I am new to database. I try to run a simple query on SQL Server 2014 and Oracle 12c.
This is the execution plan I get using SQL Server. It contains information about I/O cost and CPU cost in seconds.
However I can't find the same information using Oracle. The CPU cost shown in the execution plan is not based on execution time.
I want to do some comparison between the two databases. How I can obtain the same information in Oracle as in SQL Server? Besides, how I can know the cache hit ratio?
Thank you.
The cost estimate is in fact based on time.
It is a non-dimensionalised measurement that expresses the estimated time for the query to complete in terms of the equivalent number of logical reads, so if a logical read is expected to take 0.001 seconds then a cost of 12 is 0.012 seconds.
Although it is commonly stated that the cost between different queries cannot be compared, this was only definitively true in earlier versions. The difficulty in comparing query costs relates to how long single block and multiblock reads, writes and CPU operations take. This can depend on such a multitude of factors (other activity on the system, and activity immediately prior that affects the likelihood of blocks being cached by the instance or the i/o subsystem) that it is highly unlikely that you really expect to derive a time from a cost.
Cache hit ratios have been discredited for quite some time as a measurement of system efficiency. It is possible to improve the cache hit ratio to an arbitrary number by simply running particular types of highly inefficient queries.
Use the Oracle Database 12c: EM Express Performance Hub to get both estimates and actual values for queries and their operations. Regular explain plans are helpful, but they just show you what Oracle thinks will happen, not necessarily what will happen.
Specifically, use either the SQL Details (aggregate) or the SQL Monitor Details (last execution) information.
You're close, very close.
Run with AutoTrace.
I talk more about the feature here, or you can of course read up on the docs or the Help.
Related
I want to find those queries in MS Access which are utilizing the CPU mostly and put them in a table in descending order.
I have checked the system tables of MS Access database, but can't find any clue for this.
I am new to MS Access, please help.
For 10 or even 15 years, Access has never been CPU bound. In other words, network speeds, disk drive speeds etc. are the main factor .
In the vast majority of cases throwing more CPU at a problem will not help improve performance. If 99% of time is network or other factors, then a double of CPU will only improve by 2%.
However, I will accept that if a query is using lots of CPU, then it stands somewhat to reason that such a query is pulling a lot of data. There is no CPU logger for the Access database engine. However, you can look at rows and the query plan, and that can be done with showplan. How this works and can be used is outlined here:
How to get query plans (showplan.out) from Access 2010?
And here is a older article on showplan and how to use it:
https://www.techrepublic.com/article/use-microsoft-jets-showplan-to-write-more-efficient-queries/#
So, showplan is somewhat similar to looking at the query plan used in SQL server. It will tell you things like if a full table scan is being used to get one row, or if indexing can or was used. So, looking at the query plan, be it sql server, or in this case Access is certainly possible. However, the status on CPU usage are slim, but how much data and things like if the query plan is doing full table scans is available in a similar fashion to query plans like one would see for server based systems such as SQL server.
I have a not simple query.
When I had 10 DTUs for my database, it took about 17 seconds to execute the query.
I increased the level to 50 DTU - now the execution takes 3-4 seconds.
This ratio corresponds to the documentation - more DTU = work faster.
But!
1 On my PC I can execute the query in 1 sec.
2 In portal-statistics I see that I use only 12 DTU (max DTU percentage = 25% ).
In sys.dm_db_resource_stats I see that MAX(avg_cpu_percent) is about 25% and the other params are less.
So the question is - Why my query takes 3-4 sec to exec?
It can be executed in 1 sec. And server does not use all my DTU.
How to make server use all available resources to exec queries faster?
DTU is a combined measurement of CPU, memory, data I/O and transaction log I/O.
This means that reaching a DTU bottleneck can mean any of those.
This question may help you to measure the different aspects: Azure SQL Database "DTU percentage" metric
And here's more info on DTU: https://learn.microsoft.com/en-us/azure/sql-database/sql-database-what-is-a-dtu
On my PC I can execute the query in 1 sec
We should not be comparing our Onprem computing power with DTU.
