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When you work with Tableau Desktop 9.0 and later, your My Tableau Repository might contain a folder called Shadow Extracts. By default, the Shadow Extracts folder can contain up to five shadow extract files, which have a .ttde extension.
The contents of the Shadow Extracts folder change depending on the workbook you work with, and shadow extract files are only created when you work with workbooks that are based on non-legacy Excel or text, or statistical files.
Tableau creates and saves a shadow extract file in order to load your data more quickly. After Tableau creates five shadow extract files, Tableau deletes the oldest shadow extract file to create space when it adds a new one.
Although shadow extract files contain underlying data and other information similar to the standard Tableau extract, shadow extract files are saved in a different format, which means that they cannot be used the same way Tableau extracts are.
Have a look at this:Shadow Extracts | Tableau Software
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Shadow extracts create a faster text-based data source experience. A workbook with a large text or Excel file re-opens with improved speed.
Tableau is able to connect to a wide range of data sources, including text and Microsoft Excel files. Both text and Excel files are highly popular file formats for storing data.In the past Tableau used Microsoft Jet/Ace drivers to query text and Excel files. This approach had a number of drawbacks, such as lack of portability to other operating systems and a 4GB parsing limit. Furthermore, running analytical queries over these data sources was inherently slow because the system had to parse the file for every query.Shadow extracts have been introduced to speed up the query execution and overcome the Jet limitations. When a text or excel file is connected, Tableau extracts the data from the file, and stores them in temporary tables in the TDE. Subsequently, all queries are executed by the TDE instead of parsing the entire file each time. This greatly improves the query execution time, however, we need to pay a one-time cost of creating the temporary database. Last but not least, the system can persist extracts in workbooks to avoid recreating temporary tables at every load.
In order to effectively extract data from text and Excel files, and overcome the Jet/Ace limitations, Tableau uses an in-house parser for parsing text files and LibXL for parsing Excel files. These parsers are both more efficient, do not have the 4GB limitation, and are cross-platform. The text parser accepts a schema file as additional input if one is available. Otherwise, it attempts to discover the metadata by performing type and column name inference.