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
1k 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
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1answer
2k views

performance issues when concurrent requests

We are using SQL Server 2008 R2 on a Windows Server 2008 server. We have some main functions which uses some Stored Procedures which has a lot of sub-queries inside of it. I'll take one SP as an ...
2
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0answers
207 views

SSIS: FTP task - concurrent connections

I have created an SSIS package for downloading file from FTP server. When I debug the package, everything goes fine. But when I shedule the package as Job step, it sometimes fails to connect. The job ...
2
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1answer
223 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 ...
2
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1answer
965 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
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0answers
89 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 ...
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2answers
3k views

Return the id after insert or select

I want to build a function which will insert an email if the email value doesn't exist in the table and return the email_id of the row. How can I do this? Also how can I return the id if the email was ...
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2answers
310 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 ...
1
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1answer
50 views

Updlock hint in stored procedure

If someone runs stored procedure below very fast (few thousand execution a second), can variable @amount change while executing and execute update more than one time? CREATE PROCEDURE ...
1
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1answer
81 views

PostgreSQL optimistic reads (equivalent to SQL Server's Read Committed Snapshot)

Does PostgreSQL provide a built-in optimistic concurrency control mechanism such as SQL Server's Read Committed Snapshot Isolation? Readers can read all the rows without blocking, but writes will ...
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1answer
225 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" = ...
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2answers
503 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 ...
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1answer
130 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 ...
1
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1answer
39 views

PostgreSQL visibility of records changes from other transaction inside the current one

I have a table "tst" which has a "status" column using which we process records in that table. Once the records are processed we set the status to 'Y'; #\d tst Table "core.tst" ...
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1answer
2k views

How to view full blocking postgres queries?

I'm using a statement from this page to view blocking queries in Postgres SELECT bl.pid AS blocked_pid, a.usename AS blocked_user, ka.query ...
1
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1answer
333 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
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1answer
131 views

Can a Database request automatically remove itself from the queue after a delay?

I'm using table locking in my Oracle database. I have a small program with a function that locks my table, does some things and unlocks it again. If many instances are run and requests are made ...
1
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1answer
23 views

Using @@ROWCOUNT in multi-operation batch to check either a failure or a success of the whole transaction

I have the following script defined with hopefully explanations in comments to explain rationale in the right context. The questions are basically: Is the following usage of @@ROWCOUNT in ...
1
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2answers
59 views

SQL Server Concurrency Issue [closed]

We have a SQL server concurrency issue we are facing and would like some ideas on how to solve it. There are several procedures we have written the contain INSERT/UPDATE statements on tables in a Set ...
1
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1answer
57 views

Can sequential queries result in concurrent transactions?

I'm using PostgreSQL 9.4.4 and the default configuration. I use a JDBC connection to run multiple queries in succession. Only one Java thread is using the connection and before a new query is executed ...
1
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1answer
331 views

List MATERIALIZED VIEWs in PostgreSQL that do not have a UNIQUE index

Postgresql 9.4 allows to REFRESH materialized views CONCURRENTLY, if they have a UNIQUE index. I am looking fo a SQL-query that lists all materialized views that DO NOT have such a UNIQUE or PRIMARY ...
1
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1answer
44 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
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1answer
228 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
803 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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1answer
23 views

Paged query against timestamp populated by trigger missing items

We're having a strange problem with a PostgreSQL 9.2 database that I've been racking my brain trying to figure out. First some background. We have a "tombstone" table that is populated using a ...
1
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2answers
81 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 ...
1
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1answer
142 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
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
42 views

How to make a PostgreSQL trigger thread-safe

I want to make my PostgreSQL trigger function do something like incrementing counters etc (something complex with multiple queries etc) thread-safely (so the same function triggered concurrently won't ...
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0answers
86 views

Is it a good idea to have too many named locks?

I am using MySQL as a data store and a Java App on the top. There are many state changes that happen, and I am seeing some concurrency bugs in there. I wanted to know if I can use named locks ...
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2answers
1k views

Global Temporary Table used in Stored Procedure executed concurrently across sessions

I use dynamic SQL in a stored procedure and must use a global temp table so the temp table and its data is available outside the instance when sp_executesql happens. I need the stored procedure to be ...
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0answers
59 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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0answers
790 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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0answers
329 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 ...
1
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1answer
72 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
246 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 ...
0
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1answer
2k 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
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1answer
1k 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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2answers
3k 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 ...
0
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1answer
38 views

What does this deadlock report from MySQL mean?

Here is the output: ------------------------ LATEST DETECTED DEADLOCK ------------------------ 2015-12-23 05:57:47 7f23d73c6700 *** (1) TRANSACTION: TRANSACTION 32990178, ACTIVE 0 sec inserting mysql ...
0
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2answers
131 views

How to manage concurrencies without locking tables in MySQL?

In our php app, we have a table that records orders recieved at the reception. Since we can have up to 7 agents/active accounts, how is it possible to manage the concurrencies in the orders table ...
0
votes
1answer
38 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 ...
0
votes
1answer
549 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 ...
0
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2answers
896 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) ...
0
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1answer
492 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. ...
0
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1answer
289 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 ...
0
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1answer
44 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 ...
0
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1answer
69 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 ...
0
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1answer
2k 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 ...