Questions tagged [concurrency]

For questions about issues arising when multiple users or processes concurrently access or modify shared information in a database system.

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23 views

Measuring how long PostgreSQL waits for locks

I want to figure out how long PostgreSQL spends waiting for locks during a period of time (during this period of time, many requests are served by PostgreSQL). What I know PostgreSQL system table ...
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90 views

SQL statements to ensure that the same location cannot be reserved more than once on the same day?

I have two tables one for venues (3 locations hall1, hall2 and hall3) and one for orders (a row could be 441,2004,50895,'Requested','2021-01-02 10:53','2021-03-06 11:46',7,3) as follow: CREATE TABLE ...
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2answers
236 views

Transaction strategy for UNIQUE constraint?

How to use a transaction-based strategy to ensure that the same location cannot be booked more than once on the same day? It was suggested to me that it will different for every isolation level. Could ...
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1answer
23 views

Pessimistic locking and isolation levels

How is pessimistic locking with read and write locks related to isolation levels? Readers block writers and writers block readers. This behaviour is always constant irrespective of isolation level ...
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1answer
21 views

Is this atomic set and get in MariaDB / MySQL: `UPDATE t SET col = @my_var := col + 1 WHERE id = 123`?

I want to atomically update a single row and get its value. I discovered today that MariaDB / MySQL has this additional way of setting user variables using the assignment operator :=, described e.g. ...
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1answer
63 views

Is MySQL more scalable than PostgreSQL due to the difference in how they handle connections?

I'm trying to decide if either MySQL or PostgreSQL would be more suitable for an application that will get hit by potentially thousands of simultaneous requests at a time. During research, one fact ...
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11 views

How to avoid duplicate in mysql INNODB transaction for non index key?

I have a INNODB table with txnno column without unique index but I have primary key on another column. Now I am inserting transaction from multiple places. And there are some concurrent insertion ...
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1answer
216 views

Missing rows under read committed isolation

It's well known that Read Committed isolation is prone to different anomalies. I read The Great Paul White's series on isolation levels. The post relevant to the discussion is this one: The Read ...
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1answer
82 views

SQL Server 2019 running 100% on only 1-2 cores with MAXDOP 8 to 14

Here is the background on this issue. I am inserting slightly over 1 million records into a table from within a C++ program using ADO. One row at a time is read in from flat file and converted info a ...
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1answer
29 views

Can Postgres timestamp be used to implement client side caching?

If I have a pool of connections in a multi-threaded client sending out requests to the database server, can I rely on transaction or statement level timestamps to determine which data is most current? ...
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1answer
122 views

Can UPDATE lead to deadlocks in SQL SERVER?

Can we experience deadlock if run the same update query concurrently? Let's assume there's a table with millions of records and we need to update several thousand records at one shot. UPDATE TABLEX ...
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1answer
22 views

Concurrent Update of Array (Postgres)

I have an array that I can update from multiple workers as follows: UPDATE my_table SET arr = array_append(my_table.arr, element). The problem pops up when two workers try to update the array at the ...
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53 views

Can't refresh Materialized View Concurrently

I'm trying to run REFRESH MATERIALIZED view CONCURRENTLY my_view in Postgresql, and I'm getting the following error: SQL Error [XX000]: ERROR: unexpected attlen: -2 Where: SQL statement "SELECT ...
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The overhead of OCC validation phase

The validation phase of optimistic concurrency control has two derctions: one is backward validation, checking for conflicts with any previously validated transaction, the other is forward validation, ...
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3answers
45 views

How to improve the liveness of a system when having a long-running transaction blocking multiple reading transactions

Is there any recommended approach for improving the application liveness when having, over the same database object, multiple reading transactions, and one long-running transaction which is updating ...
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3answers
292 views

What is the difference between concurrency control in operating systems and in trasactional databases?

Recently I'm learning concurrency control techniques in transactional databases. However, I am so confused about the differences between concurrency control in operating systems and in transactional ...
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2answers
1k views

Safe to use WITH (NOLOCK) in this case?

