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

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2
votes
1answer
23 views

Do MongoDB's “read uncommit” and “write lock” conflict with each other?

I was reading the MongoDB documentation, where it says about read uncommit: MongoDB allows clients to read documents inserted or modified before it commits these modifications to disk, regardless of ...
0
votes
1answer
43 views

Oracle Enterprise Manager - top activity

I'm looking at the Top Activity view in OEM. The left axis is labeled "Active Sessions" (up to 24 in the graph I'm looking at) and there is a line labeled "Maximum CPU". The measured activities ...
0
votes
1answer
12 views

Running identical queries in sysbench

When benchmarking using sysbench and utilizing concurrency (multiple threads) for example on a table with 1,000,000 records, does sysbench run "identical" queries in a loop on the table?
0
votes
0answers
36 views

How can I obtain concurrent deletes for this table?

We have a program that is currently doing something to the effect of: SET TRANSACTION ISOLATION LEVEL READ COMMITTED BEGIN TRANSACTION UPDATE [Source].[dbo].[SourceTable] SET [ProcessedOn] = ...
1
vote
2answers
66 views

ACID violation in Postgres 9.1?

I am using a postgres DB to implement a scheduling of jobs for a large number of computers/processes. To make the story short, every job has its id, all scheduling is implemented with three tabes: all ...
-2
votes
1answer
35 views

Oracle Concurrent Insertion

We have simple case like Create table t1 (c1 number primary); session1: insert into t1 values (1); //session1 do not commit transaction session2: insert into t1 values (2); Will Session2 ...
2
votes
1answer
228 views

Why is CTE open to lost updates?

I don't understand what Craig Ringer meant when he commented: This solution is subject to lost updates if the inserting transaction rolls back; there's no check to enforce that the UPDATE affected ...
2
votes
2answers
119 views

Is SQL Server Thread Safe?

If I need to update same row with two concurrent thread then is it possible in SQL Server? Suppose I have one table that contains Id and Name columns and now I want to update the third number Id with ...
0
votes
1answer
26 views

Is reading a single record always an atomic operation?

Is reading a single record always an atomic operation? I mean, when e.g. a read (select) and write (update) query are simultaneously carried out on the same record, is it possible for the read ...
1
vote
1answer
39 views

Is it possible that records are changed while a read query is still executing?

Suppose a read (select) query R is reading some 100 records from a table, 50 records are read so far, could in the meanwhile a write query W change some of the unread records? We didn't take any ...
1
vote
1answer
97 views

MySQL consistent nonlocking reads vs. INSERT … SELECT

So, from my reading of the MySQL 5.5 documentation and some experiments I did with colleagues, I understand that the following is the case: With InnoDB tables, a simple SELECT statement (one that ...
2
votes
1answer
99 views

Does READ UNCOMMITTED have the same effect as AUTOCOMMIT=1?

AFAIK running transactions as READ UNCOMMITTED means that there is no lock, so every change caused by a transaction is immediately visible to other transactions. Is it correct to assume that this is ...
1
vote
1answer
91 views

Improve performance on concurrent UPDATEs for a timestamp column in Postgres

I am using a timestamp column called updated_at for cache invalidation. More information on this particular technique here. The queries all have the same format UPDATE "managers" SET "updated_at" = ...
0
votes
1answer
74 views

PostgreSQL - If I run multiple queries concurrently, under what circumstances would I see a speedup? Under what circumstances would I see a slowdown?

I approach you all humbly as one who is NOT a DBA, and I'm sure that my question is fraught with conceptual shortcomings and "it depends on" land mines. I'm also pretty sure that all of you who ...
2
votes
2answers
619 views

Way to do MERGE with update source when target not match?

I am trying to write a stored procedure that will take new values from a temp table, merge them into an actual table, giving me the ids of the inserted rows, and the rows that already exist matching ...
3
votes
1answer
725 views

Postgres Update, limit 1

I have a postgres database which contains details on clusters of servers, such as server status (active, standby, etc). Active servers at any time may need to fail over to a standby, and I don't care ...
1
vote
0answers
143 views

High concurrency is even slower than low concurrency in mysql? [closed]

My application is supposed to handle high concurrency insert/update, now I have only one mysql instance setup for test. My application is full asynchronous, it should be able to handle 10,000 ...
2
votes
2answers
87 views

Tuning Postgres for Single connection use? Or is postgres wrong tool?

