Avoid creating extra `relation` and `build_arel` in `_create_record` and `_update_record` (#29999)

Currently `_create_record` and `_update_record` in `Persistence` are
creating extra `unscoped` and calling `build_arel` in the relation. But
`compile_insert` and `compile_update` can be done without those
expensive operation for `SelectManager` creation. So I moved the
implementation to `Persistence` to avoid creating extra relation and
refactored to avoid calling `build_arel`.

https://gist.github.com/kamipo/8ed73d760112cfa5f6263c9413633419

Before:

```
Warming up --------------------------------------
      _update_record   150.000  i/100ms
Calculating -------------------------------------
      _update_record      1.548k (±12.3%) i/s -      7.650k in   5.042603s
```

After:

```
Warming up --------------------------------------
      _update_record   201.000  i/100ms
Calculating -------------------------------------
      _update_record      2.002k (±12.8%) i/s -      9.849k in   5.027681s
```

30% faster for STI classes.

activerecord/lib/active_record/persistence.rb

@@ -165,6 +165,38 @@ def delete(id_or_array)
         where(primary_key => id_or_array).delete_all
       end
 
+      def _insert_record(values) # :nodoc:
+        primary_key_value = nil
+
+        if primary_key && Hash === values
+          arel_primary_key = arel_attribute(primary_key)
+          primary_key_value = values[arel_primary_key]
+
+          if !primary_key_value && prefetch_primary_key?
+            primary_key_value = next_sequence_value
+            values[arel_primary_key] = primary_key_value
+          end
+        end
+
+        if values.empty?
+          im = arel_table.compile_insert(connection.empty_insert_statement_value)
+          im.into arel_table
+        else
+          im = arel_table.compile_insert(_substitute_values(values))
+        end
+
+        connection.insert(im, "#{self} Create", primary_key || false, primary_key_value)
+      end
+
+      def _update_record(values, id, id_was) # :nodoc:
+        bind = predicate_builder.build_bind_attribute(primary_key, id_was || id)
+        um = arel_table.where(
+          arel_attribute(primary_key).eq(bind)
+        ).compile_update(_substitute_values(values), primary_key)
+
+        connection.update(um, "#{self} Update")
+      end
+
       private
         # Called by +instantiate+ to decide which class to use for a new
         # record instance.
@@ -174,6 +206,13 @@ def delete(id_or_array)
         def discriminate_class_for_record(record)
           self
         end
+
+        def _substitute_values(values)
+          values.map do |attr, value|
+            bind = predicate_builder.build_bind_attribute(attr.name, value)
+            [attr, bind]
+          end
+        end
     end
 
     # Returns true if this object hasn't been saved yet -- that is, a record
@@ -671,7 +710,7 @@ def _update_record(attribute_names = self.attribute_names)
         rows_affected = 0
         @_trigger_update_callback = true
       else
-        rows_affected = self.class.unscoped._update_record attributes_values, id, id_in_database
+        rows_affected = self.class._update_record(attributes_values, id, id_in_database)
         @_trigger_update_callback = rows_affected > 0
       end
 
@@ -685,7 +724,7 @@ def _update_record(attribute_names = self.attribute_names)
     def _create_record(attribute_names = self.attribute_names)
       attributes_values = arel_attributes_with_values_for_create(attribute_names)
 
-      new_id = self.class.unscoped.insert attributes_values
+      new_id = self.class._insert_record(attributes_values)
       self.id ||= new_id if self.class.primary_key
 
       @new_record = false

activerecord/lib/active_record/relation.rb

@@ -36,67 +36,6 @@ def initialize_copy(other)
       reset
     end
 
-    def insert(values) # :nodoc:
-      primary_key_value = nil
-
-      if primary_key && Hash === values
-        primary_key_value = values[values.keys.find { |k|
-          k.name == primary_key
-        }]
-
-        if !primary_key_value && klass.prefetch_primary_key?
-          primary_key_value = klass.next_sequence_value
-          values[arel_attribute(klass.primary_key)] = primary_key_value
-        end
-      end
-
-      im = arel.create_insert
-      im.into @table
-
-      substitutes = substitute_values values
-
-      if values.empty? # empty insert
-        im.values = Arel.sql(connection.empty_insert_statement_value)
-      else
-        im.insert substitutes
-      end
-
-      @klass.connection.insert(
-        im,
-        "#{@klass} Create",
-        primary_key || false,
-        primary_key_value,
-        nil,
-      )
-    end
-
-    def _update_record(values, id, id_was) # :nodoc:
-      substitutes = substitute_values values
-
-      scope = @klass.unscoped
-
-      if @klass.finder_needs_type_condition?
-        scope.unscope!(where: @klass.inheritance_column)
-      end
-
-      relation = scope.where(@klass.primary_key => (id_was || id))
-      um = relation
-        .arel
-        .compile_update(substitutes, @klass.primary_key)
-
-      @klass.connection.update(
-        um,
-        "#{@klass} Update",
-      )
-    end
-
-    def substitute_values(values) # :nodoc:
-      values.map do |arel_attr, value|
-        bind = predicate_builder.build_bind_attribute(arel_attr.name, value)
-        [arel_attr, bind]
-      end
-    end
-
     def arel_attribute(name) # :nodoc:
       klass.arel_attribute(name, table)
     end