over an attribute that is not unique, even with the applied max +batch size, there is no guarantee that the resulting batches will not surpass it. The following +snippet demonstrates this situation when one attempt to select `Ci::Build` entries for users with +`id` between `1` and `10,000`, the database returns `1 215 178` matching rows. ```ruby [ gstg ] production> Ci::Build.where(user_id: (1..10_000)).size => 1215178 ``` -This happens because built relation is translated into following query +This happens because built relation is translated into the following query ```ruby [ gstg ] production> puts Ci::Build.where(user_id: (1..10_000)).to_sql @@ -62,12 +63,16 @@ SELECT "ci_builds".* FROM "ci_builds" WHERE "ci_builds"."type" = 'Ci::Build' AND => nil ``` -And queries which filters non-unique column by range `WHERE "ci_builds"."user_id" BETWEEN ? AND ?`, even though the range size is limited to certain threshold (`10,000` in previous example) this threshold does not translates to the size of returned dataset. That happens because when taking `n` possible values of attributes, -one can't tell for sure that the number of records that contains them will be less than `n`. +`And` queries which filter non-unique column by range `WHERE "ci_builds"."user_id" BETWEEN ? AND ?`, +even though the range size is limited to a certain threshold (`10,000` in the previous example) this +threshold does not translate to the size of the returned dataset. That happens because when taking +`n` possible values of attributes, one can't tell for sure that the number of records that contains +them will be less than `n`. ## Column definition -`EachBatch` uses the primary key of the model by default for the iteration. This works most of the cases, however in some cases, you might want to use a different column for the iteration. +`EachBatch` uses the primary key of the model by default for the iteration. This works most of the +cases, however in some cases, you might want to use a different column for the iteration. ```ruby Project.distinct.each_batch(column: :creator_id, of: 10) do |relation| @@ -78,27 +83,38 @@ end The query above iterates over the project creators and prints them out without duplications. NOTE: -In case the column is not unique (no unique index definition), calling the `distinct` method on the relation is necessary. Using not unique column without `distinct` may result in `each_batch` falling into endless loop as described at following [issue](https://gitlab.com/gitlab-org/gitlab/-/issues/285097) +In case the column is not unique (no unique index definition), calling the `distinct` method on +the relation is necessary. Using not unique column without `distinct` may result in `each_batch` +falling into an endless loop as described in following +[issue](https://gitlab.com/gitlab-org/gitlab/-/issues/285097). ## `EachBatch` in data migrations -When dealing with data migrations the preferred way to iterate over large volume of data is using `EachBatch`. +When dealing with data migrations the preferred way to iterate over a large volume of data is using +`EachBatch`. A special case of data migration is a [background migration](background_migrations.md#scheduling) -where the actual data modification is executed in a background job. The migration code that determines -the data ranges (slices) and schedules the background jobs uses `each_batch`. +where the actual data modification is executed in a background job. The migration code that +determines the data ranges (slices) and schedules the background jobs uses `each_batch`. ## Efficient usage of `each_batch` -`EachBatch` helps iterating over large tables. It's important to highlight that `EachBatch` is not going to magically solve all iteration related performance problems and it might not help at all in some scenarios. From the database point of view, correctly configured database indexes are also necessary to make `EachBatch` perform well. +`EachBatch` helps to iterate over large tables. It's important to highlight that `EachBatch` is +not going to magically solve all iteration related performance problems and it might not help at +all in some scenarios. From the database point of view, correctly configured database indexes are +also necessary to make `EachBatch` perform well. ### Example 1: Simple iteration -Let's consider that we want to iterate over the `users` table and print the `User` records to the standard output. The `users` table contains millions of records, thus running one query to fetch the users will likely time out. +Let's consider that we want to iterate over the `users` table and print the `User` records to the +standard output. The `users` table contains millions of records, thus running one query to fetch +the users will likely time out.  -This is a simplified version of the `users` table which contains several rows. We have a few smaller gaps in the `id` column to make the example a bit more realistic (a few records were already deleted). Currently we have one index on the `id` field. +This is a simplified version of the `users` table which contains several rows. We have a few +smaller gaps in the `id` column to make the example a bit more realistic (a few records were +already deleted). Currently, we have one index on the `id` field. Loading all users into memory (avoid): @@ -117,9 +133,10 @@ User.each_batch(of: 5) do |relation| end ``` -#### How does `each_batch` work? +#### How `each_batch` works -As the first step, it finds the lowest `id` (start `id`) in the table by executing the following database query: +As the first step, it finds the lowest `id` (start `id`) in the table by executing the following +database query: ```sql SELECT "users"."id" FROM "users" ORDER BY "users"."id" ASC LIMIT 1 @@ -127,9 +144,12 @@ SELECT "users"."id" FROM "users" ORDER BY "users"."id" ASC LIMIT 1  -Notice that the query only reads data from the index (`INDEX ONLY SCAN`), the table is not accessed. Database indexes are sorted so taking out the first item is a very cheap operation. +Notice that the query only reads data from the index (`INDEX ONLY SCAN`), the table is not +accessed. Database indexes are sorted so taking out the first item is a very cheap operation. -The next step is to find the next `id` (end `id`) which should respect the batch size configuration. In this example we used batch size of 5. `EachBatch` uses the `OFFSET` clause to get a "shifted" `id` value. +The next step is to find the next `id` (end `id`) which should respect the batch size +configuration. In this example we used a batch size of 5. `EachBatch` uses the `OFFSET` clause +to get a "shifted" `id` value. ```sql SELECT "users"."id" FROM "users" WHERE "users"."id" >= 1 ORDER BY "users"."id" ASC LIMIT 1 OFFSET 5 @@ -137,19 +157,25 @@ SELECT "users"."id" FROM "users" WHERE "users"."id" >= 1 ORDER BY "users"."id" A  -Again, the query only looks into the index. The `OFFSET 5` takes out the sixth `id` value: this query reads a maximum of six items from the index regardless of the table size or the iteration count. +Again, the query only looks into the index. The `OFFSET 5` takes out the sixth `id` value: this +query reads a maximum of six items from the index regardless of the table size or the iteration +count. -At this point we know the `id` range for the first batch. Now it's time to construct the query for the `relation` block. +At this point, we know the `id` range for the first batch. Now it's time to construct the query +for the `relation` block. ```sql SELECT "users".* FROM "users" WHERE "users"."id" >= 1 AND "users"."id" < 302 ``` - + -Notice the `<` sign. Previously six items were read from the index and in this query the last value is "excluded". The query will look at the index to get the location of the five `user` rows on the disk and read the rows from the table. The returned array is processed in Ruby. +Notice the `<` sign. Previously six items were read from the index and in this query, the last +value is "excluded". The query will look at the index to get the location of the five `user` +rows on the disk and read the rows from the table. The returned array is processed in Ruby. -The first iteration is done. For the next iteration, the last `id` value is reused from the previous iteration in order to find out the next end `id` value. +The first iteration is done. For the next iteration, the last `id` value is reused from the +previous iteration in order to find out the next end `id` value. ```sql SELECT "users"."id" FROM "users" WHERE "users"."id" >= 302 ORDER BY "users"."id" ASC LIMIT 1 OFFSET 5 @@ -167,7 +193,8 @@ SELECT "users".* FROM "users" WHERE "users"."id" >= 302 AND "users"."id" < 353 ### Example 2: Iteration with filters -Building on top of the previous example, we want to print users with zero sign-in count. We keep track of the number of sign-ins in the `sign_in_count` column so we write the following code: +Building on top of the previous example, we want to print users with zero sign-in count. We keep +track of the number of sign-ins in the `sign_in_count` column so we write the following code: ```ruby users = User.where(sign_in_count: 0) @@ -183,7 +210,10 @@ end SELECT "users"."id" FROM "users" WHERE "users"."sign_in_count" = 0 ORDER BY "users"."id" ASC LIMIT 1 ``` -Selecting only the `id` column and ordering by `id` is going to "force" the database to use the index on the `id` (primary key index) column, however we also have an extra condition on the `sign_in_count` column. The column is not part of the index, so the database needs to look into the actual table to find the first matching row. +Selecting only the `id` column and ordering by `id` is going to "force" the database to use the +index on the `id` (primary key index) column however, we also have an extra condition on the +`sign_in_count` column. The column is not part of the index, so the database needs to look into +the actual table to find the first matching row.  @@ -193,7 +223,11 @@ The number of scanned rows depends on the data distribution in the table. - Best case scenario: the first user was never logged in. The database reads only one row. - Worst case scenario: all users were logged in at least once. The database reads all rows. -In this particular example the database had to read 10 rows (regardless of our batch size setting) to determine the first `id` value. In a "real-world" application it's hard to predict whether the filtering is going to cause problems or not. In case of GitLab, verifying the data on a production replica is a good start, but keep in mind that data distribution on GitLab.com can be different from self-managed instances. +In this particular example, the database had to read 10 rows (regardless of our batch size setting) +to determine the first `id` value. In a "real-world" application it's hard to predict whether the +filtering is going to cause problems or not. In the case of GitLab, verifying the data on a +production replica is a good start, but keep in mind that data distribution on GitLab.com can be +different from self-managed instances. #### Improve filtering with `each_batch` @@ -207,21 +241,26 @@ This is how our table and the newly created index looks like:  -This index definition covers the conditions on the `id` and `sign_in_count` columns thus makes the `each_batch` queries very effective (similar to the simple iteration example). +This index definition covers the conditions on the `id` and `sign_in_count` columns thus makes the +`each_batch` queries very effective (similar to the simple iteration example). -It's rare when a user was never signed in so we anticipate small index size. Including only the `id` in the index definition also helps keeping the index size small. +It's rare when a user was never signed in so we a anticipate small index size. Including only the +`id` in the index definition also helps to keep the index size small. ##### Index on columns -Later on we might want to iterate over the table filtering for different `sign_in_count` values, in those cases we cannot use the previously suggested conditional index because the `WHERE` condition does not match with our new filter (`sign_in_count > 10`). +Later on, we might want to iterate over the table filtering for different `sign_in_count` values, in +those cases we cannot use the previously suggested conditional index because the `WHERE` condition +does not match with our new filter (`sign_in_count > 10`). To address this problem, we have two options: - Create another, conditional index to cover the new query. -- Replace the index with more generalized configuration. +- Replace the index with a more generalized configuration. NOTE: -Having multiple indexes on the same table and on the same columns could be a performance bottleneck when writing data. +Having multiple indexes on the same table and on the same columns could be a performance bottleneck +when writing data. Let's consider the following index (avoid): @@ -229,15 +268,18 @@ Let's consider the following index (avoid): CREATE INDEX index_on_users_never_logged_in ON users (id, sign_in_count) ``` -The index definition starts with the `id` column which makes the index very inefficient from data selectivity point of view. +The index definition starts with the `id` column which makes the index very inefficient from data +selectivity point of view. ```sql SELECT "users"."id" FROM "users" WHERE "users"."sign_in_count" = 0 ORDER BY "users"."id" ASC LIMIT 1 ``` -Executing the query above results in an `INDEX ONLY SCAN`. However, the query still needs to iterate over unknown number of entries in the index, and then find the first item where the `sign_in_count` is `0`. +Executing the query above results in an `INDEX ONLY SCAN`. However, the query still needs to +iterate over an unknown number of entries in the index, and then find the first item where the +`sign_in_count` is `0`. - + We can improve the query significantly by swapping the columns in the index definition (prefer). @@ -253,11 +295,14 @@ The following index definition is not going to work well with `each_batch` (avoi CREATE INDEX index_on_users_never_logged_in ON users (sign_in_count) ``` -Since `each_batch` builds range queries based on the `id` column, this index cannot be used efficiently. The DB reads the rows from the table or uses a bitmap search where the primary key index is also read. +Since `each_batch` builds range queries based on the `id` column, this index cannot be used +efficiently. The DB reads the rows from the table or uses a bitmap search where the primary +key index is also read. ##### "Slow" iteration -Slow iteration means that we use a good index configuration to iterate over the table and apply filtering on the yielded relation. +Slow iteration means that we use a good index configuration to iterate over the table and +apply filtering on the yielded relation. ```ruby User.each_batch(of: 5) do |relation| @@ -266,7 +311,8 @@ end ``` The iteration uses the primary key index (on the `id` column) which makes it safe from