Omit BEGIN/COMMIT statements for empty transactions

If a transaction is opened and closed without any queries being run, we
can safely omit the `BEGIN` and `COMMIT` statements, as they only exist
to modify the connection's behaviour inside the transaction. This
removes the overhead of those statements when saving a record with no
changes, which makes workarounds like `save if changed?` unnecessary.

This implementation buffers transactions inside the transaction manager
and materializes them the next time the connection is used. For this to
work, the adapter needs to guard all connection use with a call to
`materialize_transactions`. Because of this, adapters must opt in to get
this new behaviour by implementing `supports_lazy_transactions?`.

If `raw_connection` is used to get a reference to the underlying
database connection, the behaviour is disabled and transactions are
opened eagerly, as we can't know how the connection will be used.
However when the connection is checked back into the pool, we can assume
that the application won't use the reference again and reenable lazy
transactions. This prevents a single `raw_connection` call from
disabling lazy transactions for the lifetime of the connection.

1 files changed, 2 insertions, 0 deletions

diff --git a/activerecord/test/cases/test_case.rb b/activerecord/test/cases/test_case.rb
index 409b07e56c..40947767f3 100644
--- a/activerecord/test/cases/test_case.rb
+++ b/activerecord/test/cases/test_case.rb
@@ -31,6 +31,7 @@ module ActiveRecord
     end
 
     def capture_sql
+      ActiveRecord::Base.connection.materialize_transactions
       SQLCounter.clear_log
       yield
       SQLCounter.log_all.dup
@@ -48,6 +49,7 @@ module ActiveRecord
 
     def assert_queries(num = 1, options = {})
       ignore_none = options.fetch(:ignore_none) { num == :any }
+      ActiveRecord::Base.connection.materialize_transactions
       SQLCounter.clear_log
       x = yield
       the_log = ignore_none ? SQLCounter.log_all : SQLCounter.log