From 46651ce6fe013220ed397add242004d764fc0153 Mon Sep 17 00:00:00 2001 From: Daniel Baumann Date: Sat, 4 May 2024 14:15:05 +0200 Subject: Adding upstream version 14.5. Signed-off-by: Daniel Baumann --- doc/src/sgml/tsm-system-time.sgml | 71 +++++++++++++++++++++++++++++++++++++++ 1 file changed, 71 insertions(+) create mode 100644 doc/src/sgml/tsm-system-time.sgml (limited to 'doc/src/sgml/tsm-system-time.sgml') diff --git a/doc/src/sgml/tsm-system-time.sgml b/doc/src/sgml/tsm-system-time.sgml new file mode 100644 index 0000000..df6e83a --- /dev/null +++ b/doc/src/sgml/tsm-system-time.sgml @@ -0,0 +1,71 @@ + + + + tsm_system_time + + + tsm_system_time + + + + 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. + + + + 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. + + + + -- cgit v1.2.3