Only use prepared statements when bind variables are present

Prepared statements (prepare/execute/close) were being used unnecessarily
when no bind variables were present, and disabling prepared statement using
prepared_statements:false was principally broken. While bind variables were
correctly substituted with prepared_statements:false, the prepared statement
interface was still used, costing an extra two round trips per query.

In addition to making this behavioral change, I also cleaned up the internals
of exec_stmt and exec_without_stmt so that they behave the same (calling log
and constructing the ActiveRecord::Result in the same way).

Moving the check for binds.empty? to exec_query also will mean that several
code paths explicitly calling exec_without_stmt could be cleaned up to once
again call exec_query instead. I have also left the check for binds.empty? in
exec_stmt, since it is not a private method and could be called directly with
an empty binds array. For the sake of clarity in this patch, I have not made
those changes.

= The previous behavior =

When issuing a Foo.find(1) with prepared_statements:true, the bind variable
is present in the prepared query, and execute shows a value passed:

    Connect	root@localhost on rails_test
    Query	SET SQL_AUTO_IS_NULL=0
    Statistics
    Query	SHOW FULL FIELDS FROM `foos`
    Query	SHOW TABLES LIKE 'foos'
    Query	SHOW CREATE TABLE `foos`
    Prepare	SELECT  `foos`.* FROM `foos`  WHERE `foos`.`id` = ? LIMIT 1
    Execute	SELECT  `foos`.* FROM `foos`  WHERE `foos`.`id` = 1 LIMIT 1
    Close stmt
    Quit

When issuing a Foo.find(1) with prepared_statements:false, the bind variable
has already been removed and substituted with the value, but the prepared
statement interface is used anyway:

    Connect	root@localhost on rails_test
    Query	SET SQL_AUTO_IS_NULL=0
    Statistics
    Query	SHOW FULL FIELDS FROM `foos`
    Query	SHOW TABLES LIKE 'foos'
    Query	SHOW CREATE TABLE `foos`
    Prepare	SELECT  `foos`.* FROM `foos`  WHERE `foos`.`id` = 1 LIMIT 1
    Execute	SELECT  `foos`.* FROM `foos`  WHERE `foos`.`id` = 1 LIMIT 1
    Close stmt
    Quit

= With this patch applied =

When issuing a Foo.find(1) with prepared_statements:true, the bind variable
is present in the prepared query, and execute shows a value passed:

    Connect	root@localhost on rails_test
    Query	SET SQL_AUTO_IS_NULL=0
    Statistics
    Query	SHOW FULL FIELDS FROM `foos`
    Query	SHOW TABLES LIKE 'foos'
    Query	SHOW CREATE TABLE `foos`
    Prepare	SELECT  `foos`.* FROM `foos`  WHERE `foos`.`id` = ? LIMIT 1
    Execute	SELECT  `foos`.* FROM `foos`  WHERE `foos`.`id` = 1 LIMIT 1
    Close stmt
    Quit

When issuing a Foo.find(1) with prepared_statements:false, the bind variable
has been removed and substituted with the value, and the query interface is
used instead of the prepared statement interface:

    Connect	root@localhost on rails_test
    Query	SET SQL_AUTO_IS_NULL=0
    Statistics
    Query	SHOW FULL FIELDS FROM `foos`
    Query	SHOW TABLES LIKE 'foos'
    Query	SHOW CREATE TABLE `foos`
    Query	SELECT  `foos`.* FROM `foos`  WHERE `foos`.`id` = 1 LIMIT 1
    Quit

1 files changed, 40 insertions, 34 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/mysql_adapter.rb b/activerecord/lib/active_record/connection_adapters/mysql_adapter.rb
index 0b6734b010..2880282c89 100644
--- a/activerecord/lib/active_record/connection_adapters/mysql_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/mysql_adapter.rb
@@ -279,10 +279,14 @@ module ActiveRecord
       end
 
       def exec_query(sql, name = 'SQL', binds = [])
-        log(sql, name, binds) do
-          exec_stmt(sql, name, binds) do |cols, stmt|
-            ActiveRecord::Result.new(cols, stmt.to_a) if cols
-          end
+        # If the configuration sets prepared_statements:false, binds will
+        # always be empty, since the bind variables will have been already
+        # substituted and removed from binds by BindVisitor, so this will
+        # effectively disable prepared statement usage completely.
+        if binds.empty?
+          exec_without_stmt(sql, name)
+        else
+          exec_stmt(sql, name, binds)
         end
       end
 
@@ -339,41 +343,43 @@ module ActiveRecord
 
       def exec_stmt(sql, name, binds)
         cache = {}
-        if binds.empty?
-          stmt = @connection.prepare(sql)
-        else
-          cache = @statements[sql] ||= {
-            :stmt => @connection.prepare(sql)
-          }
-          stmt = cache[:stmt]
-        end
+        log(sql, name, binds) do
+          if binds.empty?
+            stmt = @connection.prepare(sql)
+          else
+            cache = @statements[sql] ||= {
+              :stmt => @connection.prepare(sql)
+            }
+            stmt = cache[:stmt]
+          end
 
-        begin
-          stmt.execute(*binds.map { |col, val| type_cast(val, col) })
-        rescue Mysql::Error => e
-          # Older versions of MySQL leave the prepared statement in a bad
-          # place when an error occurs. To support older mysql versions, we
-          # need to close the statement and delete the statement from the
-          # cache.
-          stmt.close
-          @statements.delete sql
-          raise e
-        end
+          begin
+            stmt.execute(*binds.map { |col, val| type_cast(val, col) })
+          rescue Mysql::Error => e
+            # Older versions of MySQL leave the prepared statement in a bad
+            # place when an error occurs. To support older mysql versions, we
+            # need to close the statement and delete the statement from the
+            # cache.
+            stmt.close
+            @statements.delete sql
+            raise e
+          end
 
-        cols = nil
-        if metadata = stmt.result_metadata
-          cols = cache[:cols] ||= metadata.fetch_fields.map { |field|
-            field.name
-          }
-        end
+          cols = nil
+          if metadata = stmt.result_metadata
+            cols = cache[:cols] ||= metadata.fetch_fields.map { |field|
+              field.name
+            }
+          end
 
-        result = yield [cols, stmt]
+          result = ActiveRecord::Result.new(cols, stmt.to_a)
 
-        stmt.result_metadata.free if cols
-        stmt.free_result
-        stmt.close if binds.empty?
+          stmt.result_metadata.free if cols
+          stmt.free_result
+          stmt.close if binds.empty?
 
-        result
+          result
+        end
       end
 
       def connect