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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512 views

Can a server wide setting of 'Max Degree of Parallelism' = 1 cause Brent Ozar's sp_BlitzCache to flag execution plan as 'forced-serialization'?

I am using Brent Ozar's sp_BlitzCache store procedure and I'm attempting to nail down why it is reporting: "Something in your plan is forcing a serial query. Further investigation is needed if ...
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1answer
66 views

Should I change parallelism (MaxDop and Threshold)?

Seems like I have problems with parallelism because largest wait types are CXCONSUMER and CXPACKET. Server has 8 cores. So I am planning to bump up Cost Threshold to 50 and MaxDop to 4 Currently I ...
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2answers
27 views

multiple tables INSERT and DELETE in parallel?

I have 25 tables I refresh in one transaction. Few of them refresh much longer than others. I wonder would it possible to refresh "short-timers" in parallel of "long-timers" in ...
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2answers
132 views

How can a cheap function in a SELECT make the whole query slow?

I'm using Postgres 13.3 with inner and outer queries that both only produce a single row (just some stats about row counts). I can't figure out why Query2 below is so much slower than Query1. They ...
0
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1answer
50 views

How to confirm the positive impact of changing Cost Threshold for parallelism to a value higher than default value of 5

We have a DB Server where we found a simple update query going parallel concurrently and causing blockings and deadlock. The cost of the query was around 7. CTFP on the server was set to default value ...
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1answer
51 views

How to reduce the maximum degree of parallelism at session level?

I have inherited a legacy application that includes a job that creates performance issue at instance level. The DBA told us about this and our team needs to fix the stored procedure that is the most ...
2
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0answers
87 views

Unhandled Deadlocks with SQL Server 2017 [closed]

We’re running into issues with unresolved deadlocks on the most recent version of SQL-Server 2017, has anybody seen a problem like this before? Detailed explanation: We migrated our database from ...
4
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1answer
265 views

No parallelism in Express Edition of SQL Server

I have SQL Server 2019 Express Edition (CU8) on Windows Server 2019 (1809 version) and all of my queries go serial with NoParallelPlansInDesktopOrExpressEdition in NoparallelPlanReason property. Is it ...
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51 views

Update query performance improvement (MS-SQL)

UPDATE M SET ARCHIVE_FLAG = 'N' FROM MARKPOSTED_TMP M WHERE EXISTS (SELECT 1 FROM SHFMKPSTMM S INNER JOIN SHIFT_TMP S1 ON M.MARKID = S.MARKID AND S.SHIFTID = S1.SHIFTID AND S1.ARCHIVE_FLAG <> '...
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2answers
104 views

Cardinality , Parallel Hint alternative for MS-SQL

UPDATE MARK M SET ARCHIVE_FLAG = 'N' WHERE EXISTS (SELECT /*+ cardinality(S1, 10) parallel(S1,8)*/ 1 FROM SHFASG S, SHIFT S1 WHERE S.ID = S1.ID AND M.ID = S.MARKID AND ARCHIVE_FLAG <> 'Y'); ...
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44 views

Use Python to parallelize processing of rows, or is there a native way?

As mentioned in a recent question of mine, I'm new to Oracle: Running Oracle's sqlldr on VirtualBox VM from my MacBook (client) machine I'm back, this time with a question about the best way to ...
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3answers
159 views

Slow Parallel SQL Server query, almost instant in serial

I have a SQL Server query as follows (obfuscated): UPDATE [TABLE1] SET [COLUMN1] = CAST('N' AS CHAR(1)) FROM [TABLE1] WHERE (COLUMN1 = '2' AND COLUMN2 IN('VAL1', 'VAL2', 'VAL3')) OR ...
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1answer
47 views

Does ' /*+parallel(<Specific Table >,4)*/ ' have better impact on query performance in comparison with simple parallel hint '/*+parallel(4)*/'?

I have three tables with below structure: create table customer_info (customer_num number, customer_firstName varchar2(100), customer_lastName varchar2(100), branch_code varchar2(...
0
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1answer
305 views

How to get a parallel nested loop in postgres 12?

