Maybe, the reason is that I have too few rows in my table?
Yes. For a total of 20 rows in a table a seq scan is always going to be faster than an index scan. Chances are that those rows are located in a single database block anyway, so the seq scan would only need a single I/O operation.
If you use
explain (analyze true, verbose true, buffers true) select ....
you can see a bit more details about what is really going on.
Btw: you shouldn't use text
as a column name, as that is also a datatype in Postgres (and thus a reserved word).