I have been asked to create a simple program to submit user defined queries to SQLite databases (.db). I have not worked with the offline databases before and have a question about optimizing performance.
There are a few hundred .db files that I need to query. Is it quicker to attach them all to a single query using ATTACH, or join them all into a single database and work from there? My thoughts are that there will be some trade off between how much time it takes for inital set up versus the query speed. Is there perhaps a different method that would result in better performance?
I dont think it will matter, but this will be written with C# for a windows OS desktop.
Thanks!
The documentation says:
The number of simultaneously attached databases is limited to SQLITE_MAX_ATTACHED which is set to 10 by default. [...] The number of attached databases cannot be increased above 62.
So attaching a few hundred databases will be very quick because outputting an error message can be done really fast. ☺
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I recently working with Oracle database to generate some reports. What I need is to get result sets of specific records (only SELECT statement), sometimes are large records, to be used for generating the report in excel file.
At first, the reports are queried in Views but some of them are slow (have some complex subqueries). I was asked to increase the performance and also fixed some field mapping. I also want to tidy things up, because when I query against View, I must specifically call the right column name. I want to separate the data works into database, and the web app just for passing parameters and call the right result set.
I'm new to Oracle, so which is better to do this kind of task? Using SP or Function? or in what condition that maybe View is better?
Makes no difference whether you compile your SQL in a view, SP or function. It is the SQL itself that matters.
As long as you are able to meet your requirements with the views they should be a good option. If you intend to break-up your queries into multiple ones for achieving better performance then you should go for stored procedures. If you decide to go for stored procedure then it would be advisable to create a package and bundle all the stored procedures together in the package. If your problem is performance then there may not be a silver bullet solution for the same. You will have to work on your queries and design for the same.
If the problem is performance due to complex SELECT query (queries), you can consider tuning the queries. Often you will find queries written 15-20 years ago, which do not use functionality and techniques that were introduced by Oracle in more recent versions (even if the organization spent the big bucks to buy the more recent versions - making it into a waste of money). Honestly, that may be too much of a task for you if you are new at Oracle; also, some slow queries may have been written by people just like you, many years ago - before they had a chance to learn a lot about Oracle and have experience with it.
Another thing, if the reports don't need to use the absolute current state of the underlying tables (for example, if "what was in the tables at the end of the business day yesterday" is acceptable), you can create a materialized view. It will not work any faster than a regular view, but it can run overnight (say), or every six hours, or whatever - so that the further reporting processing from there will not have to wait for the queries to complete. This is one of the main uses of materialized views.
Good luck!
I'm working on a very large query, in a inherited application. This is a large insert-query, that takes 4 tables with well over a million records. I know, I would also rather have this in SQL-server, but there is no infrastructure at this customer to do this :-)
This query has worked for over a year. However, the source-tables keep on growing, and last week it threw the dreaded 'out of system resources'-error. Bummer...!
I think it is possible to optimize this query. Working in MySQL, I would use the explain-command, to see where optimalisation might occur. Is there a equivalent of this in Access? I cannot seem to find it....
kind regards,
Paul
Probably Jet ShowPlan is closest to what you want. You will have to set a registry key. Then query plan information gets dumped to a text file named SHOWPLAN.OUT. You can read about the details in this article on TechRepublic: Use Microsoft Jet's ShowPlan to write more efficient queries
Also try the Performance Analyzer wizard. You can ask it to examine your query alone, or also ask it to examine table or other queries used by that query.
If you haven't compacted the database recently, see whether that improves performance. Compacting also updates index statistics which allows the engine to make better decisions for the query plan.
I’ve been tasked with optimizing a rather nasty stored procedure in a legacy system. It’s a database dedicated to search, and a new copy is being generate every day, with a lot of complex joins being de-normalized. No writes are being performed, only SELECTs, so I figured some easy improvements could be made by making the whole database read-only and changing the recovery model to “Simple”.
Much to my surprise, this didn’t help – at all! The stored procedure still takes the same amount of time of complete. If fact, I’m so surprised that I figured I did it wrong!
My questions:
Do I need to do anything other than setting “Database read-only” to “true”?
Am I wrong to expect significant performance improvement by making the database read-only?
Same for the recovery model: Shouldn’t “Simple” have some noticeable impact?
Are there other similar database-wide configurations that can improve performance in this scenario?
The stored procedure is huge, with temporary tables, 40+ tables joined in 20+ queries. But I’d like to optimize the database itself before I edit this proc.
Since no writes are performed by your SP, there is no reason to expect noticable performance improvement from changing recovery model and read-write mode.
As others mentioned, you should look into the query plan and optimize your queries.
Another hint: indexes in the database might get fragmented while the database is filled up. Since the data is not going to be modified any more, it might help to rebuild all the indexes with fillfactor 100 - this might help to get rid of fragmentation and to compact data.
Call this for each table in the database: ALTER INDEX ALL ON table_name REBUILD WITH (FILLFACTOR = 100).
Generally, I won't expect much of performance improvement from this, but it depends on the particular database.
Speaking of query optimization, there are very useful features in SQL Server 2005 and later: Execution Related and Index-Related Dynamic Management Views. In particular, sys.dm_exec_query_stats and missing indexes are of interest.
These give you almost the same information as Tuning Advisor, but using you real-life workload, so you don't need to simulate it and feed to the Advisor.