DTU is a combination of CPU,IO,Memory you will be getting based on your performance tier.so the comparison is not valid.
How to make server use all available resources to exec queries faster?
This is simply not possible,since when sql runs a query,memory is the only constraint ,that can prevent the query from even starting.Rest of the resources like CPU,IO speed can increase or decrease based on what query does
In summary,you will have to ensure ,queries are not constrained due to resource crunch,they can use up all resources if they need and can release them when not needed.
You also will have to look at wait types and further fine tune the query.
As Bernard Vander Beken mentioned:
DTU is a combined measurement of CPU, memory, data I/O and transaction
log I/O.
I'll also add that Microsoft does not share the formula used to calculate DTUs. You mentioned that you are not seeing DTUs peg at 100% during query execution. But since we do not know the formula, you may very well be pegging components of DTU, but not pegging DTU itself.
Azure SQL is a shared environment, and each tenant will be throttled to ensure that the minimum SLA for all tenants
What a DTU is is quite fuzzy.
We have done an experiment where we run a set of benchmarks on machines with the same amount of DTU on different data centers.
http://dbwatch.com/azure-database-performance-measured
It turns out that the actual performance varies by a factor of 5.
We have also seen instances where the performance of a repeated query on the same database varies drastically.
We provide our database performance benchmarks for free if you would like to compare the instance you run on your PC with the instance in the azure cloud.
I have a a performance tuning question on SQL server.
I have a program that needs to run every month and it takes more than 24hrs to finish. I need to tune this program in the hope that I can decrease the running time to 12 hrs or less.
As this program isn't developed by us, i can't check the program content and modify it. All i can do is just open the SQL server profiler and activity monitor to trace and analyze the sql content. I have disabled unused triggers and did some housekeeping, but the running time only decreased 1 hr.
I found that the network I/O and buffer I/O are high, but i don't know the cause and meaning of this ?
Can anyone tell me the cause of these two issues (network I/O and butter I/O)? Are there any suggestions for optimizing this program?
Thank you!
. According to your descriptions, I think your I/O is normal, your
question is only one:one procedure is too slowly. the solution:
1.open the SSMS
2.find the procedure
3.click the buttton named "Display estimated execution plan"
4.fix the procedure.
To me it seems like your application reads a lot of data into the application, which would explain the figures. Still, I would check out the following:
Is there blocking? That can easily be a huge waste of time if the process is just waiting for something else to complete. It doesn't look like that based on your statistics, but it's still important to check.
Are the tables indexed properly? Good indexes to match search criteria / joins. If there's huge key lookups, covering indexes might make a big difference. Too many indexes / unnecessary indexes can slow down updates.
You should look into plan cache to see statements responsible for the most I/O or CPU usage
Are the query plans correct for the most costly operations? You might have statistics that are outdated or other optimization issues.
If the application transfers a lot of data to and/or from the database, is the network latency & bandwidth good enough or could it be causing slowness? Is the server where the application is running a bottleneck?
If these don't help, you should probably post a new question with detailed information: The SQL statements that are causing the issues, table & indexing structure of the involved tables with row counts and query plans.
How many records are considered normal for a typical SQL sever database table? I mean, if some of the tables in database contain something like three or four million records, should I consider replacing the hardware, partitioning tables, etc? I have got a query which joins only two tables and has four conditions in its WHERE clause with an ORDER By. This query usually takes 3-4 secs to execute, but once in every 10 or 20 executions, it may take even longer (10 or 20 secs) to execute (I don't think this is related to parameter sniffing because I am recompiling the query every time). How can I improve my query to execute in less than a second? How can I know how it can be achieved? How can I know whether increasing the amount of RAM or Adding news Hard Drive or Increasing CUP Speed or even Improving indexes on tables would boost the performance?
