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#include <Columns/ColumnFixedString.h>
#include <Columns/ColumnLowCardinality.h>
#include <Columns/ColumnUnique.h>
#include <Columns/ColumnsCommon.h>
#include <Common/HashTable/HashMap.h>
#include <Common/assert_cast.h>
#include <Common/typeid_cast.h>
#include <Core/Field.h>
#include <base/TypeLists.h>
#include <DataTypes/DataTypeDate.h>
#include <DataTypes/DataTypeDateTime.h>
#include <DataTypes/DataTypeFactory.h>
#include <DataTypes/DataTypeInterval.h>
#include <DataTypes/DataTypeLowCardinality.h>
#include <DataTypes/DataTypeNullable.h>
#include <DataTypes/DataTypesNumber.h>
#include <DataTypes/Serializations/SerializationLowCardinality.h>
#include <Parsers/IAST.h>
namespace DB
{
namespace ErrorCodes
{
extern const int NUMBER_OF_ARGUMENTS_DOESNT_MATCH;
extern const int LOGICAL_ERROR;
extern const int ILLEGAL_TYPE_OF_ARGUMENT;
}
DataTypeLowCardinality::DataTypeLowCardinality(DataTypePtr dictionary_type_)
: dictionary_type(std::move(dictionary_type_))
{
auto inner_type = dictionary_type;
if (dictionary_type->isNullable())
inner_type = static_cast<const DataTypeNullable &>(*dictionary_type).getNestedType();
if (!inner_type->canBeInsideLowCardinality())
throw Exception(ErrorCodes::ILLEGAL_TYPE_OF_ARGUMENT,
"DataTypeLowCardinality is supported only for numbers, strings, Date or DateTime, but got {}",
dictionary_type->getName());
}
namespace
{
template <typename Creator>
struct CreateColumnVector
{
MutableColumnUniquePtr & column;
const IDataType & keys_type;
const Creator & creator;
CreateColumnVector(MutableColumnUniquePtr & column_, const IDataType & keys_type_, const Creator & creator_)
: column(column_), keys_type(keys_type_), creator(creator_)
{
}
template <typename T>
void operator()(TypeList<T>)
{
if (typeid_cast<const DataTypeNumber<T> *>(&keys_type))
column = creator(static_cast<ColumnVector<T> *>(nullptr));
}
};
}
template <typename Creator>
MutableColumnUniquePtr DataTypeLowCardinality::createColumnUniqueImpl(const IDataType & keys_type,
const Creator & creator)
{
const auto * type = &keys_type;
if (const auto * nullable_type = typeid_cast<const DataTypeNullable *>(&keys_type))
type = nullable_type->getNestedType().get();
WhichDataType which(type);
if (which.isString())
return creator(static_cast<ColumnString *>(nullptr));
if (which.isFixedString())
return creator(static_cast<ColumnFixedString *>(nullptr));
if (which.isDate())
return creator(static_cast<ColumnVector<UInt16> *>(nullptr));
if (which.isDate32())
return creator(static_cast<ColumnVector<Int32> *>(nullptr));
if (which.isDateTime())
return creator(static_cast<ColumnVector<UInt32> *>(nullptr));
if (which.isTime())
return creator(static_cast<ColumnVector<Int32> *>(nullptr));
if (which.isUUID())
return creator(static_cast<ColumnVector<UUID> *>(nullptr));
if (which.isIPv4())
return creator(static_cast<ColumnVector<IPv4> *>(nullptr));
if (which.isIPv6())
return creator(static_cast<ColumnVector<IPv6> *>(nullptr));
if (which.isInterval())
return creator(static_cast<DataTypeInterval::ColumnType *>(nullptr));
if (which.isInt() || which.isUInt() || which.isFloat())
{
MutableColumnUniquePtr column;
TypeListUtils::forEach(TypeListIntAndFloat{}, CreateColumnVector<Creator>(column, *type, creator));
if (!column)
throw Exception(ErrorCodes::LOGICAL_ERROR, "Unexpected numeric type: {}", type->getName());
return column;
}
throw Exception(ErrorCodes::LOGICAL_ERROR, "Unexpected dictionary type for DataTypeLowCardinality: {}",
type->getName());
}
MutableColumnUniquePtr DataTypeLowCardinality::createColumnUnique(const IDataType & keys_type)
{
auto creator = [&](auto x)
{
using ColumnType = typename std::remove_pointer_t<decltype(x)>;
return ColumnUnique<ColumnType>::create(keys_type);
};
return createColumnUniqueImpl(keys_type, creator);
}
MutableColumnUniquePtr DataTypeLowCardinality::createColumnUnique(const IDataType & keys_type, MutableColumnPtr && keys)
{
auto creator = [&](auto x)
{
using ColumnType = typename std::remove_pointer_t<decltype(x)>;
return ColumnUnique<ColumnType>::create(std::move(keys), keys_type.isNullable());
};
return createColumnUniqueImpl(keys_type, creator);
}
MutableColumnPtr DataTypeLowCardinality::createColumn() const
{
MutableColumnPtr indexes = DataTypeUInt8().createColumn();
MutableColumnPtr dictionary = createColumnUnique(*dictionary_type);
return ColumnLowCardinality::create(std::move(dictionary), std::move(indexes), /*is_shared=*/false);
}
Field DataTypeLowCardinality::getDefault() const
{
return dictionary_type->getDefault();
}
bool DataTypeLowCardinality::equals(const IDataType & rhs) const
{
if (typeid(rhs) != typeid(*this))
return false;
const auto & low_cardinality_rhs= static_cast<const DataTypeLowCardinality &>(rhs);
