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24

This is far less often a disk issue, and far more often a networking issue. You know, the N in SAN? If you go to your SAN team and start talking about the disks being slow, they're gonna show you a fancy graph with 0 millisecond latency on it and then point a stapler at you. Instead, ask them about the network path to the SAN. Get speeds, if it's ...


17

You have 4,000,000 rows and one of the columns averages 8,000 characters (16,000 bytes, I assume). SELECT CONVERT(bigint,4000000) * /* b */ 16000 / /*kb*/ 1024 / /*mb*/ 1024; ------ 61,035 If your stats are accurate, I'd expect this table to be 61 GB. (I wonder if you mean 8,000 bytes, not 8,000 characters, in which case I'd expect > 30 GB.) Other factors ...


15

We have a similar setup and recently encountered these messages in the logs. We are using a DELL Compellent SAN. Here are some things to check when receiving these messages that helped us find a solution Review your windows performance counters for your disks that the warning messages are pointing to, specifically: Disk avg. read time Disk avg. write time ...


14

Index Hint You can use an index hint to do that: SELECT * FROM dbo.Users AS u WITH (INDEX = ix_definitely_an_index) WHERE u.Reputation = 2; The downsides are: Potentially changing a lot of code If you rename an index, this breaks If you change an index definition, it might not be the best index to use anymore Plan Guides You can also use a Plan Guide, ...


13

It appears your request for an actual execution plan triggered stats updates. Since you mention this happens in the mornings, I imagine there's an overnight process that does a lot of modifications to the tables involved? Thus SQL Server uses the stats to create the plan, has hit the modification threshold, and executes automatic stats updates as part of ...


12

The easiest method to solve the problem is to query detailed timing from the PostgreSQL: EXPLAIN. For this you need to find at minimum a single query that does complete but takes longer than expected. Let's say that this line would look like delete from mydata where id='897b4dde-6a0d-4159-91e6-88e84519e6b6'; Instead of really running that command you can ...


12

Lets first get the concept of reader / writer threads out of the way During a backup, SQL Server creates one reader thread for each volume that the database files reside on. The reader thread simply reads the contents of the files. Each time that it reads a portion of the file, it stores them in a buffer. There are multiple buffers in use, so the ...


11

Alright assuming that you are talking about a local variable running from a query in SSMS since it hasn't been specified otherwise. Even if you use the same value for the AND UserStatus = @userStatus that you would use in the literal AND UserStatus = 1 you will see a difference in your execution plan due to how the cardinality estimate is generated. When ...


11

There are some defaults that exist merely because nobody really knows what the effect of changing them would be. For example, the default instance-level collation when installing on a system that uses "US English" as the OS language is SQL_Latin1_General_CP1_CI_AS. This makes no sense since the SQL_* collations are for pre-SQL Server 2000 compatibility. ...


11

Based on the data that you have provided - there are 67 total pages. Index fragmentation for such a small table would not affect the performance. I would not worry about index fragmentation for the table that you have mentioned. You should update your table statistics so that sql server can generate better query plan. I would start my troubleshooting by ...


10

If you have a non-production environment, you could simulate your insert / update workload by running SQL Query Stress. Do that before adding the index to get a baseline, and then after to see how much slower it is, and whether that change is acceptable. In addition to additional overhead for your DUI operations, another consideration is blocking. You ...


9

When I see long estimated plan times in SSMS it's one of the following in order of likelihood: The query optimizer decided that it needed to create or update statistics. The size of the estimated plan is very large (say, >10 MB) and it simply takes SSMS a long time to display it. Query compilation itself actually took a long time due to CPU usage in looking ...


9

This answer is in addition to Joe's answer as I can't be 100% certain it is the version store, however there is enough evidence so far to imply that to be part of the issue. When a secondary replica is marked as readable a good steady state for versioning information needs to first be attained so that there is a known and good starting point for all read ...


8

Disclaimer: I don't know anything about availability groups, but I do know a bit about troubleshooting queries that appear to use more CPU then they ought to. You have a CPU problem in that you're using too much of it. One important thing to say about waits is almost all of them aren't CPU busy. When a worker enters a waiting state it has yielded and is no ...


7

A literal value is known to the optimizer (so it can estimate selectivity based on that value). A parameter value (stored procedure, function) is sniffed at execution time. A subsequent execution might have some other value, for which the previously compiled plan might not be optimal. For a variable, the value is not known to the optimizer. It might be ...


