Reduce default batch sizes for background data migrations

Having these large numbers it is easier to shoot yourself in the foot. Smaller numbers are generally safer
and people can override these globally or set per specific migration.

CHANGELOG.md

@@ -1,5 +1,9 @@
 ## master (unreleased)
 
+- Reduce default batch sizes for background data migrations
+
+    `batch_size` was 20_000, now 1_000; `sub_batch_size` was 1_000, now 100
+
 - Remove deprecated code
 - Drop support for PostgreSQL < 12
 - Drop support for Ruby < 3.0 and Rails < 7.0

lib/generators/online_migrations/templates/initializer.rb.tt

@@ -100,10 +100,10 @@ OnlineMigrations.configure do |config|
   config.background_migrations.migrations_module = "OnlineMigrations::BackgroundMigrations"
 
   # The number of rows to process in a single background migration run.
-  config.background_migrations.batch_size = 20_000
+  config.background_migrations.batch_size = 1_000
 
   # The smaller batches size that the batches will be divided into.
-  config.background_migrations.sub_batch_size = 1000
+  config.background_migrations.sub_batch_size = 100
 
   # The pause interval between each background migration job's execution (in seconds).
   config.background_migrations.batch_pause = 0.seconds

lib/online_migrations/background_migrations/config.rb

@@ -13,12 +13,12 @@ class Config
       attr_accessor :migrations_module
 
       # The number of rows to process in a single background migration run
-      # @return [Integer] defaults to 20_000
+      # @return [Integer] defaults to 1_000
       #
       attr_accessor :batch_size
 
       # The smaller batches size that the batches will be divided into
-      # @return [Integer] defaults to 1000
+      # @return [Integer] defaults to 100
       #
       attr_accessor :sub_batch_size
 
@@ -61,8 +61,8 @@ class Config
       def initialize
         @migrations_path = "lib"
         @migrations_module = "OnlineMigrations::BackgroundMigrations"
-        @batch_size = 20_000
-        @sub_batch_size = 1000
+        @batch_size = 1_000
+        @sub_batch_size = 100
         @batch_pause = 0.seconds
         @sub_batch_pause_ms = 100
         @batch_max_attempts = 5

lib/online_migrations/background_migrations/migration_helpers.rb

@@ -335,8 +335,8 @@ def perform_action_on_relation_in_background(model_name, conditions, action, upd
       #     defaults to `SELECT MIN(batch_column_name)`
       # @option options [Integer] :max_value Value in the column the batching will end at,
       #     defaults to `SELECT MAX(batch_column_name)`
-      # @option options [Integer] :batch_size (20_000) Number of rows to process in a single background migration run
-      # @option options [Integer] :sub_batch_size (1000) Smaller batches size that the batches will be divided into
+      # @option options [Integer] :batch_size (1_000) Number of rows to process in a single background migration run
+      # @option options [Integer] :sub_batch_size (100) Smaller batches size that the batches will be divided into
       # @option options [Integer] :batch_pause (0) Pause interval between each background migration job's execution (in seconds)
       # @option options [Integer] :sub_batch_pause_ms (100) Number of milliseconds to sleep between each sub_batch execution
       # @option options [Integer] :batch_max_attempts (5) Maximum number of batch run attempts

lib/online_migrations/schema_statements.rb

@@ -12,7 +12,7 @@ module SchemaStatements
     # @param column_name [String, Symbol]
     # @param value value for the column. It is typically a literal. To perform a computed
     #     update, an Arel literal can be used instead
-    # @option options [Integer] :batch_size (1000) size of the batch
+    # @option options [Integer] :batch_size (1_000) size of the batch
     # @option options [String, Symbol] :batch_column_name (primary key) option is for tables without primary key, in this
     #     case another unique integer column can be used. Example: `:user_id`
     # @option options [Proc, Boolean] :progress (false) whether to show progress while running.