Have you tried using the Database Engine Tuning Advisor included in SQL Server? It will analyze your query and suggest new indexes that will improve the performance of the query. Some of them will be good, some will be bad (for example, I've seen it suggest adding every column in a table to an index, sometimes like 30 of them!), so I don't follow it blindly. Generally I'll add a few indexes and then retest, to find the suggestions that are the most important. I've used it to optimize many queries that I thought I had properly indexed, only to find I could get a lot more performance out of them.
I had a similar setup, large stored procedures with lots of large temp tables.
Our problem was that the joins with and between the temp tables was very slow.
I recommend that you look at your execution plan and try to add relevant indexes to the temp tables too if you have not already.
Looking for a bit of advice on how to optimise one of our projects. We have a ASP.NET/C# system that retrieves data from a SQL2008 data and presents it on a DevExpress ASPxGridView. The data that's retrieved can come from one of a number of databases - all of which are slightly different and are being added and removed regularly. The user is presented with a list of live "companies", and the data is retrieved from the corresponding database.
At the moment, data is being retrieved using a standard SqlDataSource and a dynamically-created SQL SELECT statement. There are a few JOINs in the statement, as well as optional WHERE constraints, again dynamically-created depending on the database and the user's permission level.
All of this works great (honest!), apart from performance. When it comes to some databases, there are several hundreds of thousands of rows, and retrieving and paging through the data is quite slow (the databases are already properly indexed). I've therefore been looking at ways of speeding the system up, and it seems to boil down to two choices: XPO or LINQ.
LINQ seems to be the popular choice, but I'm not sure how easy it will be to implement with a system that is so dynamic in nature - would I need to create "definitions" for each database that LINQ could access? I'm also a bit unsure about creating the LINQ queries dynamically too, although looking at a few examples that part at least seems doable.
XPO, on the other hand, seems to allow me to create a XPO Data Source on the fly. However, I can't find too much information on how to JOIN to other tables.
Can anyone offer any advice on which method - if any - is the best to try and retro-fit into this project? Or is the dynamic SQL model currently used fundamentally different from LINQ and XPO and best left alone?
Before you go and change the whole way that your app talks to the database, have you had a look at the following:
Run your code through a performance profiler (such as Redgate's performance profiler), the results are often surprising.
If you are constructing the SQL string on the fly, are you using .Net best practices such as String.Concat("str1", "str2") instead of "str1" + "str2". Remember, multiple small gains add up to big gains.
Have you thought about having a summary table or database that is periodically updated (say every 15 mins, you might need to run a service to update this data automatically.) so that you are only hitting one database. New connections to databases are quiet expensive.
Have you looked at the query plans for the SQL that you are running. Today, I moved a dynamically created SQL string to a sproc (only 1 param changed) and shaved 5-10 seconds off the running time (it was being called 100-10000 times depending on some conditions).
Just a warning if you do use LINQ. I have seen some developers who have decided to use LINQ write more inefficient code because they did not know what they are doing (pulling 36,000 records when they needed to check for 1 for example). This things are very easily overlooked.
Just something to get you started on and hopefully there is something there that you haven't thought of.
Cheers,
Stu
As far as I understand you are talking about so called server mode when all data manipulations are done on the DB server instead of them to the web server and processing them there. In this mode grid works very fast with data sources that can contain hundreds thousands records. If you want to use this mode, you should either create the corresponding LINQ classes or XPO classes. If you decide to use LINQ based server mode, the LINQServerModeDataSource provides the Selecting event which can be used to set a custom IQueryable and KeyExpression. I would suggest that you use LINQ in your application. I hope, this information will be helpful to you.
I guess there are two points where performance might be tweaked in this case. I'll assume that you're accessing the database directly rather than through some kind of secondary layer.
First, you don't say how you're displaying the data itself. If you're loading thousands of records into a grid, that will take time no matter how fast everything else is. Obviously the trick here is to show a subset of the data and allow the user to page, etc. If you're not doing this then that might be a good place to start.
Second, you say that the tables are properly indexed. If this is the case, and assuming that you're not loading 1,000 records into the page at once and retreiving only subsets at a time, then you should be OK.
But, if you're only doing an ExecuteQuery() against an SQL connection to get a dataset back I don't see how Linq or anything else will help you. I'd say that the problem is obviously on the DB side.
So to solve the problem with the database you need to profile the different SELECT statements you're running against it, examine the query plan and identify the places where things are slowing down. You might want to start by using the SQL Server Profiler, but if you have a good DBA, sometimes just looking at the query plan (which you can get from Management Studio) is usually enough.
I need every day to transfer large amounts of data (about several millions records) from db2 to oracle database. Could u suggest the best perfoming method to do that?
DB2 will allow you to select Oracle as a replication target. This is probably the most efficient and easiest way to do it every day, it also removes the "intermediate container" objection that you have.
See this introduction (and more from the documentation online) for more.
If you're only talking speed then do this.
Time how long it takes to dump the DB2 data to a flatfile.
Time how long it takes to suck that flatfile into Oracle.
there's your baseline and it's free. If you can beat that with an ETL tool, then decide if the cost of the tool is worth it.
For a simple ETL like this, there's little that I've found that can beat this on time.
The downside of this is just general file manipulation BS...
how do you know when to read from the file
how do you know that you got all the rows
how do you resume when something breaks
All those little "niceties" usually get paid for with speed. Of course, I'm joking a bit. They aren't always a little nicety. They are often essential for a smooth running process.
Dump out data to delimited file. Load to Oracle via DIRECT load sqlldr job. Not sexy, but fast. If you can be on same subnet that would be best (pushing data across the network is not what you want). Set this up on a cron, add email alerts on errors