F.41. tsm_system_rows
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.
F.41.1. Examples
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).