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

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1answer
192 views

timestamp based concurrency

I am wondering if someone could help me out with a question I have about time stamp based concurrency. I found the following practice problem on a website It basically asks which of the following ...
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1answer
645 views

Which Optimistic Concurrency Control method will work out best in the long run?

These are the options I've thought of: DotNet Dave: UPDATE mytable set f1=newval where id=666 and f1 := f1ValueFromSelect Charlie Checksum: UPDATE mytable set f1=newval where id=666 and ...
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2answers
28 views

Does InnoDB allow adding columns to a table with concurrent READ/WRITE of rows?

I have a large (3M rows) InnoDB table in MySQL 5.6.21 in production and I want to add some columns. From the MySQL Documentation on Overview of Online DDL I understand that, even if this is an ...
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1answer
417 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 ...
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1answer
96 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 ...
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1answer
3k 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 ...
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1answer
2k 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. ...
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1answer
2k views

Differences between SQL Server 2005 and SQL Server 2008 deadlock handling

We recently upgraded a server to SQL Server 2008 R2. The server had been running SQL Server 2005. We are using Transactional Replication Management Data Warehouse and Data Collector on the ...
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0answers
82 views

How to identify concurrency issues in Oracle 11g?

For a short period in time we had a concurrency issue that lead to subsequent faults in our application (Java, Spring, Hibernate). Since we are using dictionary managed tablespaces, I first thought ...
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1answer
97 views

Work queue with complex select and long processing: Ensuring concurrency

I have a queue table from which worker processes claim records, one at a time. It is crucial that no two workers claim same item to process. Normally this kind of thing is done with select top (1) ...
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0answers
197 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 ...
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1answer
57 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 ...
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0answers
183 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 ...
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1answer
1k 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, ...
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1answer
31 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 ...
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2answers
1k 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 ...
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1answer
324 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 ...
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2answers
606 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) ...
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1answer
395 views

How could I handle heavy load and consistency for product inventory in online retailers? [closed]

I have posted that question on Stackoverflow and they suggest me to repost my question here, as it would more likely to get help from a DB community (regarding this issue). Hope you guys don't mind. ...
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1answer
27 views

How to serialize on SQL server metadata?

I tried to find an answer in Kalen Delaney's SQL Server Concurrency but there is no mention of metadata. The problem is how to ensure that only one of these reads and index creates executes from one ...
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1answer
56 views

How to avoid duplications using a function

I am using this function in a bulk insert to avoid duplication of url paths. CREATE OR REPLACE FUNCTION "univ"."gc_landing"(IN _name text, OUT landing_id int4) RETURNS "int4" AS $BODY$ DECLARE ...
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1answer
367 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
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1answer
267 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 ...
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1answer
205 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 ...
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1answer
191 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 ...
0
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1answer
338 views

Moving a collection to a new database (v2.2)

To take advantage of the improved concurrency in Mongodb 2.2--namely database level locking--we would like to move a sharded collection to a new database. I have been unable to find any documentation ...
0
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1answer
203 views

Why we use low numbers for concurrent in MySQL benchmarks or HTTP_LOAD? [closed]

I've seen so many times that we use numbers like 5,6 or 10 at most. What is this concurrency? Doesn't it refer to number of simultaneous users in bench? EDIT I'm not talking about any specific ...
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0answers
23 views

How to execute two transactions concurrently in Postgresql?

I want to execute two transactions concurrently in PGAdmin: --The first transaction should modify the age of Paul BEGIN; DECLARE newage INTEGER; --the original age of Paul is 32 newage := Select age ...
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0answers
15 views

Can a slow SELECT statement in READ COMMITTED isolation level read content from concurrent committed UPDATE?

Let's say we have a big table with hundreds of million of rows (or more). On one connection, we perform a query requiring full table scan under READ COMMITTED isolation level: SELECT * FROM ...
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1answer
27 views

InnoDB index concurrent inserts

I have an InnoDB table with an index on it. I have a few million rows to insert, so I thought I might take advantage of the row-level locking and do it in parallel. I'm inserting in batches of 10000, ...
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0answers
57 views

Rowversion column in every table - Bad practice?

I'm currently providing my application with concurrency checks. For that, I'm using Entity Framework combined with SQL Server's RowVersion / Timestamp column. Now I've thought about adding a ...
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1answer
127 views

Concurrent SELECT and UPDATE in multiprocessing python

I have to randomly match about 20 million rows in two tables without using the same row twice using PostgreSQL 9.3.5. Both tables have the following structure CREATE TABLE de_sim_points_end ( -- ...
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1answer
31 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?
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0answers
55 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] = ...
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0answers
54 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 ...
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0answers
126 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 ...
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0answers
64 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 ...
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0answers
46 views

Oracle User Profile for REST API

What sort of profile would you recommend for an Oracle user that is manifested through a RESTful API; particularly where there will be a lot of asynchronous requests? So we're talking about a ...
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4answers
294 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 ...
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1answer
42 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 ...