Questions tagged [parallelism]

Reducing run times by breaking a problem into parts that can be completed by separate execution paths simultaneously.

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Postgres parallel seq scan making high CPU Utilization

When I run a select query which runs on a view in Postgres, it is starting 3 processes for a single query, which is causing high CPU utilization when multiple job runs in parallel. How to avoid using ...
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1answer
48 views

Part of parallel execution plan has serial portion

I have a big linq to sql query which runs in parallel mode. It is returning results in 8 seconds and it is pretty good. I noticed in the execution plan that the extreme right part(from where execution ...
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Parallel insert and delete into staging tables

I work on a small system that receives quite a lot of data from multiple sources (hundreds of them). Depending on few factors I can get anywhere from few rows to a couple of thousands (in fact ...
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1answer
34 views

Assign foreign keys only after populating database

I am porting a MongoDB database over to a PostgreSQL (relational) database. I would like to perform this task concurrently, and I have developed code with parallelism in mind, but dependencies between ...
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2answers
451 views

How can I get rid of an unhelpful parallel branch when unpivoting a single row?

Consider the following query that unpivots a few handfuls of scalar aggregates: SELECT A, B FROM ( SELECT MAX(CASE WHEN ID = 1 THEN 1 ELSE 0 END) VAL1 , MAX(CASE WHEN ID = 2 THEN 1 ...
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3answers
314 views

Converted a Scalar function to a TVF function for parallel execution-Still running in Serial mode

One of my query in was running in serial execution mode after a release and i noticed that two new functions were used in a view which is referenced in the LINQ to SQL Query generated from the ...
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2answers
103 views

Parallel replication is not working on a 5.7 slave from a 5.7 master

Note: Following are the parameters that we turned ON on the 5.7 replica for parallel replication to happen: slave_preserve_commit_order = 1 slave_parallel_type = LOGICAL_CLOCK slave_parallel_workers =...
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1answer
122 views

In SQL Server, How Does Parallelism Change Memory Grants?

I've heard conflicting things about memory grants for parallel select queries: Memory grants get multiplied by DOP Memory grants get divided by DOP Which is it?
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53 views

Can a query whose execution plan uses parallelization use the cores of a processor on a another server that's part of the same Availability Group?

If I have two servers (server A and server B) that are part of the same Availability Group failover cluster, and a query running on server A has an execution plan that uses parallelization, is it ...
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43 views

Complex query won't run after migration [closed]

I have migrated an old, ugly query from a 2008 SQL Server to a new 2016 instance. The query runs fine on the old database even when the query plan has to be recreated. On the new database the query ...
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1answer
928 views

How can I disable parallel queries in PostgreSQL?

I want to benchmark some things in PostgreSQL. When I do this I normally turn off some of the features the database is currently using, just for my session, to see what the second-best plan is. ...
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1answer
245 views

Identify Query Waiting on resources

I have some query with CXCONSUMER wait time when i ran sp_whoisactive as shown in the screenshot below. When i checked the status it is suspended.That means it is waiting for some process to release ...
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Read Deadlock XML - Intra-query Parallelism

I encountered several deadlocks in my application over the last 2 months and i checked the Extended Events to find the details. I tried to understand the dealock XML and i came across this great ...
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42 views

GTID ignore error on parallel replication

Im getting an error in replication where GTID and parallel replications are enabled. Now I want to skip one transaction. I didn't find a way to do that. Last_SQL_Error: Worker 48 failed executing ...
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1answer
165 views

Does CREATE/REFRESH MATERIALIZED VIEW in PostgreSQL use the parallel planner?

I am wondering whether CREATE (with data) or REFRESH MATERIALIZED VIEW uses the parallel planner? When I EXPLAIN ANALYZE the query by itself (any query with parallel potential works here) without ...
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1answer
158 views

Eliminating a intra query deadlock, can I avoid restricting parallelism?

