Question

I am trying to understand isolation/locks in SQL Server.

I have following scenario in READ COMMITTED isolation level(Default)

We have a table.

create table Transactions(Tid int,amt int)

with some records

insert into Transactions values(1, 100)
insert into Transactions values(2, -50)
insert into Transactions values(3, 100)
insert into Transactions values(4, -100)
insert into Transactions values(5, 200)

Now from msdn i understood

When a select is fired shared lock is taken so no other transaction can modify data(avoiding dirty read).. Documentation also talks about row level, page level, table level lock. I thought of following scenarion

Begin Transaction

select * from Transactions

/*
some buisness logic which takes 5 minutes

*/

Commit

What I want to understand is for what duration of time shared lock would be acquired and which (row, page, table).

Will lock will be acquire only when statement select * from Transactions is run or would it be acquire for whole 5+ minutes till we reach COMMIT.

Answer 1

lock will only acquire when select * from Transaction is run

You can check it with below code

open a sql session and run this query

Begin Transaction

select * from Transactions

 WAITFOR DELAY '00:05'
/*
some buisness logic which takes 5 minutes

*/

Commit

Open another sql session and run below query

Begin Transaction
Update Transactions
Set = ...
where ....
commit