We Keep Coding

How to display millions of rows in a DataGridView? [duplicate]

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.

C# Winforms Fastest Way To Query MS Access

This may be a dumb question, but I wanted to be sure. I am creating a Winforms app, and using c# oledbconnection to connect to a MS Access database. Right now, i am using a "SELECT * FROM table_name" and looping through each row to see if it is the row with the criteria I want, then breaking out of the loop if it is. I wonder if the performance would be improved if I used something like "SELECT * FROM table_name WHERE id=something" so basically use a "WHERE" statement instead of looping through every row?

The best way to validate the performance of anything is to test. Otherwise, a lot of assumptions are made about what is the best versus the reality of performance.
With that said, 100% of the time using a WHERE clause will be better than retrieving the data and then filtering via a loop. This is for a few different reasons, but ultimately you are filtering the data on a column before retrieving all of the columns, versus retrieving all of the columns and then filtering out the data. Relational data should be dealt with according to set logic, which is how a WHERE clause works, according to the data set. The loop is not set logic and compares each individual row, expensively, discarding those that don’t meet the criteria.
Don’t take my word for it though. Try it out. Especially try it out when your app has a lot of data in the table.

yes, of course.
if you have a access database file - say shared on a folder. Then you deploy your .net desktop application to each workstation?
And furthermore, say the table has 1 million rows.
If you do this:
SELECT * from tblInvoice WHERE InvoiceNumber = 123245
Then ONLY one row is pulled down the network pipe - and this holds true EVEN if the table has 1 million rows. To traverse and pull 1 million rows is going to take a HUGE amount of time, but if you add criteria to your select, then it would be in this case about 1 million times faster to pull one row as opposed to the whole table.
And say if this is/was multi-user? Then again, even on a network - again ONLY ONE record that meets your criteria will be pulled. The only requirement for this "one row pull" over the network? Access data engine needs to have a useable index on that criteria. Of course by default the PK column (ID) always has that index - so no worries there. But if as per above we are pulling invoice numbers from a table - then having a index on that column (InvoiceNumber) is required for the data engine to only pull one row. If no index can be used - then all rows behind the scenes are pulled until a match occurs - and over a network, then this means significant amounts of data will be pulled without that index across that network (or if local - then pulled from the file on the disk).

Millions of rows in the database, only so much needed

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.

SQL - Better two queries instead of one big one

I am working on a C# application, which loads data from a MS SQL 2008 or 2008 R2 database. The table looks something like this:
ID | binary_data | Timestamp
I need to get only the last entry and only the binary data. Entries to this table are added irregular from another program, so I have no way of knowing if there is a new entry.
Which version is better (performance etc.) and why?
//Always a query, which might not be needed
public void ProcessData()
{
byte[] data = "query code get latest binary data from db"
}
vs
//Always a smaller check-query, and sometimes two queries
public void ProcessData()
{
DateTime timestapm = "query code get latest timestamp from db"
if(timestamp > old_timestamp)
data = "query code get latest binary data from db"
}
The binary_data field size will be around 30kB. The function "ProcessData" will be called several times per minutes, but sometimes can be called every 1-2 seconds. This is only a small part of a bigger program with lots of threading/database access, so I want to the "lightest" solution. Thanks.

Luckily, you can have both:
SELECT TOP 1 binary_data
FROM myTable
WHERE Timestamp > #last_timestamp
ORDER BY Timestamp DESC
If there is a no record newer than #last_timestamp, no record will be returned and, thus, no data transmission takes place (= fast). If there are new records, the binary data of the newest is returned immediately (= no need for a second query).

I would suggest you perform tests using both methods as the answer would depend on your usages. Simulate some expected behaviour.
I would say though, that you are probably okay to just do the first query. Do what works. Don't prematurely optimise, if the single query is too slow, try your second two-query approach.

Two-step approach is more efficient from overall workload of system point of view:
Get informed that you need to query new data
Query new data
There are several ways to implement this approach. Here are a pair of them.
Using Query Notifications which is built-in functionality of SQL Server supported in .NET.
Using implied method of getting informed of database table update, e.g. one described in this article at SQL Authority blog

I think that the better path is a storedprocedure that keeps the logic inside the database, Something with an output parameter with the data required and a return value like a TRUE/FALSE to signal the presence of new data

Paging in ASP web page?

I'm new to this so forgive me if this is trivial. Anyway, I'm creating a simple ASP web page that reports data from a table in a database that has 5 items in it. I use a GridView to display the data and that works fine, but I'd like to limit the results to three items so that I can allow paging. I tried configuring the select statement from something like "select * from country" to
select * from country limit 3
but I get a "There was an error executing the query" message when I try to test it. Is there some other way that I have to do this?

set Gridview's property PageSize=3

Try to use TOP keyword instead if you use SQL Server database:
SELECT TOP 3 * FROM COUNTRY;

To do this in SQL Server you need to use top
select top 3 * from country

Depending on the total number of records you are expecting to have in the country table a simple solution would be to use the asp:DataPager control without any limits on the number of rows returned from the SQL.
This will handle all the paging for you, however the full data set is retained in the page view state and so is not an appropriate solution is you are expecting 1000's of records.