ClickHouse/src/TableFunctions/ITableFunction.h

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#pragma once
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#include <Parsers/IAST_fwd.h>
#include <Storages/IStorage_fwd.h>
#include <Storages/ColumnsDescription.h>
#include <Access/Common/AccessType.h>
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#include <memory>
#include <string>
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namespace DB
{
class Context;
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/** Interface for table functions.
*
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* Table functions are not relevant to other functions.
* The table function can be specified in the FROM section instead of the [db.]Table
* The table function returns a temporary StoragePtr object that is used to execute the query.
*
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* Example:
* SELECT count() FROM remote('example01-01-1', merge, hits)
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* - go to `example01-01-1`, in `merge` database, `hits` table.
*
*
* When creating table AS table_function(...) we probably don't know structure of the table
* and have to request if from remote server, because structure is required to create a Storage.
* To avoid failures on server startup, we write obtained structure to metadata file.
* So, table function may have two different columns lists:
* - cached_columns written to metadata
* - the list returned from getActualTableStructure(...)
* See StorageTableFunctionProxy.
*/
class ITableFunction : public std::enable_shared_from_this<ITableFunction>
{
public:
static inline std::string getDatabaseName() { return "_table_function"; }
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/// Get the main function name.
virtual std::string getName() const = 0;
/// Returns true if we always know table structure when executing table function
/// (e.g. structure is specified in table function arguments)
virtual bool hasStaticStructure() const { return false; }
/// Returns false if storage returned by table function supports type conversion (e.g. StorageDistributed)
virtual bool needStructureConversion() const { return true; }
virtual void parseArguments(const ASTPtr & /*ast_function*/, ContextPtr /*context*/) {}
/// Returns actual table structure probably requested from remote server, may fail
virtual ColumnsDescription getActualTableStructure(ContextPtr /*context*/) const = 0;
/// Check if table function needs a structure hint from SELECT query in case of
/// INSERT INTO FUNCTION ... SELECT ...
/// It's used for schema inference.
virtual bool needStructureHint() const { return false; }
/// Set a structure hint from SELECT query in case of
/// INSERT INTO FUNCTION ... SELECT ...
/// This hint could be used not to repeat schema in function arguments.
virtual void setStructureHint(const ColumnsDescription &) {}
/// Create storage according to the query.
StoragePtr
execute(const ASTPtr & ast_function, ContextPtr context, const std::string & table_name, ColumnsDescription cached_columns_ = {}, bool use_global_context = false) const;
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virtual ~ITableFunction() = default;
private:
virtual StoragePtr executeImpl(
const ASTPtr & ast_function, ContextPtr context, const std::string & table_name, ColumnsDescription cached_columns) const = 0;
virtual const char * getStorageTypeName() const = 0;
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virtual AccessType getSourceAccessType() const;
};
using TableFunctionPtr = std::shared_ptr<ITableFunction>;
}