Remove potentially heavy queries used to get the ranges of a backgrou…

…nd migration
fatkodima · Jan 18, 2024 · 111746e · 111746e
1 parent 2ab0784
commit 111746e
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Showing 3 changed files with 12 additions and 15 deletions.
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -1,5 +1,7 @@
 ## master (unreleased)
 
+- Remove potentially heavy queries used to get the ranges of a background migration
+
 ## 0.12.0 (2024-01-18)
 
 - Require passing model name for background migration helpers when using multiple databases

diff --git a/lib/online_migrations/background_migrations/migration.rb b/lib/online_migrations/background_migrations/migration.rb
@@ -195,9 +195,8 @@ def next_batch_range
         on_shard do
           # rubocop:disable Lint/UnreachableLoop
           iterator.each_batch(of: batch_size, column: batch_column_name, start: next_min_value) do |relation|
-            min = relation.arel_table[batch_column_name].minimum
-            max = relation.arel_table[batch_column_name].maximum
-            batch_range = relation.pick(min, max)
+            arel_column = relation.arel_table[batch_column_name]
+            batch_range = relation.pick(arel_column.minimum, arel_column.maximum)
 
             break
           end
@@ -249,14 +248,10 @@ def set_defaults
               self.min_value = self.max_value = self.rows_count = -1 # not relevant
             else
               on_shard do
-                self.min_value ||= migration_relation.minimum(batch_column_name)
-                self.max_value ||= migration_relation.maximum(batch_column_name)
-
-                # This can be the case when run in development on empty tables
-                if min_value.nil?
-                  # integer IDs minimum value is 1
-                  self.min_value = self.max_value = 1
-                end
+                # Getting exact min/max values can be a very heavy operation
+                # and is not needed practically.
+                self.min_value ||= 1
+                self.max_value ||= migration_model.unscoped.maximum(batch_column_name) || self.min_value
 
                 count = migration_object.count
                 self.rows_count = count if count != :no_count

diff --git a/test/background_migrations/migration_test.rb b/test/background_migrations/migration_test.rb
@@ -89,14 +89,14 @@ def test_status_transitions
     end
 
     def test_sets_defaults
-      user1 = User.create!
+      _user1 = User.create!
       user2 = User.create!
 
       m = create_migration
 
       assert m.enqueued?
       assert_equal "id", m.batch_column_name
-      assert_equal user1.id, m.min_value
+      assert_equal 1, m.min_value
       assert_equal user2.id, m.max_value
       assert_not m.composite?
       assert_nil m.parent
@@ -350,15 +350,15 @@ def test_creates_child_migrations_for_sharded_migration
       child1, child2, child3 = children
 
       assert_equal "default", child1.shard
-      assert_equal 10, child1.min_value
+      assert_equal 1, child1.min_value
       assert_equal 100, child1.max_value
       assert_equal 2, child1.rows_count
 
       # Everything else is same as for "default".
       assert_equal "shard_one", child2.shard
 
       assert_equal "shard_two", child3.shard
-      assert_equal 200, child3.min_value
+      assert_equal 1, child3.min_value
       assert_equal 300, child3.max_value
       assert_equal 3, child3.rows_count
     end