Postgres Npgsql.PostgresException deadlock detected - postgresql

Question

We are developing a group chat application and using PostgreSQL to store chat messages.
CREATE TABLE public.chatmessage
(
chatmessageid uuid NOT NULL DEFAULT uuid_generate_v4(),
text character varying COLLATE pg_catalog."default",
planid uuid NOT NULL,
userid uuid NOT NULL,
createdat timestamp with time zone NOT NULL DEFAULT timezone('utc'::text, now()),
updatedat timestamp with time zone,
deleted boolean NOT NULL DEFAULT false,
viewedallstatus boolean,
vieweduserids uuid[],
alloweduserids uuid[],
CONSTRAINT chatmessage_pkey PRIMARY KEY (chatmessageid)
)
To manage read status, we are storing the userIds of viewed participants in vieweduserIds column.
While heavily using the group chat with 5 or more participants, we are getting the following exception,
Npgsql.PostgresException (0x80004005): 40P01: deadlock detected
at Npgsql.NpgsqlConnector.<DoReadMessage>d__157.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
at System.Runtime.CompilerServices.ValueTaskAwaiter`1.GetResult()
at Npgsql.NpgsqlConnector.<ReadMessage>d__156.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at Npgsql.NpgsqlConnector.<ReadMessage>d__156.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
at System.Runtime.CompilerServices.ValueTaskAwaiter`1.GetResult()
at Npgsql.NpgsqlDataReader.<NextResult>d__32.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at Npgsql.NpgsqlDataReader.<<NextResultAsync>b__31_0>d.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at Npgsql.NpgsqlCommand.<Execute>d__71.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
at System.Runtime.CompilerServices.ValueTaskAwaiter`1.GetResult()
at Npgsql.NpgsqlCommand.<ExecuteNonQuery>d__84.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at Npgsql.NpgsqlCommand.<>c__DisplayClass83_0.<<ExecuteNonQueryAsync>b__0>d.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at Dapper.SqlMapper.<ExecuteImplAsync>d__37.MoveNext() in C:\projects\dapper\Dapper\SqlMapper.Async.cs:line 646
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
I feel since the application simultaneously updates the same column this issue is coming. The query which updates vieweduserIds column is as follows,
"Update ChatMessage Set ViewedUserIds = ViewedUserIds || #UserId Where PlanId = #PlanId And #UserId = Any(AllowedUserIds) And Not (#UserId = Any(ViewedUserIds)) And Deleted = False;"
How can we resolve this issue?

Answer 1

A deadlock is no problem unless it happens frequently.
The correct solution is for the application not to panic, but simply to retry the database transaction.
If deadlocks happen too frequently, you need to investigate and remedy the cause. Usually that means to keep your transactions short and small and to always lock objects in a certain order.
If you need specific help, read the PostgreSQL log file to find out which queries are involved. You'll have to understand which locks conflict to determine the root cause.