Some databases have sampling capabilities and Tableau is able to use those. It's like TOP but more random.

Here's Oracle documentation:

The sample_clause lets you instruct the database to select from a random sample of data from the table, rather than from the entire table.

Hello Dmitry, how does sampling work on MySQL database? I'd like to use it, but only if it can get representative sample (I have data where top N rows definitely isn't a random sample). Thanks!

For MySQL we are running this query:

     SELECT * FROM (<your relation>) as AS `TableauSampleRecords` ORDER BY RAND()

It does give a pretty good representative sample however it's known for not being very efficient with large row count.

You can read about it here:

MySQL: ORDER BY RAND() – a case study of alternatives | Robert Hartung http://www.roberthartung.de/mysql-order-by-rand-a-case-study-of-alternatives/

ORDER BY RAND() - ~jk

MySQL :: MySQL 5.7 Reference Manual :: 9.2.1.15 ORDER BY Optimization

Thanks! I don't care much for efficiency, happy to know that it is representative sample .