diesel::pg::expression::dsl

Trait TablesampleDsl

source
pub trait TablesampleDsl: Table {
    // Provided methods
    fn tablesample_bernoulli(
        self,
        portion: i16,
    ) -> Tablesample<Self, BernoulliMethod> { ... }
    fn tablesample_system(self, portion: i16) -> Tablesample<Self, SystemMethod> { ... }
}
Available on crate feature postgres_backend only.
Expand description

The tablesample method

The TABLESAMPLE clause is used to select a randomly sampled subset of rows from a table.

This is only implemented for the Postgres backend. While TABLESAMPLE is standardized in SQL:2003, in practice each RDBMS seems to implement a superset of the SQL:2003 syntax, supporting a wide variety of sampling methods.

Calling this function on a table (mytable.tablesample(...)) will result in the SQL FROM mytable TABLESAMPLE ...mytable.tablesample(...) can be used just like any table in diesel since it implements Table.

The BernoulliMethod and SystemMethod types can be used to indicate the sampling method for a TABLESAMPLE method(p) clause where p is specified by the portion argument. The provided percentage should be an integer between 0 and 100.

To generate a TABLESAMPLE ... REPEATABLE (f) clause, you’ll need to call .with_seed(f).

Example:

let random_user_ids = users::table
    .tablesample_bernoulli(10)
    .select((users::id))
    .load::<i32>(connection);

Selects the ids for a random 10 percent of users.

It can also be used in inner joins:

users::table
    .tablesample_system(10).with_seed(42.0)
    .inner_join(posts::table)
    .select((users::name, posts::title))
    .load::<(String, String)>(connection);

That query selects all of the posts for all of the users in a random 10 percent storage pages, returning the same results each time it is run due to the static seed of 42.0.

Provided Methods§

source

fn tablesample_bernoulli( self, portion: i16, ) -> Tablesample<Self, BernoulliMethod>

See the trait-level docs.

source

fn tablesample_system(self, portion: i16) -> Tablesample<Self, SystemMethod>

See the trait-level docs.

Object Safety§

This trait is not object safe.

Implementors§