`change_column_default` should be executed after type changing

If do not execute a type changing first, filling in default value may be
failed.

```
% ARCONN=postgresql be ruby -w -Itest test/cases/migration/compatibility_test.rb -n test_legacy_change_column_with_null_executes_update
Using postgresql
Run options: -n test_legacy_change_column_with_null_executes_update --seed 20459

E

Error:
ActiveRecord::Migration::CompatibilityTest#test_legacy_change_column_with_null_executes_update:
StandardError: An error has occurred, this and all later migrations canceled:

PG::StringDataRightTruncation: ERROR:  value too long for type character varying(5)
: UPDATE "testings" SET "foo"='foobar' WHERE "foo" IS NULL
```

2 files changed, 18 insertions, 14 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb b/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb
index 2e9e74403b..bf5fbb30e1 100644
--- a/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb
+++ b/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb
@@ -393,9 +393,9 @@ module ActiveRecord
         end
 
         def change_column(table_name, column_name, type, options = {}) #:nodoc:
-          quoted_table_name = quote_table_name(table_name)
+          clear_cache!
           sqls, procs = change_column_for_alter(table_name, column_name, type, options)
-          execute "ALTER TABLE #{quoted_table_name} #{sqls.join(", ")}"
+          execute "ALTER TABLE #{quote_table_name(table_name)} #{sqls.join(", ")}"
           procs.each(&:call)
         end
 
@@ -646,8 +646,7 @@ module ActiveRecord
             end
           end
 
-          def change_column_for_alter(table_name, column_name, type, options = {})
-            clear_cache!
+          def change_column_sql(table_name, column_name, type, options = {})
             quoted_column_name = quote_column_name(column_name)
             sql_type = type_to_sql(type, options)
             sql = "ALTER COLUMN #{quoted_column_name} TYPE #{sql_type}".dup
@@ -661,7 +660,11 @@ module ActiveRecord
               sql << " USING CAST(#{quoted_column_name} AS #{cast_as_type})"
             end
 
-            sqls = [sql]
+            sql
+          end
+
+          def change_column_for_alter(table_name, column_name, type, options = {})
+            sqls = [change_column_sql(table_name, column_name, type, options)]
             procs = []
             sqls << change_column_default_for_alter(table_name, column_name, options[:default]) if options.key?(:default)
             sqls << change_column_null_for_alter(table_name, column_name, options[:null], options[:default]) if options.key?(:null)
@@ -673,7 +676,6 @@ module ActiveRecord
 
           # Changes the default value of a table column.
           def change_column_default_for_alter(table_name, column_name, default_or_changes) # :nodoc:
-            clear_cache!
             column = column_for(table_name, column_name)
             return unless column
 
@@ -689,7 +691,6 @@ module ActiveRecord
           end
 
           def change_column_null_for_alter(table_name, column_name, null, default = nil) #:nodoc:
-            clear_cache!
             "ALTER #{quote_column_name(column_name)} #{null ? 'DROP' : 'SET'} NOT NULL"
           end
 
diff --git a/activerecord/lib/active_record/migration/compatibility.rb b/activerecord/lib/active_record/migration/compatibility.rb
index da307c1723..bd8c054c28 100644
--- a/activerecord/lib/active_record/migration/compatibility.rb
+++ b/activerecord/lib/active_record/migration/compatibility.rb
@@ -16,13 +16,16 @@ module ActiveRecord
       V5_2 = Current
 
       class V5_1 < V5_2
-        if adapter_name == "PostgreSQL"
-          def change_column(table_name, column_name, type, options = {})
-            unless options[:null] || options[:default].nil?
-              column = connection.send(:column_for, table_name, column_name)
-              connection.execute("UPDATE #{connection.quote_table_name(table_name)} SET #{connection.quote_column_name(column_name)}=#{connection.quote_default_expression(options[:default], column)} WHERE #{connection.quote_column_name(column_name)} IS NULL") if column
-            end
-            connection.change_column(table_name, column_name, type, options)
+        def change_column(table_name, column_name, type, options = {})
+          if adapter_name == "PostgreSQL"
+            clear_cache!
+            sql = connection.send(:change_column_sql, table_name, column_name, type, options)
+            execute "ALTER TABLE #{quote_table_name(table_name)} #{sql}"
+            change_column_default(table_name, column_name, options[:default]) if options.key?(:default)
+            change_column_null(table_name, column_name, options[:null], options[:default]) if options.key?(:null)
+            change_column_comment(table_name, column_name, options[:comment]) if options.key?(:comment)
+          else
+            super
           end
         end
       end