Struct ColumnLowCardinality 

pub struct ColumnLowCardinality { /* private fields */ }

Column for LowCardinality type (dictionary encoding)

Stores unique values in a dictionary and uses indices to reference them, providing compression for columns with many repeated values.

Reference Implementation: See clickhouse-cpp/clickhouse/columns/lowcardinality.cpp

Implementations

impl ColumnLowCardinality

pub fn new(type_: Type) -> Self

Create a new empty LowCardinality column for the given type.

pub fn dictionary<T: Column + 'static>(&self) -> &T

Get a reference to the dictionary column as a specific type

Example

let col: ColumnLowCardinality = /* ... */;
let dict: &ColumnString = col.dictionary();

pub fn dictionary_mut<T: Column + 'static>(&mut self) -> &mut T

Get mutable reference to the dictionary column as a specific type

Example

let mut col: ColumnLowCardinality = /* ... */;
let dict_mut: &mut ColumnString = col.dictionary_mut();

pub fn dictionary_ref(&self) -> ColumnRef

Get the dictionary column as a ColumnRef (Arc<dyn Column>)

pub fn dictionary_size(&self) -> usize

Get the number of unique values in the dictionary

pub fn index_at(&self, index: usize) -> u64

Get the index at position

pub fn len(&self) -> usize

Returns the number of values (rows) in this column.

pub fn is_empty(&self) -> bool

Returns true if the column contains no values.

pub fn append_unsafe(&mut self, value: &ColumnValue) -> Result<()>

Append a value with hash-based deduplication (like C++ AppendUnsafe) This is the core method for adding values to LowCardinality columns

pub fn append_values<I>(&mut self, values: I) -> Result<()>

where I: IntoIterator<Item = ColumnValue>,

Bulk append values from an iterator with deduplication

Trait Implementations

impl Column for ColumnLowCardinality

fn column_type(&self) -> &Type

Get the type of this column

fn size(&self) -> usize

Get the number of rows in this column

fn clear(&mut self)

Clear all data from the column

fn reserve(&mut self, new_cap: usize)

Reserve capacity for at least new_cap elements

fn append_column(&mut self, other: ColumnRef) -> Result<()>

Append another column’s data to this column

fn load_prefix(&mut self, buffer: &mut &[u8], _rows: usize) -> Result<()>

Load column prefix from byte buffer (for types that need prefix data) Default implementation is a no-op. Override for types like LowCardinality. This matches C++ clickhouse-cpp’s LoadPrefix pattern.

fn load_from_buffer(&mut self, buffer: &mut &[u8], rows: usize) -> Result<()>

Load column data from byte buffer

fn save_prefix(&self, buffer: &mut BytesMut) -> Result<()>

Save column prefix to byte buffer (for types that need prefix data) Default implementation is a no-op. Override for types like LowCardinality, Array with special nested types. This matches C++ clickhouse-cpp’s SavePrefix pattern.

fn save_to_buffer(&self, buffer: &mut BytesMut) -> Result<()>

Save column data to byte buffer

fn clone_empty(&self) -> ColumnRef

Create an empty clone of this column (same type, no data)

fn slice(&self, begin: usize, len: usize) -> Result<ColumnRef>

Create a slice of this column

fn as_any(&self) -> &dyn Any

Downcast to a concrete column type

fn as_any_mut(&mut self) -> &mut dyn Any

Downcast to a mutable concrete column type