I have a checkboxlist in my C#/asp.net project and I'm populating it with a dataTable that gets data from a query to my database. The query returns a large amount of data and I want to restrict the number of elements that it shows initially before I filter the data. (To, say, the top 1000). How would I go about doing this?
There are two places where you can limit the number of data.
In the database (assuming you use SQL Server) you can modify the query to return the top 1000 rows.
SELECT TOP 1000 * FROM SomeTable
Or you can filter the data after it arrives using Linq.
var newData = dataTable.AsEnumerable().Take(1000);
I would prefer the first method, so you don't truck around useless data. But the second definitely works as well if you need that data elsewhere.
You can use the Take<> generic IEnumerable method:
var data = someQuery.Exec();
var limitedData = data.Take(1000).ToArray();
Related
I have a web application in which I get data from my database and show in a datatable. I am facing an issue doing this as the data that I am fetching has too many rows(200 000). So when I query something like select * from table_name;
my application gets stuck.
Is there a way to handle this problem with JavaScript?
I tried pagination but I cannot figure how would i do that as datatable creates pagination for already rendered data?
Is there a way through which I can run my query through pagination at
the backend?
I have come across the same problem when working with mongodb and angularjs. I used server side paging. Since you have huge number of records, You can try using the same approach.
Assuming a case that you are displaying 25 records in one page.
Backend:
Get the total count of the records using COUNT query.
select * from table_name LIMIT 25 OFFSET
${req.query.pageNumber*25} to query limited records based on the page number;
Frontend:
Instead of using datatable, display the data in HTML table it self.
Define buttons for next page and previous page.
Define global variable in the controller/js file for pageNumber.
Increment pageNumber by 1 when next page button is clicked and
decrement that by 1 when prev button is pressed.
use result from COUNT query to put upper limit to pageNumber
variable.(if 200 records are there limit will be 200/25=8).
So basically select * from table_name LIMIT 25 OFFSET
${req.query.pageNumber*25} will limit the number of records to 25. when req.query.pageNumber=1, it will offset first 25records and sends next 25 records. similarly if req.query.pageNumber=2, it will offset first 2*25 records and sends 51-75 records.
There are two ways to handle.
First way - Handling paging in client side
Get all data from database and apply custom paging.
Second way - Handling paging in server side
Every time you want to call in database and get records according to pagesize.
You can use LIMIT and OFFSET constraints for pagination in MySQL. I understand that at a time 2 lacs data makes performance slower. But as you mention that you have to use JS for that. So make it clear that if you wants js as frontend then it is not going to help you. But as you mention that you have a web application, If that application is on Node(as server) then I can suggest you the way, which can help you a lot.
use 2 variables, named var_pageNo and var_limit. Now use the row query of mysql as
select * form <tbl_name> LIMIT var_limit OFFSET (var_pageNo * var_limit);
Do code according to this query. Replace the variable with your desire values. This will make your performance faster, and will fetch the data as per your specified limit.
hope this will helpful.
We have a 4 column data grid display on a page. I would like to perform a query based on each of the values within each cell of the grid i.e. 4 queries for each row.
This is so I can populate the cell with count of records in db that matches that value.
When each row gets populated by jqgrid, it fires off an ajax call for each cell.
I think it's a very bad idea since I have already discovered the browser limits the number of ajax calls to the same server.
Are there similar limits for ado.net?
I would like to batch these queries together so I do fewer calls to the db, is this what you would do?
How would you approach this?
You could combine your AJAX calls into one, the resulting object contains an array, or multiple properties for each result set, and then you run your SQL in parallel on the server.
Check out this QA for how to use options on how to use TPL and SQL.
Parallel.Foreach SQL querying sometimes results in Connection
me would suggest you to select the associated id data in the first request and populate the values in the UI.
You could either use join or left join in your first query based on your requirement and architecture and fetch the specific column value/ count(id) ( Here you mentioned as the count of records ).
Problem summary:
C# (MVC), entity framework 5.0 and Oracle.
I have a couple of million rows in a view which joins two tables.
I need to populate dropdownlists with filter-posibilities.
The options in these dropdownlists should reflect the actual contents
of the view for that column, distinct.
I want to update the dropdownlists whenever you select something, so
that the new options reflect the filtered content, preventing you
from choosing something that would give 0 results.
Its slow.
Question: whats the right way of getting these dropdownlists populated?
Now for more detail.
-- Goal of the page --
The user is presented with some dropownlists that filter the data in a grid below. The grid represents a view (see "Database") where the results are filtered.
Each dropdownlist represents a filter for a column of the view. Once something is selected, the rest of the page updates. The other dropdownlists now contain the posible values for their corresponding columns that complies to the filter that was just applied in the first dropdownlist.
Once the user has selected a couple of filters, he/she presses the search button and the grid below the dropdownlists updates.