statement -timeouts. The filter (`sign_in_count: 0`) is applied on the `relation` where the `id` is already constrained (range). The number of rows are limited. +timeouts. The filter (`sign_in_count: 0`) is applied on the `relation` where the `id` is already +constrained (range). The number of rows is limited. Slow iteration generally takes more time to finish. The iteration count is higher and one iteration could yield fewer records than the batch size. Iterations may even yield @@ -285,18 +331,19 @@ projects.each_batch do |relation| end ``` -The iteration uses the `id` column of the `projects` table. The batching does not affect the subquery. -This means for each iteration, the subquery is executed by the database. This adds a constant "load" -on the query which often ends up in statement timeouts. We have an unknown number of confidential -issues, the execution time and the accessed database rows depends on the data distribution in the -`issues` table. +The iteration uses the `id` column of the `projects` table. The batching does not affect the +subquery. This means for each iteration, the subquery is executed by the database. This adds a +constant "load" on the query which often ends up in statement timeouts. We have an unknown number +of confidential issues, the execution time and the accessed database rows depend on the data +distribution in the `issues` table. NOTE: Using subqueries works only when the subquery returns a small number of rows. #### Improving Subqueries -When dealing with subqueries, a slow iteration approach could work: the filter on `creator_id` can be part of the generated `relation` object. +When dealing with subqueries, a slow iteration approach could work: the filter on `creator_id` +can be part of the generated `relation` object. ```ruby projects = Project.all @@ -306,7 +353,8 @@ projects.each_batch do |relation| end ``` -If the query on the `issues` table itself is not performant enough, a nested loop could be constructed. Try to avoid it when possible. +If the query on the `issues` table itself is not performant enough, a nested loop could be +constructed. Try to avoid it when possible. ```ruby projects = Project.all @@ -320,7 +368,8 @@ projects.each_batch do |relation| end ``` -If we know that the `issues` table has many more rows than `projects`, it would make sense to flip the queries, where the `issues` table is batched first. +If we know that the `issues` table has many more rows than `projects`, it would make sense to flip +the queries, where the `issues` table is batched first. ### Using `JOIN` and `EXISTS` @@ -331,7 +380,8 @@ When to use `JOINS`: - `projects` - `project_settings` - `users` - `user_details` - `users` - `user_statuses` -- `LEFT JOIN` works well in this case. Conditions on the joined table need to go to the yielded relation so the iteration is not affected by the data distribution in the joined table. +- `LEFT JOIN` works well in this case. Conditions on the joined table need to go to the yielded +relation so the iteration is not affected by the data distribution in the joined table. Example: @@ -353,7 +403,8 @@ end ### Complex queries on the relation object -When the `relation` object has several extra conditions, the execution plans might become "unstable". +When the `relation` object has several extra conditions, the execution plans might become +"unstable". Example: @@ -370,10 +421,11 @@ end Here, we expect that the `relation` query reads the `BATCH_SIZE` of user records and then filters down the results according to the provided queries. The planner might decide that using a bitmap index lookup with the index on the `confidential` column is a better way to -execute the query. This can cause unexpectedly high amount of rows to be read and the query -could time out. +execute the query. This can cause an unexpectedly high amount of rows to be read and the +query could time out. -Problem: we know for sure that the relation is returning maximum `BATCH_SIZE` of records, however the planner does not know this. +Problem: we know for sure that the relation is returning maximum `BATCH_SIZE` of records +however, the planner does not know this. Common table expression (CTE) trick to force the range query to execute first: @@ -394,4 +446,132 @@ end ### `EachBatch` vs `BatchCount` -When adding new counters for usage ping, the preferred way to count records is using the `Gitlab::Database::BatchCount` class. The iteration logic implemented in `BatchCount` has similar performance characteristics like `EachBatch`. Most of the tips and suggestions for improving `BatchCount` mentioned above applies to `BatchCount` as well. +When adding new counters for Service Ping, the preferred way to count records is using the +`Gitlab::Database::BatchCount` class. The iteration logic implemented in `BatchCount` +has similar performance characteristics like `EachBatch`. Most of the tips and suggestions +for improving `BatchCount` mentioned above applies to `BatchCount` as well. + +## Iterate with keyset