F.43. tsm_system

F.43. tsm_system_rows
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F.43. tsm_system_rows

F.43.1. Examples

+ 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.43.1. 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). +

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