return dictionary_type->equals(*low_cardinality_rhs.dictionary_type);
}
void DataTypeLowCardinality::updateHashImpl(SipHash & hash) const
{
dictionary_type->updateHash(hash);
}
SerializationPtr DataTypeLowCardinality::doGetSerialization(const SerializationInfoSettings &) const
{
return SerializationLowCardinality::create(dictionary_type);
}
void DataTypeLowCardinality::forEachChild(const ChildCallback & callback) const
{
callback(*dictionary_type);
dictionary_type->forEachChild(callback);
}
static DataTypePtr create(const ASTPtr & arguments)
{
if (!arguments || arguments->children.size() != 1)
throw Exception(ErrorCodes::NUMBER_OF_ARGUMENTS_DOESNT_MATCH,
"LowCardinality data type family must have single argument - type of elements");
return std::make_shared<DataTypeLowCardinality>(DataTypeFactory::instance().get(arguments->children[0]));
}
void registerDataTypeLowCardinality(DataTypeFactory & factory)
{
factory.registerDataType("LowCardinality", create, DataTypeFactory::Case::Sensitive, Documentation{
.description = R"DOCS_MD(
Changes the internal representation of other data types to be dictionary-encoded.
## Syntax {#syntax}
```sql
LowCardinality(data_type)
```
**Parameters**
- `data_type` — [String](../../sql-reference/data-types/string.md), [FixedString](../../sql-reference/data-types/fixedstring.md), [Date](../../sql-reference/data-types/date.md), [DateTime](../../sql-reference/data-types/datetime.md), and numbers excepting [Decimal](../../sql-reference/data-types/decimal.md). `LowCardinality` is not efficient for some data types, see the [allow_suspicious_low_cardinality_types](../../operations/settings/settings.md#allow_suspicious_low_cardinality_types) setting description.
## Description {#description}
`LowCardinality` is a superstructure that changes a data storage method and rules of data processing. ClickHouse applies [dictionary coding](https://en.wikipedia.org/wiki/Dictionary_coder) to `LowCardinality`-columns. Operating with dictionary encoded data significantly increases performance of [SELECT](../../sql-reference/statements/select/index.md) queries for many applications.
The efficiency of using `LowCardinality` data type depends on data diversity. If a dictionary contains less than 10,000 distinct values, then ClickHouse mostly shows higher efficiency of data reading and storing. If a dictionary contains more than 100,000 distinct values, then ClickHouse can perform worse in comparison with using ordinary data types.
Consider using `LowCardinality` instead of [Enum](../../sql-reference/data-types/enum.md) when working with strings. `LowCardinality` provides more flexibility in use and often reveals the same or higher efficiency.
## Example {#example}
Create a table with a `LowCardinality`-column:
```sql
CREATE TABLE lc_t
(
`id` UInt16,
`strings` LowCardinality(String)
)
ENGINE = MergeTree()
ORDER BY id
```
## Related Settings and Functions {#related-settings-and-functions}
Settings:
- [low_cardinality_max_dictionary_size](../../operations/settings/settings.md#low_cardinality_max_dictionary_size)
- [low_cardinality_use_single_dictionary_for_part](../../operations/settings/settings.md#low_cardinality_use_single_dictionary_for_part)
- [low_cardinality_allow_in_native_format](../../operations/settings/settings.md#low_cardinality_allow_in_native_format)
- [allow_suspicious_low_cardinality_types](../../operations/settings/settings.md#allow_suspicious_low_cardinality_types)
- [output_format_arrow_low_cardinality_as_dictionary](/operations/settings/formats#output_format_arrow_low_cardinality_as_dictionary)
Functions:
- [toLowCardinality](../../sql-reference/functions/type-conversion-functions.md#toLowCardinality)
## Related content {#related-content}
- Blog: [Optimizing ClickHouse with Schemas and Codecs](https://clickhouse.com/blog/optimize-clickhouse-codecs-compression-schema)
- Blog: [Working with time series data in ClickHouse](https://clickhouse.com/blog/working-with-time-series-data-and-functions-ClickHouse)
- [String Optimization (video presentation in Russian)](https://youtu.be/rqf-ILRgBdY?list=PL0Z2YDlm0b3iwXCpEFiOOYmwXzVmjJfEt). [Slides in English](https://github.com/ClickHouse/clickhouse-presentations/raw/master/meetup19/string_optimization.pdf)
)DOCS_MD",
.syntax = "LowCardinality(T)",
.examples = {},
.related = {},
});
}
DataTypePtr removeLowCardinality(const DataTypePtr & type)
{
if (const auto * low_cardinality_type = typeid_cast<const DataTypeLowCardinality *>(type.get()))
return low_cardinality_type->getDictionaryType();
return type;
}
DataTypePtr removeLowCardinalityAndNullable(const DataTypePtr & type)
{
return removeNullable(removeLowCardinality(type));
};
}