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.
This module is considered “trusted”, that is, it can be
installed by non-superusers who have CREATE privilege
on the current database.
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).