Trait diesel::pg::expression::dsl::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> { ... }
}
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§
sourcefn tablesample_bernoulli(
self,
portion: i16,
) -> Tablesample<Self, BernoulliMethod>
fn tablesample_bernoulli( self, portion: i16, ) -> Tablesample<Self, BernoulliMethod>
See the trait-level docs.
sourcefn tablesample_system(self, portion: i16) -> Tablesample<Self, SystemMethod>
fn tablesample_system(self, portion: i16) -> Tablesample<Self, SystemMethod>
See the trait-level docs.