Lock escalation on table with no key/index

Question

Hi,
I've got the table with single column only (int) and no indexes nor pk/uq, just auto-created statistics.
now, in transaction, there is a command: SELECT <COL> FROM <TABLE> WITH (ROWLOCK UPDLOCK) WHERE <COL>=100
Table has variable num of records but it doesn't go over 200k, usually below 60k. Column values are mostly unique but can be duplicated in minor portion.
Question 1: when this select executes, what will be locked - row, page or entire table?
Question 2: if I have PK/UQ/index and execute same SELECT as above, what will locked then?

Answer 1

Hi,
1. Please note that every lock taken by sql server has some memory associated with it so more the number of locks more memory it will use. To avoid this unnecessary usage sql server uses Lock escalation when it deems necessary that it would be better to
lock whole table instead of locking whole row.
There is no use here using UPDLOCK with select statement, it would just change locking behavior and would force U lock lets see example
I have a table having structure as
CREATE TABLE [dbo].[Logging](
[Id] [int] IDENTITY(1,1) NOT NULL,
[c1] [char](50) NOT NULL DEFAULT ('SOME MORE DATA')
) ON [PRIMARY]
GO
SET ANSI_PADDING OFF
GO
it ha some 20 K records now lets concentrate on query which you asked
begin transaction
select @@SPID,ID,C1 from dbo.Logging with (ROWLOCK,UPDLOCK) where id=12
--commit
To see locks taken by this query we would run below query in other SSMS window
Select
resource_type,
request_mode,
request_status,
request_session_id
from sys.dm_tran_locks
where resource_database_id=11--databse ID
below is result
You can see update lock bein taken on row and IX intent exclusive lock on object that is table and shared lock on database.
Now if we change the query to scan whole table like below
Now you can see Only exclusive lock is taken on whole table instead of taking on every row. If you see the query every row has predicate 'some more data' so ultimately a table scan would be required .
Above is assuming default Isolation level
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