SQL server update locks complete table instead of single rows

All we need is an easy explanation of the problem, so here it is.

Consider a simple table with an Id and Item:

Column  date type
------------------------
Id      int primary key
Item    varchar(50)

Insert three items:

INSERT INTO [Items] ([Item]) VALUES ('first item'); // Id 1
INSERT INTO [Items] ([Item]) VALUES ('seconditem'); // Id 2
INSERT INTO [Items] ([Item]) VALUES ('third item'); // Id 3

Now I update a single row in a transaction (which I explicitely don’t commit for testing purposes):

// query 1
BEGIN TRANSACTION
UPDATE Items SET
  Item = 'updated'
WHERE Id = 2;

Now in parallel I run a second query:

// query 2
SELECT Id, Item From [Items];

query 2 hangs indefinitely. I don’t understand why. Shouldn’t this query be independent of query 1, just reading committed data and ignoring the ongoing update? Any way to optimize/configure this? Or do I have to live with SQL Server blocking whole tables during updates?

How to solve :

I know you bored from this bug, So we are here to help you! Take a deep breath and look at the explanation of your problem. We have many solutions to this problem, But we recommend you to use the first method because it is tested & true method that will 100% work for you.

Method 1

As David Browne mentions, this is the default behavior of the default isolation level in SQL Server. You can look into using alternative isolation levels such as READ_COMMITTED_SNAPSHOT.

Also to clarify, UPDATE operations don’t lock the entire table necessarily, in the default isolation level. In your example, likely only a row level lock is occuring on the row with Id = 2 but because your second query requests that row (since it’s selecting the entire table) it sits waiting on the lock for just the row with Id = 2 before it can return any results. If you modified your test, so your second query said SELECT Id, Item From [Items] WHERE Id = 1 then you’ll likely get results instantly.

Method 2

Turn on READ_COMMITTED_SNAPSHOT if you want readers and writers to not block each other.

See: Row Versioning-based Isolation Levels in the SQL Server Database Engine

Note: Use and implement method 1 because this method fully tested our system.
Thank you 🙂

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0

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