Sql-server – SELECT blocks UPDATE in Snapshot Isolation

snapshot-isolationsql serversql-server-2016tempdb-version-storetransaction

We have a database running on SQL Server 2016 SP1 Enterprise and on which Allow Snapshot Isolation is enabled. In this scenario readers do not block writers and writers do not block readers.

Today we saw that a SELECT-query was blocking an UPDATE-query.
These are the queries at issue:

First query:

IF @@TRANCOUNT = 0 SET TRANSACTION ISOLATION LEVEL SNAPSHOT
SELECT TOP 10 Field1, Field2, Field3 FROM [dbo].[TABLE1] 
WHERE ([Field1] = @SV1 AND (1=1))
ORDER BY A.[Field1] ASC,A.[Field2] ASC
OPTION (KEEP PLAN, KEEPFIXED PLAN, LOOP JOIN)

Second query:

UPDATE [dbo].[Table1] SET [Field4]= @V128, [Field5]= @V140, [Field6]= @V141 WHERE [Field3] = @ID0

If a understand versioning correctly, a SELECT-query do not use any locks because it works on a snapshot of the data (taken at the start of the transaction). How is it possible that the SELECT-query blocks the UPDATE?

Best Answer

if @@trancount <> 0 then that SELECT will run with the current transaction isolation level. Which (unless it already happens to be SNAPSHOT) will require S locks for reading data.

If you want all SELECTS using the default READ COMMITTED isolation level to use row versioning instead of S locks, set READ COMMITTED SNAPSHOT on the database.

Related Solutions

Postgresql – Read Committed Isolation Level

The difference lies between a query and a transaction. A transaction can contain any number of queries. To illustrate the difference, I set up a small example:

CREATE TABLE table_to_be_updated (
    id serial PRIMARY KEY,
    other_column text,
    column_changing text
);

INSERT INTO table_to_be_updated (other_column, column_changing)
VALUES
('value', 'old_value'),
('value', 'other_value'),
('nonvalue', 'doesnt matter');

Then run two transactions concurrently (issuing the commands one by one, the middle line wants to depict the timeline):

                                | <-- BEGIN;
                                |
                                |
                                |     UPDATE table_to_be_updated
BEGIN; -----------------------> |     SET column_changing = 'new_value'
                                |     WHERE
                                |         other_column = 'value' AND
                                |         column_changing = 'old_value';
                                |
                                |
SELECT column_changing -------> |     -- update not yet committed
FROM table_to_be_updated        |
WHERE other_column = 'value';   | <-- COMMIT;
                                |
                                |
SELECT column_changing -------> |
FROM table_to_be_updated        |
WHERE other_column = 'value';   |
                                |
                                |
COMMIT; ----------------------> |

Running these in READ COMMITTED isolation level, the first query returns a row with 'old_value', while the second one shows a row with 'new_value'. On an other run, I change the left-hand-side transaction isolation level:

SET transaction ISOLATION LEVEL REPEATABLE READ;

(The command must be the first statement in a transaction.) Now both SELECTs return the same rowset, while a third one after committing both transactions will show the new row.

Sql-server – How do these snapshot isolation level configurations interact on a SQL Server 2005 instance

As @AlexKuznetsov noted, SNAPSHOT and READ_COMMITTED_SNAPSHOT are two different isolation levels. As such, snapshot_isolation_state indicates the former, while is_read_committed_snapshot_on the latter. Here are a few comments that I found that summarize the differences between the two:

READ COMMITTED SNAPSHOT does optimistic reads and pessimistic writes. In contrast, SNAPSHOT does optimistic reads and optimistic writes.

~Bill Paetzke here

and ...

[READ_COMMITTED_SNAPSHOT] differs from the SNAPSHOT isolation level in that instead of providing a reader with the last committed version of the row that was available when the transaction started (SNAPSHOT ISOLATION), a reader gets the last committed version of the row that was available when the statement started (READ_COMMITTED_SNAPSHOT).

~Uri Dimant (emphasis mine), here

Best Answer

Related Solutions

Postgresql – Read Committed Isolation Level

Sql-server – How do these snapshot isolation level configurations interact on a SQL Server 2005 instance

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