Caveman's Blog

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File upload – attachment size validation

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Restricting the size of a file upload is an important validation that needs to be performed by an online application so avoid the risk of filling up the server disk space by a malicious intent. ASP.Net provides an upload control that only provides a server side validation of file size. By the time the control validates the size of the uploaded file, the physical file would have been already been copied to the server, which is tool late in avoiding the issue.

Client side technologies comes to the rescue in this scenario, where the validation of an attachment size can be implemented using a browser run-time like Flash, Silverlight, ActiveX, HTML5 etc. This way, if attempts were made to upload files with unsupported sizes, the run-time plug-in can thwart the attempt without any impact on your web server. Following are two free tools that can be employed for this purpose:

  • SWFUpload is a flash based tool.
  • PLUpload is a versatile plugin that can support multiple run-times. This plugin slices a large file in small chunks and will send them out one by one to the server. You can then safely collect them on the server and combine into the original file. The size of the chunks and the acceptable file formats can be defined in the plugin UI definition.

We have implemented PLUpload with good success. This plugin also support multiple file uploads. Visit the plugin homepage to see the other rich features that are supported. The online forum is a treasure trove, where you can find the various implementations, code snippets and will be able to participate in contributing to the community.

PLUpload

References:
1. SWFUpload
2. PLUpload
3. PLUpload Forums

Written by cavemansblog

February 20, 2013 at 10:02 am

SQL Server – Clean Buffers

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Use DBCC DROPCLEANBUFFERS to test queries with a cold buffer cache without shutting down and restarting the server. To drop clean buffers from the buffer pool, first use CHECKPOINT to produce a cold buffer cache. This forces all dirty pages for the current database to be written to disk and cleans the buffers. After you do this, you can issue DBCC DROPCLEANBUFFERS command to remove all buffers from the buffer pool. [1]

CHECKPOINT;
GO
DBCC DROPCLEANBUFFERS;
GO

It is recommended these commands should not be executed in a production environment where the SQL Server caching helps in gaining performance. Running these commands could adversely impact the server performance.

Reference:
1. DBCC DROPCLEANBUFFERS (Transact-SQL)

Written by cavemansblog

December 10, 2012 at 11:34 pm

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Sql Server: Temp table vs table variable

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Temp table vs table variable. One question that lingers in my mind is; how to decide between using one over the other?  I will present some points that I have gathered that could aid in helping you make a decision in picking one over the other:

1. Reseed the index of a table variable is not allowed.

2. We will not be able to explicitly create Indexes on a table variable.

3. Altering the schema of a table variable is not possible

4. Table variables do not have to drop it after usage, they will be automatically cleaned up when they are out of scope.

5. Transactions involving table variables last only for the duration of an update on the table variable. Therefore, table variables require less locking and logging resources. They are not affected by transaction rollbacks.

6. Table variables contribute to more efficient Query Processing.

7. CHECK constraints, DEFAULT values and computed columns in the table type declaration cannot call user-defined functions.

8. The table declaration includes column definitions, names, data types, and constraints. The only constraint types allowed are PRIMARY KEY, UNIQUE, NULL, and CHECK.

Table variables will offer best performance when the data size is small and the . The next time you have to make a choice, choose wisely between the two and make a wise choice for the usage to derive optimal performance from your queries.

References:
1. Table (Transact SQL) – Microsoft SQL Server

Written by cavemansblog

October 24, 2012 at 12:03 am

SQL Server: Case sensitive string comparison

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Here is a useful tidbit. We can use one of the following two methods to perform a case sensitive string/field comparison when a SQL Server database does not support case sensitivity.

declare @a as nvarchar(20) = 'Test'
declare @b as nvarchar(20) = 'TEST'

--Method1:
if(convert(varbinary,@a) = convert(varbinary,@b))
   select 'identical'
else
   select 'non-identical'

--Method 2:
if(@a = @b COLLATE Latin1_General_CS_AS)
   select 'identical'
else
   select 'non-identical'

case_sensitive_comparison

Written by cavemansblog

August 15, 2012 at 10:26 pm

SQL Server: Data transfer for Excel using Linked Server

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Introduction

I will demonstrate yet another way to perform data transfer to and from an excel file using a Linked Server in SQL Server 2008. In this demo I will show you how to:

  • Create a Linked Server using Excel as the data server
  • Import data to a SQL Server table from a spreadsheet
  • Export data to a spreadsheet from a SQL Server table

Create a Data Source

As the first step in this process I have created an Excel file with a spreadsheet named “Employee”, followed by defining the headers and inserting 5 records as showing in following illustration:

Create a Linked Server

Now that we have a data source let us create a linked server using an the Excel file created in the previous step as the data source. Open SQL Server Management Studion (SSMS) and expand “Server Objects” under the intended SQL Server, to find the Linked Servers item.Right click on Linked Servers and click on “New Linked Server” to see following dialog box. EXCEL_LINKED_SERVER is the name that I have chosen to call this linked server. Then we need to populate the Provider, Product Name, Data Source and Provider String like in this example and click the OK button.

A linked server should have been created successfully at this time. Right click on the “Linked Server” tree view item and click on refresh to see the newly created linked server. Expand the linked server to see the Exployee$ spreadsheet under the list of tables.

Import Data

At this point you should be able to access the Excel spread sheet just like any other SQL Server database table.  Let us import data into a table named DataFeed in the Demo database. It is important to note that the spreadsheet name in the SQL query has to be accessed only by preceding spreadsheet name  with “…”  Only one dot is used to represent the current database, two dots to represent another database in the SQL Server instance and three dots represent a Linked Server. When we would have ran the following SQL query, a table named “DataFeed” would have been created and 5 records from this table would be displayed in the results pane.

Export Data

Let us insert a couple of records into the DataFeed table that was created in the above step, followed by exporting those two rows to the spreadsheet. When the following query us run, you will observe that the spread sheet will have 7 records.

Now Let us look at the new data in the spreadsheet:

Written by cavemansblog

July 28, 2012 at 5:47 pm