pagination + +There are a few special cases where iterating with `EachBatch` does not work. `EachBatch` +requires one distinct column (usually the primary key), which makes the iteration impossible +for timestamp columns and tables with composite primary keys. + +Where `EachBatch` does not work, you can use +[keyset pagination](database/pagination_guidelines.md#keyset-pagination) to iterate over the +table or a range of rows. The scaling and performance characteristics are very similar to +`EachBatch`. + +Examples: + +- Iterate over the table in a specific order (timestamp columns) in combination with a tie-breaker +if column user to sort by does not contain unique values. +- Iterate over the table with composite primary keys. + +### Iterate over the issues in a project by creation date + +You can use keyset pagination to iterate over any database column in a specific order (for example, +`created_at DESC`). To ensure consistent order of the returned records with the same values for +`created_at`, use a tie-breaker column with unique values (for example, `id`). + +Assume you have the following index in the `issues` table: + +```sql +idx_issues_on_project_id_and_created_at_and_id" btree (project_id, created_at, id) +``` + +### Fetching records for further processing + +The following snippet iterates over issue records within the project using the specified order +(`created_at, id`). + +```ruby +scope = Issue.where(project_id: 278964).order(:created_at, :id) # id is the tie-breaker + +iterator = Gitlab::Pagination::Keyset::Iterator.new(scope: scope) + +iterator.each_batch(of: 100) do |records| + puts records.map(&:id) +end +``` + +You can add extra filters to the query. This example only lists the issue IDs created in the last +30 days: + +```ruby +scope = Issue.where(project_id: 278964).where('created_at > ?', 30.days.ago).order(:created_at, :id) # id is the tie-breaker + +iterator = Gitlab::Pagination::Keyset::Iterator.new(scope: scope) + +iterator.each_batch(of: 100) do |records| + puts records.map(&:id) +end +``` + +### Updating records in the batch + +For complex `ActiveRecord` queries, the `.update_all` method does not work well, because it +generates an incorrect `UPDATE` statement. +You can use raw SQL for updating records in batches: + +```ruby +scope = Issue.where(project_id: 278964).order(:created_at, :id) # id is the tie-breaker + +iterator = Gitlab::Pagination::Keyset::Iterator.new(scope: scope) + +iterator.each_batch(of: 100) do |records| + ApplicationRecord.connection.execute("UPDATE issues SET updated_at=NOW() WHERE issues.id in (#{records.dup.reselect(:id).to_sql})") +end +``` + +NOTE: +To keep the iteration stable and predictable, avoid updating the columns in the `ORDER BY` clause. + +### Iterate over the `merge_request_diff_commits` table + +The `merge_request_diff_commits` table uses a composite primary key (`merge_request_diff_id, +relative_order`), which makes `EachBatch` impossible to use efficiently. + +To paginate over the `merge_request_diff_commits` table, you can use the following snippet: + +```ruby +# Custom order object configuration: +order = Gitlab::Pagination::Keyset::Order.build([ + Gitlab::Pagination::Keyset::ColumnOrderDefinition.new( + attribute_name: 'merge_request_diff_id', + order_expression: MergeRequestDiffCommit.arel_table[:merge_request_diff_id].asc, + nullable: :not_nullable, + distinct: false, + ), + Gitlab::Pagination::Keyset::ColumnOrderDefinition.new( + attribute_name: 'relative_order', + order_expression: MergeRequestDiffCommit.arel_table[:relative_order].asc, + nullable: :not_nullable, + distinct: false, + ) +]) +MergeRequestDiffCommit.include(FromUnion) # keyset pagination generates UNION queries + +scope = MergeRequestDiffCommit.order(order) + +iterator = Gitlab::Pagination::Keyset::Iterator.new(scope: scope) + +iterator.each_batch(of: 100) do |records| + puts records.map { |record| [record.merge_request_diff_id, record.relative_order] }.inspect +end +``` + +### Order object configuration + +Keyset pagination works well with simple `ActiveRecord` `order` scopes +([first example](iterating_tables_in_batches.md#iterate-over-the-issues-in-a-project-by-creation-date). +However, in special cases, you need to describe the columns in the `ORDER BY` clause (second example) +for the underlying keyset pagination library. When the `ORDER BY` configuration cannot be +automatically determined by the keyset pagination library, an error is raised. + +The code comments of the +[`Gitlab::Pagination::Keyset::Order`](https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/pagination/keyset/order.rb) +and [`Gitlab::Pagination::Keyset::ColumnOrderDefinition`](https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/pagination/keyset/column_order_definition.rb) +classes give an overview of the possible options for configuring the `ORDER BY` clause. You can +also find a few code examples in the +[keyset pagination](database/keyset_pagination.md#complex-order-configuration) documentation. |