-- Database --
I have a view that selects almost all columns from two tables, nothing fancy there. Like this:
SELECT tbl1.blabla, tbl2.blabla etc etc
FROM table1 tbl1, table2 tbl2
WHERE bsl.bvz_id = bvz.id AND bsl.einddatum IS NULL;
There is a total of 22 columns. 13 VARCHARS (mostly small, 1 - 20, one of em has a size of 2000!), 6 DATES and 3 NUMBERS (one of them size 38 and one of them 15,2).
There are a couple of indexes on the tables, among which the relevant ID's for the WHERE clause.
Important thing to know: I cannot change the database. Maybe set an index here and there, but nothing major.
-- Entity Framework --
I created a Database first EDMX in my solution and also mapped the view. There are also classes for both tables, but I need data from both of them, so I don't know if I need them. The problem by selecting things from either table would be that you can't apply half of the filtering, but maybe there are smart way's I didn't think of yet.
-- View --
My view is strongly bound to a viewModel. In there I have a IEnumerable for each dropdownlist. The getter for these gets its data from a single IEnumerable called NameOfViewObjects. Like this:
public string SelectedColumn1{ get; set; }
private IEnumerable<SelectListItem> column1Options;
public IEnumerable<SelectListItem> Column1Options
{
get
{
if (column1Options == null)
{
column1Options= NameOfViewObjects.Select(item => item.Column1).Distinct()
.Select(item => new SelectListItem
{
Value = item,
Text = item,
Selected = item.Equals(SelectedColumn1, StringComparison.InvariantCultureIgnoreCase)
});
}
return column1Options;
}
}
The two solutions I've tried are:
- 1 -
Selecting all columns in a linq query I need for the dropdownlists (the 2000 varchar is not one of them and there are only 2 date columns), do a distinct on them and put the results into a Hashset. Then I set NameOfViewObjects to point towards this hashset. I have to wait for about 2 minutes for that to complete, but after that, populating the dropdownlists is almost instant (maybe a second for each of them).
model.Beslissingen = new HashSet<NameOfViewObject>(dbBes.NameOfViewObject
.DistinctBy(item => new
{
item.VarcharColumn1,
item.DateColumn1,
item.DateColumn2,
item.VarcharColumn2,
item.VarcharColumn3,
item.VarcharColumn4,
item.VarcharColumn5,
item.VarcharColumn6,
item.VarcharColumn7,
item.VarcharColumn8
}
)
);
The big problem here is that the object NameOfViewObject is probably quite large, and even though using distinct here, resulting in less than 100.000 results, it still uses over 500mb of memory for it. This is unacceptable, because there will be a lot of users using this screen (a lot would be... 10 max, 5 average simultaniously).
- 2 -
The other solution is to use the same linq query and point NameOfViewObjects towards the IQueryable it produces. This means that every time the view wants to bind a dropdownlist to a IEnumerable, it will fire a query that will find the distinct values for that column in a table with millions of rows where most likely the column it's getting the values from is not indexed. This takes around 1 minute for each dropdownlist (I have 10), so that takes ages.
Don't forget: I need to update the dropdownlists every time one of them has it's selection changed.
-- Question --
So I'm probably going at this the wrong way, or maybe one of these solutions should be combined with indexing all of the columns I use, maybe I should use another way to store the data in memory, so it's only a little, but there must be someone out there who has done this before and figured out something smart. Can you please tell me what would be the best way to handle a situation like this?
Acceptable performance:
having to wait for a while (2 minutes) while the page loads, but
everything is fast after that.
having to wait for a couple of seconds every time a dropdownlist
changes
the page does not use more than 500mb of memory
Of course you should have indexes on all columns and combinations in WHERE clauses. No index means table scan and O(N) query times. Those cannot scale under any circumstance.
You do not need millions of entries in a drop down. You need to be smarter about filtering the database down to manageable numbers of entries.
I'd take a page from Google. Their type ahead helps narrow down the entire Internet graph into groups of 25 or 50 per page, with the most likely at the top. Maybe you could manage that, too.
Perhaps a better answer is something like a search engine. If you were a Java developer you might try Lucene/SOLR and indexing. I don't know what the .NET equivalent is.
First point you need to check is your DB, make sure you have to right indexes and entity relations in place,
next if you want to dynamical build your filter options then you need to run the query with the existing filters to obtain what the next filter can be. there are several ways to do this,
firstly you can query the data and extract the values from the return, this has a huge load time and wastes time returning data you don't want (unless you are live updating the results with the filter and dont have paging, in which case you might aswell just get all the data and use linqToObjects to filter)
a second option is to have a parallel queries for each filter that returns the possible filters, so filter A = all possible values of A from data, filter b = all possible values of B when filtered by A in the data, C = all possible values of C when filtered by A & B in the data, etc. this is better than the first but not by much
another option is the use aggregates to speed things up, ie you have a parallel query as above but instead of returning the data you return how many records are returned, aggregate functions are always quicker so this will cut your load time dramatically but you are still repeatedly querying a huge dataset to it wont be exactly nippy.
you can tweak this further using exist to just return a 0 or 1.
in this case you would look at a table with all possible filters and then remove the ones with no values from the parallel query
the next option will be the fastest by a mile is to cache the filters in the DB, with a separate table
then you can query that and say from Cache, where filter = ABC select D, the problem with this maintaining the cache, which you would have to do in the DB as part of the save functions, trigggers etc.
