The Drop table function is used to remove a table from your Qlikview Document and is usually done to temporary tables that you do not need in your final data structure. By dropping unnecesary tables you decrease your model size and ultimetly improve performance.
A QVD is basically a Qlikview data file which can be used to store data in. This can be imported into a model as a table and should also be dropped when not required.
When creating a QVD, it will export the table to a .qvd file. You will then have to drop the table after exporting it to a QVD and then import it back into the model when required later on.
We usually make use of a QVD file to export historical data to which we would later import again. Be it the same model or to other data models. It is especially usefull if the historical data does not change and you can quickly load the data from a QVD than having to run it from the main database.
Hope this helps,
There are two kind of use of Drop Table.
1. When you do you raw data extraction from database and store them into qvds you can extract multiple tables and there would be possibility that it will create mutiple Synthetic keys or circular loop and may hang the system. Since purpose of extraction is to extract data and store them into qvds hence it is good practice to drop table after storing into qvd.
2. You may need Iteration or transformation of a table 2-3 times and for this you need to do resident load. After transforming the table you do not need the previous table so you need to drop them.