MySQL: `ROW_FORMAT=DYNAMIC` create table option by default

Since MySQL 5.7.9, the `innodb_default_row_format` option defines the
default row format for InnoDB tables. The default setting is `DYNAMIC`.

The row format is required for indexing on `varchar(255)` with `utf8mb4`
columns.

As long as using MySQL 5.6, CI won't be passed even if MySQL server
setting is properly configured the same as MySQL 5.7
(`innodb_file_per_table = 1`, `innodb_file_format = 'Barracuda'`, and
`innodb_large_prefix = 1`) since InnoDB table is created as the row
format `COMPACT` by default on MySQL 5.6, therefore indexing on string
with `utf8mb4` columns aren't succeeded.

Making `ROW_FORMAT=DYNAMIC` create table option by default for legacy
MySQL version would mitigate the indexing issue on the user side, and it
makes CI would be passed on MySQL 5.6 which is configured properly.

1 files changed, 18 insertions, 0 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/mysql/schema_statements.rb b/activerecord/lib/active_record/connection_adapters/mysql/schema_statements.rb
index 66db4e4149..47b5c4b9ec 100644
--- a/activerecord/lib/active_record/connection_adapters/mysql/schema_statements.rb
+++ b/activerecord/lib/active_record/connection_adapters/mysql/schema_statements.rb
@@ -77,6 +77,10 @@ module ActiveRecord
           super
         end
 
+        def create_table(table_name, options: default_row_format, **)
+          super
+        end
+
         def internal_string_options_for_primary_key
           super.tap do |options|
             if !row_format_dynamic_by_default? && CHARSETS_OF_4BYTES_MAXLEN.include?(charset)
@@ -104,6 +108,20 @@ module ActiveRecord
             end
           end
 
+          def default_row_format
+            return if row_format_dynamic_by_default?
+
+            unless defined?(@default_row_format)
+              if query_value("SELECT @@innodb_file_per_table = 1 AND @@innodb_file_format = 'Barracuda'") == 1
+                @default_row_format = "ROW_FORMAT=DYNAMIC"
+              else
+                @default_row_format = nil
+              end
+            end
+
+            @default_row_format
+          end
+
           def schema_creation
             MySQL::SchemaCreation.new(self)
           end