Another solution that can be added in addition to the previous suggestions is to use the /*+ result_cache */ hint, if your version of Oracle supports it (Oracle version 11g or later). If the output of the query is small enough for a drop-down list, then when a user enters criteria that matches the same criteria another user used, the results are returned in a few milliseconds instead of a few seconds or minutes. Result cache is wonderful for queries that return a small set of rows out of millions.
select /*+ result_cache */ item_desc from some_table where item_id ...
The result cache is automatically flushed when any insert/updates/deletes occur on the database tables.
I've done something 'kind of' similar in the past - if you can add a table to the database then I'd explore introducing a 'scratchpad' type table where results are temporarily stored as the user refines their search. Since multiple users could be working simultaneously the table would have to have an additional column for identifying the user.
I'd think you'd see some performance benefit since all processing is kept server-side and your app would simply be pulling data from this table. Since you're adding this table you would also have total control over it.
Essentially I'd imagine the program flow would go something like:
User selects some filters and clicks 'Search'.
Server populates scratchpad table with results from that search.
App populates results grid from scratchpad table.
User further refines search and clicks 'Search'.
Server removes/adds rows to scratchpad table as necessary.
App populates results grid from scratchpad table.
And so on.
Rather than having all the users results in one 'scratchpad' table you could possibly explore having temporary 'scratchpad' tables per user.
I have a table called driver and i want to update the drivers' position fields ('pos_x' and pos_y) with random numbers and what i did was once select the data from table (to see how many drivers do i have) then update their position then select the data again is there another way to do this thing?
If you create a class to hold the driver information, then you can eliminate the last step (selecting the data again).
The steps would be:
1) Read the data into a List.
2) Update the values in the List.
3) Write the data from the List to the database.
I want to update the drivers' position fields (pos_x and pos_y) with random numbers
You can do this quite easily just using SQL.
UPDATE Person
SET Pos_X = ABS(CHECKSUM(NEWID())) % 1000
, Pos_Y = ABS(CHECKSUM(NEWID())) % 1000
As this is all done on the SQL server it means you won't incur a network overhead shipping data back and forth. Of course you will need to select the result to work with afterwards.
Why ABS-CHECKSUM-NEWID? I tried it with T-SQL's RAND() function with less than satisfactory results!
I've got an web app in ASP.NET 2.0 in which I need to do paging. My method of data access is to pull a DataSet out of a database call, then convert that to a List<Foo> (where Foo is my type I'm pulling out of the DB) and bind my GridView to it. My reason for this is that I didn't want to be having to use string indexers on DataTables all through my application, and that I could separate the display logic from the database by implementing display logic as properties on my classes. This also means I'm doing sorting in .NET instead of SQL.
To implement paging, then, I need to pull all Foo out of the database, sort the list, then take what I want out of the full list to display:
List<Foo> myFoo = MyDB.GetFoos();
myFoo.Sort(new Foo.FooComparer());
List<Foo> toDisplay = new List<Foo>();
for (int i = pageIndex * pageSize; i < (pageIndex + 1) * pageSize && i < myFoo.Count; i++)
{
toDisplay.Add(myFoo[i]);
}
//bind grid
With enough elements, this becomes a source of delay; on my development machine connecting to the test database, it takes almost 0.5 seconds to bind one grid on the screen when pulling 5000 records from the DB.
To solve this problem, am I going to have to move all my display logic to SQL so the sorting and paging can take place there, or is there a better way?
Also, does Linq to SQL solve this? If I'm sorting by a custom property implemented in my .NET class and then using .Skip(pageIndex * pageSize).Take(pageSize), will it convert that to SQL as noted in this question?
Yes - I would recommend you move your record selection to SQL (sorting and paging) - the classic way to perform paging in SQL is to use a CTE. I'll find you a good example and update my answer. There's a good example here http://softscenario.blogspot.com/2007/11/sql-2005-server-side-paging-using-cte.html - I googled for "sql paging cte".
Linq to SQL will use the Row_Number approach as described in the article and in general it will have performant paging queries to the database.
However there are limits to the amount of data that SQL can page for you and still be performant.
If you have a table with millions or rows in it, the paging functions need to limit the amount of data queried and subsequently paged with the Row_Number approach.
Let's just say you want to page this query :
Select column1, column2, column3 from table1 where column1 > 100
Now let's say that returns 1,000,000 rows. SQL Server still has to run its paging routine over a million rows. That will take a few seconds to page out the result set of the initial query. And it has to do this for every query.
To make sure the performance is maintained, you need to limit the amount of records returned that SQL will page.
Select TOP 10000 column1, column2, column3 from table1 where column1 > 100
Now, even though 1 million records match the query, only 10000 will be paged and this will speed things up to sub second response. In this scenario, the user should be notified that the query that they ran to the database was too broad and they need to narrow the search criteria becauase not all possible results were results.