Any rules of thumb for work_mem, maintenance_work_mem, shared_buffer, etc. for a database that DOESN'T anticipate concurrent connections and that is doing lots of aggregate functions? I'm a social ...
0
votes
0answers
48 views

Why does the delete/update block each other in this case?

The following drives me crazy. I have: session1>begin; session2>begin; session2>update dummy set salary=1000 where id=11; session1>delete from dummy where id=1; id is the PK ...
3
votes
1answer
103 views

Can't update trigger because of activity

Trying to add a line to a trigger that is attached to a table that is constantly in use ALTER TRIGGER [dbo].[name_of_trigger] ON [dbo].[name_of_table] AFTER UPDATE BEGIN IF ORIGINAL_LOGIN() in ...
0
votes
2answers
667 views

How to maintain MySQL performance at high concurrency

The following has been observed using "mysqlslap". mysqlslap -u root -p --create-schema=matrix --query=monthly_kpi.sql --iterations=1 --concurrency=1 --delimiter=";" Benchmark Average number ...
1
vote
1answer
77 views

How does the transactions interact if you set a different isolation level per connection?

I read that I can have different isolation levels per connection and in the server. The default isolation level is REPEATABLE-READ. So if I have a transaction that issues a SERIALIZED to the client ...
1
vote
2answers
84 views

Transaction basics. What is the result of 2 transactions running concurrently?

I have a rather basic question on transactions. Assume a table Account with a column amount. Now we have the following code: BEGIN $amount = SELECT amount FROM ACCOUNT WHERE id = 123; $amount = ...
0
votes
0answers
103 views

Handling concurrent updates, inserting if and only if no record already exists

I'm writing a Java application to update a relational database (currently H2, but we'll likely switch to PostgreSQL at some point). I have a process which does the following: Poll an external system ...
1
vote
1answer
227 views

DB Locks - How would one handle long transactions and maintain high concurrency?

We're maintaining a POS/WMS Application built in powerbuilder that runs on SQL Server 2008 R2 backend. Recently, we've been having a lot of locking issues and I believe it has to do with how we handle ...
0
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0answers
335 views

MySQL Innodb Concurrent Inserts and Selects - can't use multiple threads on FreeBSD

We have an application that needs to be dumping rows into some tables at a much faster rate than it currently is. It is a dual processor xeon DB server with 48gb of ram and a 4 TB disk RAID 10 (for a ...
4
votes
1answer
760 views

How is SQL Server returning both a new value and old value during an UPDATE?

We've had issues, during high concurrency, of queries returning non-sensical results - results the violate the logic of the queries being issued. It took a while to reproduce the issue. I've managed ...
0
votes
1answer
131 views

What causes lost updates being observed in my SQL Azure database?

I have a SQL Azure database with the following tables: CREATE TABLE [dbo].[UserBalances] ( [UserId] UNIQUEIDENTIFIER UNIQUE NOT NULL DEFAULT NEWID(), [AvailableMoney] INT NOT NULL ...
1
vote
1answer
78 views

How, when and why does implicit changing of transaction isolation level happen?

RCSI (Read Committed Snapshot Isolation) is the setting of a database changing the behavior of (the default) Read Committed isolation level in SQL Server. This is, as I understand, the context of ...
0
votes
0answers
78 views

MySQL + PHP : avoid dirty read

It's possible to avoid dirty read on a MySql database by using a PHP script? i have three two fields (user, incoming_request , outcoming_request) on my table that keep information (as string) about ...
1
vote
1answer
53 views

Patterns/Designs to help with concurrency/performance [closed]

I'm working on a SQL Server 2008 R2 DB and I'm experiencing some performance/concurrency issues with probably the main table in the DB. Problems like slow reads, deadlocks, poor performance in ...
0
votes
1answer
879 views

Why are timestamps not always increasing with concurrent inserts?

I'm seeing some unexpected behavior with timestamp (rowversion) columns . I created a test table: create table Test ( Test_Key int identity(1,1) primary key clustered, Test_Value int, ...
0
votes
0answers
53 views

MySQL Multiple threads inserting *SAME* record a Bad Thing?