I have a lateral join, and I would like each subselect to execute in parallel. According to the docs (https://www.postgresql.org/docs/12/parallel-plans.html), nested loops can be parallelized: Just ...
1
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1answer
43 views

how to sum times in EXPLAIN for Postgres parallel query

I'm having trouble understanding this EXPLAIN on Postgres 12.3: EXPLAIN (ANALYZE, VERBOSE, BUFFERS) SELECT count(1) FROM mytable WHERE page ~ 'foo'; This is a 22GB table with 30 million rows, on a ...
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2answers
121 views

importing postgres tar file throws parallel not supported in postgres 12

I'm trying to import a db that someone sent me: testdb.tar (43GB size) First I've imported using: pg_restore --host localhost --port 5432 --username postgres --dbname testdb --role postgres --no-...
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0answers
97 views

Bottleneck when inserting many rows into different tables

TLDR: I want to bulk insert into many initially empty tables at once, without the performance of each insert being affected too badly. I seem to be hitting a performance bottleneck I can't yet explain....
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3answers
39 views

Is there performance loss in out of sequence inserted rows (MySQL InnoDB)

I am trying to migrate from a MySQL AWS RDS instance with a huge SSD and too much excess space down to a small one, and data migration is the only method. There are four tables in the range of 330GB-...
0
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1answer
217 views

How many parallel workers can I have on Postgres?

I working on a server with Ubuntu, 320 GB SSD, 6 cores and 16GB Ram, but Postgres is having issues some queries that take very long time to run, and with parallel queries, that are increasing the ...
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1answer
38 views

Should an update statement query plan ever go parallel?

If an update statement's query plan does go parallel, does it guarantee a communication buffer resource deadlock, or is that determined by whether the parallel plan is using all cores?
6
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1answer
814 views

In SQL Server, what is “Partitioned Table Parallelism” that is only available in Enterprise Edition?

I am reviewing the differences between Standard and Enterprise editions of SQL Server and can't reproduce the differences advertised in this demo that explains the differences - the performance I ...
3
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1answer
245 views

How can INDEX rebuilds be going parallel when MAXDOP is set to 1

I am periodically running into a THREADPOOL wait issue with a SharePoint data store of hundreds of databases using a SQL Server 2008R2 Standard Edition instance (recently migrated to an Azure VM). It ...
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142 views

parallelism in sql server in user defined function

a four page long query (that creates two temporal tables, and then joins them) in sql server runs in a dozen of seconds (thanks to parallel execution). The same query packed into a user defined ...
3
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2answers
204 views

How many threads being used while query execution?

I have AdventureWorks2014 database installed on my test SQL Server 2014. I am planning to execute the following query: SELECT * FROM Sales.SalesOrderDetail sod INNER JOIN Production.Product p ON sod....
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0answers
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Calculating forecast data in parallel

I'd like to ask about how to calculate forecast data in parallel in a relational DB like PostgreSQL, which seems like a very typical problem when doing forecasts. Let's say we're trying to get the ...
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1answer
507 views

Run stored procedures in other schemas from stored procedure [closed]

I have a current process: a large procedure in one schema. There is a plan to have A, B1, B2, B3 schemas (B-schemas are identical, could be B-n schemas) in Oracle 12c database. Schemas B1, B2, B3 have ...
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2answers
593 views

Is PostgreSQL function parallel safety intended for functions which update data?

I am having some trouble understanding function parallel safety. I can understand what it does - which is to specify whether the function will be executed by the parallel master node or whether the ...
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0answers
111 views

How to trigger Mysql parallel query function to start?

Please help,how to trigger Mysql parallel query function to start? MySQL version is 8.0.19. mysql> show variables like 'innodb_parallel_read_threads'; +------------------------------+-------+ | ...
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1answer
261 views

Is each query batch in a dynamic SQL statement executed sequentially or can it be parallelized, when using sp_ExecuteSql?

