The tsm_system_rows module provides the table sampling method SYSTEM_ROWS, which can be used in the TABLESAMPLE clause of a SELECT command.
This table sampling method accepts a single integer argument that is the maximum number of rows to read. The resulting sample will always contain exactly that many rows, unless the table does not contain enough rows, in which case the whole table is selected.
Like the built-in SYSTEM sampling method, SYSTEM_ROWS performs block-level sampling, so that the sample is not completely random but may be subject to clustering effects, especially if only a small number of rows are requested.
SYSTEM_ROWS does not support the REPEATABLE clause.
Here is an example of selecting a sample of a table with SYSTEM_ROWS. First install the extension:
CREATE EXTENSION tsm_system_rows;
Then you can use it in a SELECT command, for instance:
SELECT * FROM my_table TABLESAMPLE SYSTEM_ROWS(100);
This command will return a sample of 100 rows from the table my_table (unless the table does not have 100 visible rows, in which case all its rows are returned).