Okay, as I've noted in other questions, I'm not a sophisticated database user even though I took my first course in relational algebra from ted Codd. I've now inherited an application that ...
0
votes
1answer
135 views

When and how would MyISAM concurrent insert = always be a problem?

I wish to set concurrent_insert to always on a MyISAM log table, so that inserts wouldn't have to wait for selects to perform when there are some holes in the table. We're talking about 10-20 inserts ...
1
vote
1answer
178 views

How to insert and or update a variable number of records and prevent concurrency issues

I currently have the following stored procedure. However I am seeing some race conditions where I end up with incorrect data which I think is caused because the procedure isn't attomic and it can be ...
1
vote
0answers
138 views

UPLOCK to avoid multiple reads

Given the following table structure... Orders ====== OrderId int PK OrderEvent =========== OrderEventId int PK OrderId int FK EventTypeId int FK I've got multiple concurrent processes SELECT-ing ...
2
votes
1answer
1k views

oracle blocking sessions

I am trying to investigate some concurrency issues on Oracle 11 RAC. So far the most helpful tool I have is "blocking sessions" view of the Enterprise Manager, which tells me which query is blocking ...
0
votes
1answer
1k views

Putting a Select statement in a transaction

What is the difference between these 2 queries: start transaction; select * From orders Where id=1; UPDATE orders SET username="John" Where id=1; commit; And without transaction: select * From ...
0
votes
1answer
233 views

How to create fault tolerant databases? [closed]

I have a local application running on a LAN. Sometimes a database gets shutdown. In such a scenario how can I keep the application working? Can I have two concurrent databases so that one picks up the ...
5
votes
2answers
145 views

Inconsistency in repeatable read

http://www.postgresql.org/docs/9.2/static/transaction-iso.html The Repeatable Read mode provides a rigorous guarantee that each transaction sees a completely stable view of the database. However, ...
6
votes
1answer
977 views

LATCH_EX Waits on Resource METADATA_SEQUENCE_GENERATOR

We have a process that generates an inventory report. On the client side, the process splits of a configurable number of worker threads to build a chunk of data for the report that corresponds to one ...
3
votes
1answer
1k views

PostgreSQL concurrent increment of counter

I need to maintain a statistical table for a project, composed by a list of items and their usage (Think about something like a website in which you would like to count pageviews). Each time an item ...
3
votes
1answer
153 views

Read-only on an SQLite database which is being modified by another program

I wrote a program that has an SQLite database, modifying it every few seconds. Now I want to write a small gadget program that would just read a bit of info from that database (read-only) and display ...
2
votes
1answer
716 views

unique constraint violated (ORA-00001) in concurrent insert [closed]

I have a procedure that is called from concurrent transactions: //Some actions here INSERT INTO table1 (table1_id, table1_val1, table1_val2, table1_val3) VALUES ...
2
votes
0answers
80 views

How to safely take an SQL job out of the schedule?

There is a table | jobID | status | startTime | endTime | and a job, that is scheduled to run every X seconds, it will take the first undone job ID, put it in stored proc and run it. Stored proc takes ...
5
votes
1answer
954 views

How to improve performance on PostgreSQL when using multiple concurrent processes?

Seeing query performance which is not satisfactory on our Python application, which runs several processes that use SQLAlchemy core to access a PostgreSQL 9.2 database. We may have around 100 - 200 ...
-1
votes
4answers
240 views

Importing data with a lot of SELECTS during transaction

I'm using SQL server 2008 R2 and I have a products table with an auto increment integer id column as primary key and a product_no column (unique) and 6 tables like articles and product_assets which ...
3
votes
1answer
388 views

Isolation and atomiticy of SQL Server UPDATE

Is there any difference in atomicity and isolation between the two following SQL statements? Batch A: UPDATE Orders SET InProgress = 1 WHERE Completed = 0 and Batch B: UPDATE Orders SET ...
1
vote
1answer
1k views

Show processlist / Max concurrent connections seem to max out at 131

When I put my database under a load test (basically just a basic write and basic read from our app), and run show processlist; in the middle of it, there is always 131 rows in the set and that's it. ...
0
votes
2answers
431 views

MySQL concurrent INSERTs

I have a MySQL database with InnoDB tables. There are different client processes making SELECT (to check the existence of a value) and INSERT or UPDATE (depending on the result of the select) ...