When trying to connect to a data source based on the Microsoft Jet database engine, such as Excel, Access or a plain text (CSV) file, you may encounter unexpected issues that are related to limitations in the Jet database engine. Microsoft Jet technology is used with Tableau Desktop prior to 8.2 for Excel and CSV files or when using the legacy connector option with Tableau Desktop 8.2 and higher.
Known data or file size limitations
Working with large CSV files can be problematic.
There is no explicit or documented file-size limit, but as a general guideline, keep a CSV file size smaller than 4GB. Even if you can connect to a CSV file larger than 4GB, taking an extract might cause an error such as
Query too complex
In addition, when you try to connect to a large CSV file, an error might occur that suggests that your computer does not have enough temporary disk space, even if it does.
Table size is limited to 255 columns (fields). In most cases, Jet fails when it reaches the 255 column limit.
Union contains more than 255 columns (fields). If a union contains more than 255 columns, the following error occurs:
Database error 0x800004005: Too many fields defined
Note: Unions may contain more than 255 columns if using a UNION ALL statement in your custom SQL query. If using UNION ALL, the total number of columns in the union are equal to the number of columns counted each time the UNION ALL statement is used. This can cause the number of columns to exceed the column limit. For example, suppose you have UNION ALL on 3 tables with 102 columns each. The statement, SELECT * FROM "Table1" UNION ALL SELECT * FROM "Table2" UNION ALL SELECT * FROM “Table3” will count 306 columns and therefore exceed the Jet column limit.
Memo fields in Access data sources might be truncated after 255 characters.
- Table and file names are limited to 64 characters. If a table or file name exceeds the 64-character limit, when you connect to the data source from Tableau, you might get any of the following error messages:
- An error occurred while communicating with data source <data-source-file-name>… with an additional suggestion to check your permissions.
- <data-source-file-name> is not a valid file name…. Check for punctuation or other invalid characters in the file name.
- Unable to open the text file…. Check the file type and your permissions.
- The table <table-name> does not exist.
Field names are limited to 255 characters. Although Jet can read fields with names that are longer, many operations that you perform on those fields cause an "Unknown" error. This limitation also causes fields to disappear without indication when you create an extract.