Scenario I have a table that deals with concurrent SELECT's and DELETE's. I'm getting a few deadlocks on my SELECT statements. I assume that the DELETE from the other transaction is getting an ...
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1answer
26 views

Why use an intent lock at a higher level in the hierarchy, why not just a regular lock?

This article suggests intent locks are used at a higher level of the lock hierarchy to indicate that a lock has been taken on a lower level. The idea of this mechanism is that SQL Server doesn't need ...
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2answers
94 views

Dirty read or phantom row

We are having dirty reads or phantom rows in our data warehouse. My question is not how to solve it (it is a long story and I am currently working on it), but I want to know what exactly happens to ...
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1answer
33 views

What is the proper way to validate that a many-to-many relationship will not surpass a certain limit?

In a Postgres DB, I have a relationship as follows: A Group table has a column userEntranceLimits. A UserEntrance table has a column userId and a column groupId. The count of a UserEntrance's ...
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1answer
34 views

Enforcing maximum number of relations for a record

I am trying to figure out the way to enforce a maximum number of relations in a highly concurrent system. The following is an example use case. // Job table job_id | ... | num_positions xx-xx-xx | ....
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1answer
182 views

Does SQL Server Support Parallel Bulk Inserts From Multiple Connections?

Does SQL Server support parallel bulk inserts into the same database table? Some clarifications for my particular use case: The table has a clustered index composed of two distinct index keys, and ...
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1answer
913 views

What is “special” about PostgreSQL update vs delete+insert

My understanding is that an update locks a tuple, marks it as deleted, and then adds a new tuple. In other words, update = delete + insert. Or so I had believe. But it appears there something ...
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19 views

Query doesn't return data that's written recently

I have two micro services where one write to DB and another read from DB. The first micro service is written in C++ while the other is written in NodeJS. When querying for data say for a IP the ...
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21 views

Problem in model to save score

I am building an application in which users will have a score in which they can receive it (+10 XP) when they do certain activities, but they can also lose it when using that score to make "...
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0answers
61 views

Order of foreign keys/cascades processing in SQL Server

I have a parent table and 2 (or more) children tables with foreign key defined with some cascade option. So when an entry from a parent table is being deleted, SQL Server will check 2 (or more) ...
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1answer
157 views

Postgres CREATE OR REPLACE VIEW… can it block?

I'm looking for a way for a table to be swapped while it's being used. Say we have a view CREATE OR REPLACE VIEW my_data AS SELECT * FROM my_data_v1; that is being used, e.g. lots of SELECT * FROM ...
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1answer
213 views

Prevent unique constraint violations caused by concurrent inserts

I have a situation where I need to insert a unique record into the database: SELECT user FROM users WHERE id = 'some_randomly_generated_id' If user does not exist, INSERT INTO users WHERE id = '...
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1answer
214 views

How to ensure no duplicate records are added when using multiple Node JS processes

We are working on an appointment scheduler. We have a table Appointment that stores an appointment with starttime, endtime, assigneeId and eventId. We are using Bookshelf.js as an ORM with Node JS. ...
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1answer
316 views

How to determine right value for innodb_sync_array_size

How do I determine the right value for innodb_sync_array_size in MySQL? We have version 5.7 deployed on AWS RDS with 32 vCPUs, and it sees about 26 concurrent threads on average. When I measure waits ...
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2answers
554 views

UPDATE statement behavior

I have a question regarding inner workings of UPDATE statements with regards to SQL Server. I am trying to understand what will happen if the following 2 update statements are received or serviced at ...
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1answer
782 views

How to prevent implicit casting from bytea to text during the current connection in postgres?

In Postgres bytea values are automatically converted to text when inserted to text/varchar columns, based on the bytea_output setting. I am working with some program code which automatically converts ...
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2answers
231 views

PostgreSQL sometimes fails to detect serialization failures

I have an http-server in application language that looks roughly like this: item = SELECT item FROM table WHERE field = 'value' if (item) { UPDATE another field on item } else { INSERT item VALUE ...
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2answers
50 views

PostgreSQL concurrency control and Serializable transactions

Im building an application that needs to do some concurrent read/write operations on specific tables. Even after reading the official documentation (and many, many articles), the trouble persists, no ...
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1answer
238 views

Are SqlServer Sequences, guaranteed to return different values for concurrent transactions?