7

Why storing the data on a SAN? What's the point? All database performance is tied to Disk I/O and you are using 3 servers with only one device for the I/O behind them. That makes no sense... and unfortunately so common. I spend my life encountering poorly designed hardware platforms where people just try to design a large scale computer. All CPU power here, ...


7

Tl;dr at the bottom Why was the bad plan chosen The main reason for choosing one plan over the other is the Estimated total subtree cost. This cost was lower for the bad plan than for the better performing plan. The total estimated subtree cost for the bad plan: The total estimated subtree cost for your better performing plan The operator estimated ...


6

After several hours of research and examining the current situation I think I managed to solve the issue. (Many thanks to fellow user ypercube for the inspiration and for Erwin Brandstetter who in parallel came to the same solution.) So there were several layers of the problem. 1.) UPGRADE Upgrading with pg_upgrade 9.3.2 --> 10.5 should be made in two ...


6

In a comment from Zane on your question, he stated: ...It seems like part of your problem is you're reading 50 million rows in order to return 20K in the plan. This is, indeed, the problem. There's no index available to push some, or all, of the predicates down to the storage engine. Microsoft recommends this baseline indexing strategy for temporal ...


6

Examples Consider your query with 8 filter predicates in your IN clause on a dataset of 10K records. select UniqueToken from Books where UniqueToken in ( 'Unique token 1', 'Unique token 2', 'Unique token 3', 'Unique token 4', 'Unique token 5', 'Unique token 6', 'Unique token 9999', 'Unique token 5000' -- 10,000 items ...


6

Network I/O is included in the rpc_completed duration*, so I would expect you'll see improvement in the workload you've described. I enabled TCP/IP on my local SQL Server 2016 instance, and then ran a series of queries through a .NET application that uses an ORM. Here's a comparison of sp_statement_completed and rpc_completed Extended Events targets for ...


5

As the issue self-resolved, I'm left to speculate its cause (rhyming not intentional). Based on Sean's post and the fact that an open Query Store transaction looks to have been the root cause to my increased version store size (e.g. the cause to the HADR_DATABASE_WAIT_FOR_TRANSITION_TO_VERSIONING waits), I can only assume the Query Store had a part in the ...


5

Both do the same and you won't be able to measure any difference in performance. Details This is a string literal or string constant: '5' The manual: A string constant in SQL is an arbitrary sequence of characters bounded by single quotes ('), for example 'This is a string'. To include a single-quote character within a string constant, write two ...


5

A common query rewrite that helps with OR predicates looks like this: SELECT tTitulo.CdContaCartao, tTitulo.CdStatus, MAX(DiariaMaxima) INTO #DiariaMaxima FROM Sistema.Titulo AS tTitulo CROSS APPLY ( SELECT MAX(DiariaMaxima) FROM ( SELECT tTIPM.DtDiaria FROM ...


5

We're going to have to move back to a disk table for now, and I'd like to not log anything to the transaction log for reads on this table. Reads aren't logged because they don't change anything, so you already get what you want.


5

This question has the [Performance] tag so I suspect that you may be thinking of a hash index. In SQL Server, the maximum key length for nonclustered indexes is 1700 bytes. It is not possible to create a nonclustered index with a long string column as a key column. For example, for the following table: DROP TABLE IF EXISTS #HASH_INDEX_DEMO; CREATE TABLE #...


5

Don't delete more than 1000 at a time. All the rows being deleted are saved in case of a crash (or reboot) so that they can be restored. (cf Atomicity.) This also explains why the table was non-responsive after the reboot. Index updates are delayed (cf Change Buffering). This may explain why subsequent deletes got slower -- the updates to the indexes ...


5

BACKUP DATABASE successfully processed 19696388 pages in 1945.648 seconds (79.088 MB/sec). The speed you see here is a result of simple division of the whole backup duration per volume of data processed. In your case backup duration is 1945.648 s, data volume processed is 19696388 pages * 8Kb / 1024 = 153.878,03125 Mb The speed = 153.878,03125 Mb / ...


5

What is currently happening When running your query, the table scan, stream agg & compute scalar operators are not evaluated at runtime. Why is it happening The apply NL join means that for each row in #Docs, return a row from #Docsitems that matches the predicate. This predicate should be WHERE IDDocs = D.ID But the compute scalar operator (EXPR1007)...


4

It can be an O(1) operation if only the name is changing, see: https://dev.mysql.com/doc/refman/8.0/en/alter-table.html and search for: Renaming, Redefining, and Reordering Columns That is in a section that explains that the operation only modifies table metadata. Back in MySQL 5.5 the manual says it works there too, except for InnoDB tables.


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