F.45. tsm_system_time - the SYSTEM_TIME sampling method for TABLESAMPLE
The
tsm_system_time
module provides the table sampling method
SYSTEM_TIME
, which can be used in
the
TABLESAMPLE
clause of a
SELECT
command.
This table sampling method accepts a single floating-point argument that is the maximum number of milliseconds to spend reading the table. This gives you direct control over how long the query takes, at the price that the size of the sample becomes hard to predict. The resulting sample will contain as many rows as could be read in the specified time, unless the whole table has been read first.
Like the built-in
SYSTEM
sampling
method,
SYSTEM_TIME
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 selected.
SYSTEM_TIME
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.45.1. Examples #
Here is an example of selecting a sample of a table with
SYSTEM_TIME
. First install the extension:
CREATE EXTENSION tsm_system_time;
Then you can use it in a
SELECT
command, for instance:
SELECT * FROM my_table TABLESAMPLE SYSTEM_TIME(1000);
This command will return as large a sample of
my_table
as
it can read in 1 second (1000 milliseconds). Of course, if the whole
table can be read in under 1 second, all its rows will be returned.