In documentation it is told that: Duplicate validation only occurs once a record is fully populated. This can result in some cases where the same number is used for more than one record during ...
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1answer
75 views

How to improve performance for read only innodb MySQL Database

I have a particular task which is to maximize the concurrent performance. There is only one particular type of query, which is select * from table where col1 between ? and ? and col2 between ? and ? ...
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2answers
336 views

Is there a memory overhead using partitions in postgres vs. a single table?

Background I am ingesting time series data totalling to about 10M rows per day, where each row has a timestamp, a player id and some other columns. That data is then queried via an API for a ...
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1answer
53 views

Why aren't read-only queries executing concurrently? [closed]

I have a Django app using either sqlite or Postgres. (I've tried both and encountered the same problem.) Requesting a particular endpoint results in a read-only query like this that takes 1.5-3 ...
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1answer
36 views

Prevent failure in conditional insert in mysql database

The infrastructure of our system looks like this. An AWS lambda function receives requests such as (accountId, .....). It creates an entry in the MySQL database using a newly generated UUID as caseId....
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0answers
223 views

Using a Table as a Queue without sp_getapplock/sp_releaseapplock

I have a list of commands I need to execute, all of which are contained within a table I've named myQueue. This table is a little unique in that some commands should be grouped together such that ...
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0answers
32 views

What is correct way to ensure that row is not deleted in concurrent transaction

I have 2 tables: Parent (PK id) Child (PK id, parent_id) Note that parent_id is not defined a foreign key (actually they are not parent/child, I use those names just for example). I need to handle ...
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0answers
50 views

Repro for missed rows during Allocation Order Scan + Page Splits

In Read Uncommitted, an index scan can execute as an allocation order scan. If a page split occurs during the scan, it is possible that a row will be read twice if it moves to a later position in the ...
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1answer
654 views

What standard SQL isolation levels do the SQLite isolation levels correspond to?

There are four standard SQL isolation levels: Read Uncommitted, Read Committed, Repeated Read, and Serializable. (c.f. Database System Concepts 6ed.) In SQLite, there are three isolation levels: ...
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53 views

Do concurrent in-memory databases exist?

I spent days searching the internet but all the databases I found are at most in-memory but not concurrent. By concurrency I mean that they run on multiple threads and can do multiple reads ...
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1answer
113 views

Is SELECT FOR … UPDATE *necessary* to DELETE or UPDATE multiple rows without deadlock?

Consider the following when the transaction isolation level is Read Committed (the default): UPDATE example SET foo = 'bar' WHERE ... and DELETE FROM example WHERE ... Neither DELETE nor UPDATE ...
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2answers
2k views

Why am I getting a deadlock for a single UPDATE query?

I have two processes which execute code like this in parallel: begin; update foos set unread=false where owner_id=123 and unread=true; commit; This results in deadlocks. My understanding of what ...
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1answer
463 views

Update query race condition

Consider this simple query UPDATE [SomeTable] SET Value = Value + 1 WHERE Id = 1 For almost any programming language this has a potential risk of race condition if it is executed by more than one ...
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2answers
2k views

Will my SQL query use stale data? How can I prevent it?

I have two tables (SJob & SJobDependent) that I need to join for some logic in a stored procedure. They both have a column (job) that connects them in a one-to-many relationship - one SJob record ...
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42 views

How does MySql database handles multiple update operation

I am new to database and the way they work so my question might be a bit dumb, please be nice. I have always been thinking about how database update work in general and I think I understand the ...
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1answer
54 views

Postgres Search Path between connections

I'm a noob to Postgres, and am setting up a multi-tenant database using one schema per tenant. To target each tenant I need to set search_path accordingly. I'm worried about accidentally executing ...

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