Help Center> MapReduce Service> Component Operation Guide (LTS)> Using HetuEngine> HetuEngine SQL Syntax> SQL Syntax> DQL Syntax> TABLESAMPLE
Updated on 2024-05-29 GMT+08:00
View PDF

TABLESAMPLE

There are two sampling methods: BERNOULLI and SYSTEM.

Neither of these sampling methods allows you to limit the number of rows returned by the result set.

BERNOULLI

Each row is selected to the sample table based on the specified sampling rate. When the Bernoulli method is used to sample a table, all physical blocks of the table are scanned and some rows are skipped (based on the comparison between the sampling percentage and the random value calculated at run time). The probability that the result contains one row is irrelevant to any other rows. This does not reduce the time required to read the sample table from disk. If the sampling output is further processed, the total query time may be affected.

SELECT * FROM users TABLESAMPLE BERNOULLI (50);

SYSTEM

This sampling method divides a table into logical segments of data and samples the table based on the granularity. This sampling method either selects all rows from a particular data segment or skips it (based on a comparison between the sampling percentage and a random value calculated at run time). The selection of rows in the system sampling depends on the connector used. For example, if the Hive data source is used, this depends on the data layout on HDFS. This sampling method cannot ensure an independent sampling probability.

SELECT * FROM users TABLESAMPLE SYSTEM (75);
Parent topic: DQL Syntax