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I have a simple query like this
SELECT * FROM MY_TABLE;
When I run it, SQL Server Management Studio hangs.
Other tables and views are working fine.
What can cause this? I've had locks while running UPDATE statements before, and I know how to approach those. But what could cause a SELECT to lock?
I have run the "All Blocking Transactions" report, and it says there are none.
853
A SELECT in SQL Server will place a shared lock on a table row - and a second SELECT would also require a shared lock, and those are compatible with one another. So no - one SELECT cannot block another SELECT .
SELECT statements get a shared lock on the entire table. Other statements get exclusive locks on the entire table, which are released when the transaction commits. SELECT statements get shared locks on a range of rows. UPDATE and DELETE statements get exclusive locks on a range of rows.
Transaction locks are the most common cause of blocked processes. The stronger (least concurrent) the isolation level, the more likely it is to cause a blocked process. Try to use less granular lock for your queries, as the less granular the lock, the more likely a blocked process or deadlock will not occur.
It is probably not the select that is locking up, but some other process that is editing (udpate/delete/insert) the table that is causing the locks.
You can view which process is blocking by runing exec sp_who2 on your SQL Server.
Alternatively, if you are OK with dirty reads, you can do one of two things
SELECT * FROM Table WITH (NOLOCK)
OR
SET Transaction Isolation Level Read Uncommitted
SELECT * FROM Table
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