Any advice on this would be appreciated :)
4 million records is not a lot. Even Microsoft Access might manage that.
Even 3-4 seconds for a query is a long time. 95% of the time when you come across performance issues like this it comes down to:
Lack of appropriate indexes;
Poorly written query;
A data model that doesn't lend itself to writing performant queries;
Unparameterized queries thrashing the query cache;
MVCC disabled and you have long-running transactions that are blocking SELECTS (out of the box this is how SQL Server acts). See Better concurrency in Oracle than SQL Server? for more information on this.
None of which has anything to do with hardware.
Unless the records are enormous or the throughput is extremely high then hardware is unlikely to be the cause or solution to your problem.
Unless you're doing some heavy-weight joins, 3-4 million rows do not require any extraordinal hardware. I'd first investigated if there are appropriate indexes, if they are used correctly, etc.
It's my understanding that SQL Server 2005 does some sort of result or index caching. I'm currently profiling complex select statements which take several seconds to several minutes to complete. My problem is that a second run of a query never takes more than a second to run even if I don't alter it. I'm currently using SQL Server Management Studio Express to execute the queries against a SQL Server 2005 server.
My question is, Is there any way to avoid or clear the cache that is causing my queries to execute so quickly on a second run?
There are a couple different things that could at play here, the 3 that come to mind initially (in probable-ish order) would be as follows - if you would like some help interpreting the results, follow the instructions below and paste the stat outputs in the question:
Your query/batch is taking a long time to compile an execution plan. Execution plans are determined and cached (see this post on serverfault for an overview of understanding for how long, when they are rebuilt, etc.)
To verify this, turn on statistics time output, which will provide you information on how long the engine is taking to generate a query plan. For the query/batch in question:
DBCC FREEPROCCACHE
SET STATISTICS TIME ON
Execute the batch, capture the stats output
Execute the batch again, capture the stats output
Compare the 2 stat outputs, paying particular attention to the parse/compile time differences between the 2 executions.
If this is the problem, you can take a couple of approaches to resolving the issue, including specifying a plan guide, specifying a static plan with use plan, or possibly other options like creating a scheduled job to simply compile the plan every few minutes (not as good on option on Sql 2k5 as the others).
Your query/batch is touching a lot of data - on the first execution the data may not be in the buffer pool (basically the cached pages of data the server needs) and the query is performing physical IO operations as opposed to logical IO operations (i.e. reads from disk vs. reads from cache).
To verify this, turn on statistics io output, which will provide you information on the types of IOs and how many of those the engine is performing for the batch. For the query/batch in question:
DBCC DROPCLEANBUFFERS
SET STATISTICS IO ON
Execute the batch, capture the stats output
Execute the batch again, capture the stats output
Compare the 2 stat outputs, paying particular attention to the physical/read-ahead and logical IO outpus between the 2 executions.
To resolve this, you've basically got only 1 option - optimize the query in question so it performs fewer IO operations. You could consider creating a scheduled job that runs the query every so ofter to keep the data in the buffer pool, but this wouldn't be as good an option.
Your query/batch is getting a poor execution plan and/or a poor execution plan choice for different variable values - is this a batch/query that is using a parameterized statement (i.e. you are using variables/static values in the where/join clauses?)? If so, are you seeing the difference in execution times for the same values or different values? If for the same values, the answer is likely #1 or #2 - if for different values, this is potentially your problem. If you think this is the issue after researching #1 and #2, repost with the .sqlplan, the TSQL, and the different parameter values you are using.
I've found the only realiable metrics for performance tuning come from SQL Server's Profiler application. When looking at CPU Time, and Reads in particular, you become much more sheltered from 'other influences'.
For example, the OS being busy, or multiple users being active will reduce your share of CPU and so increase the time to execute. And you may or may not get parallelism over multiple CPUs. But either way, the total CPU Time (as opposed to execution time) will stay approximately the same.
Do this:
CHECKPOINT
DBCC DROPCLEANBUFFERS