I have about 500 databases that are all structured the same but the data is unique in each database. Sometimes I write dynamic SQL statements to hit all (or a subset) of those databases and will ...
3
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1answer
3k views

Query runs in parallel but it is showing as blocked by itself

I have a query which runs in parallel with a MAXDOP of 8. When i looked at sp_who2 i see that the same session ID is being repeated several times(>8) with different connection id. I used the below ...
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1answer
73 views

which command to execute to run multiple OLAP queries?

I have 10 sql files that contains different OLAP queries. I want to run them as a batch. Meaning by batch, Postgres will consider each file as a batch and execute the queries at once (depends on its ...
11
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2answers
696 views

Why/when does SQL Server evaluate the probe side of an inner hash join when the build side was empty?

Setup DROP TABLE IF EXISTS #EmptyTable, #BigTable CREATE TABLE #EmptyTable(A int); CREATE TABLE #BigTable(A int); INSERT INTO #BigTable SELECT TOP 10000000 CRYPT_GEN_RANDOM(3) FROM sys....
3
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1answer
108 views

Save CPU cycles on SQL Server Query Plans (Better without Stats than with on wide tables)

Assume that a query in question has a very wide set of columns. The query looks like the following: set statistics io,time on; Select a.Col1, a.Col2, b.Col3, b.Col4, c.Col5 From FirstWideTable a ...
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1answer
187 views

Multiple SPIDs in sp_who2, Deadlocks and Parallelism

There were reports of slowness on our web application - screens hanging, some unable to log in. It's an OLTP system with approx 50 concurrent users. Upon investigation I found a few things: Number ...
2
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1answer
628 views

Is parallel execution enabled or disabled by default in Oracle Standard Edition database version 12.1?

Oracle Standard Edition 12.1 on Red Hat Linux 7 According to the documentation, parallel execution is enabled by default in Oracle SE 12.1. However, when looking at the database parameters, the ...
0
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1answer
1k views

Number of Worker threads available in PostgreSQL

In SQL Server we can have many worker threads per CPU core. Does anyone know if the same is true in PostgreSQL and can point me at a reference? (assume version 11). The background is that we have ...
5
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1answer
615 views

When to mark functions as PARALLEL RESTRICTED vs PARALLEL SAFE?

The manual for Postgres 12 says: The following operations are always parallel restricted. Scans of common table expressions (CTEs). Scans of temporary tables. ... Further down on ...
0
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1answer
39 views

Is there a way to have SQL work in parallelism with two computers?

Amateur here. I have a couple of the same old 4 core computers that I am trying to use to beef up some automatic query scripts that run every evening for my hobby. I recently found MPICH to have the ...
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2answers
806 views

How to run INSERT INTO AS SELECT in parallel on Oracle?

I have a very large table AUD$ that I want to copy it to another empty one with the same structure AUD_BACKUP. INSERT INTO TEST_USER.AUD_BACKUP SELECT * FROM SYS.AUD$; COMMIT; How do I use ...
0
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1answer
206 views

Query runs in serial and parallel mode in two identical databases on the same instance [duplicate]

I have two databases in the same SQL Server instance. One is a test database with a smaller number of records and total size of 500GB. Other one is production database with a size of 2.2 TB. Both ...
3
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2answers
118 views

Is it normal to share CPUs on a VM of SQL Server?

Our IT set up a SQL Server as a VM on a big VMWare box that contains other VMs. The CPUs are setup as shared. As a result, any queries that might need multiple CPUs take 30x longer than if I limited ...
4
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1answer
87 views

DMVs - Relating tasks to branches

In this excellent article, Paul White demonstrates branches in parallel plans. https://sqlperformance.com/2013/10/sql-plan/parallel-plans-branches-threads At the most fine grained level, we can look ...
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1answer
129 views

SQL Server - Update with Output not working in Parallel environment

I know that the output parameter does not guarantees parallelism logic. And the logic I want to implement is failing in a multi thread environment. To test this, just open 2 windows and execute them ...
0
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1answer
455 views

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 ...
0
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1answer
67 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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0answers
270 views

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 ...
0
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1answer
71 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 ...
9
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2answers
480 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 ...
10
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3answers
558 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
744 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 =...