Concurrency and Locks

Locks are used in SQream DB to provide consistency when there are multiple concurrent transactions updating the database.

Read only transactions are never blocked, and never block anything. Even if you drop a database while concurrently running a query on it, both will succeed correctly (as long as the query starts running before the drop database commits).

Locking Modes

SQream DB has two kinds of locks:

exclusive - this lock mode prevents the resource from being modified by other statements

This lock tells other statements that they’ll have to wait in order to change an object.

DDL operations are always exclusive. They block other DDL operations, and update DML operations (insert and delete).
inclusive - For insert operations, an inclusive lock is obtained on a specific object. This prevents other statements from obtaining an exclusive lock on the object.

This lock allows other statements to insert or delete data from a table, but they’ll have to wait in order to run DDL.

When are Locks Obtained?

Operation	SELECT	INSERT	DELETE, TRUNCATE	DDL
SELECT	Concurrent	Concurrent	Concurrent	Concurrent
INSERT	Concurrent	Concurrent	Concurrent	Wait
DELETE, TRUNCATE	Concurrent	Concurrent	Wait	Wait
DDL	Concurrent	Wait	Wait	Wait

Statements that wait will exit with an error if they hit the lock timeout. The default timeout is 3 seconds, see statementLockTimeout.

Monitoring Locks

Monitoring locks across the cluster can be useful when transaction contention takes place, and statements appear “stuck” while waiting for a previous statement to release locks.

The utility SHOW_LOCKS can be used to see the active locks.

In this example, we create a table based on results (CREATE TABLE AS), but we are also effectively dropping the previous table (by using OR REPLACE which also drops the table). Thus, SQream DB applies locks during the table creation process to prevent the table from being altered during it’s creation.

t=> SELECT SHOW_LOCKS();
statement_id | statement_string                                                                                | username | server       | port | locked_object                   | lockmode  | statement_start_time | lock_start_time
-------------+-------------------------------------------------------------------------------------------------+----------+--------------+------+---------------------------------+-----------+----------------------+--------------------
287          | CREATE OR REPLACE TABLE nba2 AS SELECT "Name" FROM nba WHERE REGEXP_COUNT("Name", '( )+', 8)>1; | sqream   | 192.168.1.91 | 5000 | database$t                      | Inclusive | 2019-12-26 00:03:30  | 2019-12-26 00:03:30
287          | CREATE OR REPLACE TABLE nba2 AS SELECT "Name" FROM nba WHERE REGEXP_COUNT("Name", '( )+', 8)>1; | sqream   | 192.168.1.91 | 5000 | globalpermission$               | Exclusive | 2019-12-26 00:03:30  | 2019-12-26 00:03:30
287          | CREATE OR REPLACE TABLE nba2 AS SELECT "Name" FROM nba WHERE REGEXP_COUNT("Name", '( )+', 8)>1; | sqream   | 192.168.1.91 | 5000 | schema$t$public                 | Inclusive | 2019-12-26 00:03:30  | 2019-12-26 00:03:30
287          | CREATE OR REPLACE TABLE nba2 AS SELECT "Name" FROM nba WHERE REGEXP_COUNT("Name", '( )+', 8)>1; | sqream   | 192.168.1.91 | 5000 | table$t$public$nba2$Insert      | Exclusive | 2019-12-26 00:03:30  | 2019-12-26 00:03:30
287          | CREATE OR REPLACE TABLE nba2 AS SELECT "Name" FROM nba WHERE REGEXP_COUNT("Name", '( )+', 8)>1; | sqream   | 192.168.1.91 | 5000 | table$t$public$nba2$Update      | Exclusive | 2019-12-26 00:03:30  | 2019-12-26 00:03:30

For more information on troubleshooting lock related issues, see Lock Related Issues.