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