I have an update statement that is generating intra query parallelism deadlock. Based on this link I believe my two options to resolve this issue is to force SQL Server to avoid parallelism via an ...
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2answers
62 views

Functions causing Serial plan.What to use instead? [closed]

I have few stored procedures in my SQL Server database.I see that these stored procedures are running in serial mode from their execution plan. For example : CREATE PROCEDURE [dbo].[DemoProc] @...
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1answer
243 views

Forced serialization on inline table valued function

Apologies if this is a stupid question. We are attempting to convert our multi-statement table valued functions (MSTVFs) to inline table valued functions (ITVFs), to avoid the forced serialization of ...
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1answer
2k views

MS SQL Server: Do multiple queries in a batch ever execute in parallel and if so what happens when the second query is dependent on the first?

If I have a batch of two different SELECT statements, is it possible for them to be executed in parallel (if the SQL Optimizer deems it to be the most efficient execution method)? If the first SELECT ...
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2answers
3k views

MAXDOP = 1, Query Hints and Cost Threshold For Parallelism

If an instance has MAXDOP set at 1 and query hints are used to allow specific queries to go parallel, is the Cost Threshold For Parallelism value still used by SQL to decide whether or not to actually ...
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1answer
62 views

Finding Serial plan from Plan cache

My application goes slow sometimes and when i check the query plan plan for a particular operation,i see that serial pans got generated and gets stored in the plan cache and this is being taken for ...
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1answer
231 views

Why might a parallel pg_restore command take hours longer to complete its non-parallel equivalent?

I used pg_restore to restore a 50 GB db from a directory backup using the following command which used 4 jobs: pg_restore -d analytics -U postgres -j 4 -v "D:\Program Files\PostgreSQL\10\backups\...
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3answers
231 views

Serial Plan in plan cache

I have a query which was executing slowly earlier. Later I found out that it was not running parallel and this is making the query execution slow. The query involves a big view and then querying the ...
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1answer
298 views

How to handle SQL Server Intrasession Parallelism Deadlock?

I am struggling to deal with a intrasession deadlock coming from parallelism on SQL Server 2017. The query is not handwritten but generated by an ORM. Please have a look at the query plan over here....
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2answers
408 views

What can I do about high CXPACKET waits?

We are experiencing some slowness on our MS SQL Server 2016 database, I have been using Brent Ozar's first aid kit to do some initial troubleshooting. I am seeing a high amounts of CXPACKET wait ...
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2answers
65 views

Index Seeks cost of OR operator

I have a query which is slow and it takes 15 seconds to execute. The query fetches data from a view and in the where clause, I have few conditions which includes OR and AND. exec sp_executesql N'...
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2answers
47 views

Very simple read-only SELECT not run in parallel in postrgresql

I am trying to do a simple SELECT on a large table, but it is not being run in parallel: my_db=> explain (analyze, buffers) select value from my_table; Seq Scan on my_table (cost=0.00..94433357.34 ...
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133 views

NonParallelPlanReason = MaxDOPSetToOne when maxdop set to 4

Running SQL Server 2014 on both servers. While I was attempting to find out why a procedure was running faster on Server B than it was on Server A I found that on Server B the query plan says the ...
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1answer
91 views

CPU vs Elapsed & Parallelism

I have read threads here and here and I get that elapsed time is the total duration of the task - and if the elapsed time is less than CPU time, the query went parallel. After typing that, I was ...
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1answer
125 views

What's the easiest and most accurate way to visualize parallel thread usage in SQL Server?

Following up on my other question here, I'd like an easy way to visualize the answer. I've tried using a query like this, but the number of "running" threads can exceed DOP sometimes. SELECT ost....
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2answers
356 views

In SQL Server, is parallelism per operator, or something else?

I work with a really old DBA who says a lot of weird stuff. Dude has an O'Reilly book that only has an amoeba on the cover. At lunch we were talking about parallelism, because our new server has 24 ...
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1answer
89 views

Degree of parallelism and scheduler usage

Is it ever possible for a SQL Server query running with an effective degree of parallelism x to have parallel workers assigned to more than x distinct schedulers? Even if the execution plan has many ...
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1answer
228 views

How do I synchronize parallel imports with COPY? [closed]

I have ~20 tables that get built at the same time but using different psql processes (via bash script) at the first stage with something like: create table foo_1 as (select ....); -- via psql process ...
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1answer
693 views

Did Microsoft change query optimizer regarding number of files and parallelism

Did Microsoft change query optimizer regarding number of files and parallelism? Does the optimizer no longer factor in the number of files to determine the degree of parallelism for a query? If so, ...
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1answer
663 views

Parallel Sequential Scan on SELECT statement, but not on SELECT … INTO [duplicate]

The following query EXPLAIN SELECT c.global_id FROM centers_res17 AS c LEFT JOIN regions_res17 AS r ON ST_Contains(r.wkb_geometry, c.wkb_geometry) WHERE r.global_id IS NULL; says that a Parallel ...
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2k views

If a Parallelism Exchange Event deadlock is victim-less, is it a problem?

