Understanding SQL Server query execution and transactions - sql-server

Question

I'm quite experienced with SQL databases but mostly with Oracle and MySQL.
Now I'm dealing with SQL Server 2012 (Management Studio 2008) and facing a weird behaviour that I cannot explain.
Considering these 3 queries and an origin table made of 400k rows:
SELECT ID_TARJETA
INTO [SGMENTIA_TEMP].[dbo].[borra_borra_]
FROM [DATAMART_SEGMENTIA].[DESA].[CLIENTES]
ALTER TABLE [SGMENTIA_TEMP].[dbo].[borra_borra_]
ADD PRIMARY KEY (ID_TARJETA)
SELECT COUNT(*)
FROM [SGMENTIA_TEMP].[dbo].[borra_borra_]
If I run them one after the other it runs OK. (total: ~7sec).
If I select them all and run all the queries at once it runs BAD. (total: ~60sec)
Finally if I wrap it all with a transaction it runs OK again
BEGIN TRANSACTION;
SELECT ID_TARJETA
INTO [SGMENTIA_TEMP].[dbo].[borra_borra_]
FROM [DATAMART_SEGMENTIA].[DESA].[CLIENTES]
ALTER TABLE [SGMENTIA_TEMP].[dbo].[borra_borra_]
ADD PRIMARY KEY(ID_TARJETA)
SELECT COUNT(*)
FROM [SGMENTIA_TEMP].[dbo].[borra_borra_]
COMMIT;
The whole picture makes no sense to me, considering that creating transactions looks quite expensive the first scenario should be a slow one, and the second one should work far better, am I wrong?
The question is quite important for me since, I'm building programatically (jdbc) this sort of packages of queries and I need a way to tweak its performance.

Answer 1

The only difference between the two snippet provided, is that the first uses the default transaction mode and the second uses an Explicit Transaction.
Since SQL Server default transaction mode is Autocommit Transactions, each individual statement is a transaction.
You can find more information about transaction modes here.
You can try this to see if it run in 60 sec too:
BEGIN TRANSACTION;
SELECT ID_TARJETA
INTO [SGMENTIA_TEMP].[dbo].[borra_borra_]
FROM [DATAMART_SEGMENTIA].[DESA].[CLIENTES];
COMMIT;
BEGIN TRANSACTION;
ALTER TABLE [SGMENTIA_TEMP].[dbo].[borra_borra_]
ADD PRIMARY KEY(ID_TARJETA);
COMMIT;
BEGIN TRANSACTION;
SELECT COUNT(*)
FROM [SGMENTIA_TEMP].[dbo].[borra_borra_]
COMMIT;