We're seeing a lot of these Intra-Query Parallel Thread Deadlocks in our Production environment (SQL Server 2012 SP2 - yes...I know...), however when looking at the Deadlock XML that has been captured ...
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1answer
86 views

SQL Server Parallelism Disabled

I came up with a strange issue where the performance of select queries was degraded after creating missing indexes on tables. Initially, it looks like all the queries are affected but when looked into ...
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1answer
2k views

SQL Parallel Stored Procedure execution using CLR - Performance

We have a main analytics SP that calls 100 other sub SPs, all of which act upon the same set of data (Claims), check it for some business rules, and output discrepancies. The SPs need not all be run ...
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2answers
251 views

Insert result of a parallellized select query into a table, in Postgresql

According to https://www.postgresql.org/docs/current/static/when-can-parallel-query-be-used.html, "Even when it is in general possible for parallel query plans to be generated, the planner will ...
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1k views

UNION Queries run in Parallel form or not

I have three tables and I want to select data from these three tables in parallel form to increase the performance. I'm running a query like this. SELECT * FROM table1 UNION ALL SELECT * FROM table2 ...
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2answers
2k views

Is there a way to prevent Scalar UDFs in computed columns from inhibiting parallelism?

Much has been written about the perils of Scalar UDFs in SQL Server. A casual search will return oodles of results. There are some places where a Scalar UDF is the only option, though. As an ...
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1answer
646 views

Oracle data pump import parallel configuration

I am trying to track down a cause for a difference in oracle impdp processing, and I am not finding anything, so I am wondering if anyone here can explain the cause of the differences. At times I ...
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2answers
923 views

Round robin vs Demand Parallelism partitioning

This is related to having a common key driver on top of the query with a nested loops, and the parallelism for the rows coming out of driver being either demand type or round robin type. I would have ...
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1answer
272 views

Can multiple cores be used by PostgreSQL to run code in sections (cursors) of a table effecting parallelism?

Let's say I have a 1 million record table. Each record is uniquely identifiable by id. If the table was divided into 10 cursors and 10 requests were made by a web client for each cursor to say change ...
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215 views

Cost threshold for parallelism and wait stats

We have an Stored Proc on our prod server (SQL Server 2014) that takes 3hr 40m (on a Sunday: low server activity) to run (collection job costs on 220 jobs with each requiring 6 SP calls, so 13200 SP ...
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1answer
4k views

SQL Server Threads and Degree Of Parallelism

My understanding about SQL Server threads is that SQL Server breaks down an execution request within a session into several tasks and associates each task with a worker thread. So SQL Server always ...
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1answer
88 views

SQL Server using Parallel Plan for Cost less than 5

I have a pivot query that uses a parallel plan even though the cost is less than 5. The Cost Threshold for Parallelism is set to default of 5, and MAXDOP is 0. SELECT Column1, Column2 Column3, ...
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228 views

Parallel LOAD DATA to different memory tables very slow compared to single one

I'm using a fairly simple bash script to execute mysql load data infile to 11 different MEMORY engine tables. Example: load.sh echo "start $@ $(date)" mysql -p"mypass" -e "use db; set ...
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6answers
14k views

Run stored procedures in parallel

I'm looking to try and run the same stored procedure multiple times with different parameters but at the same time. I'm using SQL 2014 The reason for this is that the procedure takes around 7 hours ...
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1answer
441 views

Why does my parallel query sometimes process billions of extra rows?

I was running some data prep queries when I noticed that one of them sometimes ran much longer than expected. The query should have finished in seconds but sometimes would take minutes to run. Here is ...