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37 lines
1.3 KiB
C++
37 lines
1.3 KiB
C++
#pragma once
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#include <DB/Parsers/IAST.h>
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namespace DB
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{
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class Context;
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class WriteBuffer;
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class TypeAndConstantInference;
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/** Transform GROUP BY, ORDER BY and LIMIT BY sections.
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* (LIMIT BY is an extension to SQL language, do not be confused with ordinary LIMIT)
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*
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* Remove constant expressions (like ORDER BY concat('hello', 'world')).
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* For GROUP BY, unwrap injective functions (like GROUP BY toString(x) -> GROUP BY x).
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* For GROUP BY, remove deterministic functions of another keys (like GROUP BY x + 1, x -> GROUP BY x).
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* TODO For ORDER BY, remove deterministic functions of previous keys (like ORDER BY num, toString(num) -> ORDER BY num),
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* but only if no collation has specified.
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* As a special case, remove duplicate keys.
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* For LIMIT BY, apply all the same as for GROUP BY.
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*
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* TODO We should apply something similar for DISTINCT,
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* but keys for DISTINCT are specified implicitly (as whole SELECT expression list).
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*
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* This should be run after CollectAliases, because some aliases will be lost from AST during this transformation.
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* This should be run after TranslatePositionalArguments for positional arguments like ORDER BY 1, 2 not to be confused with constants.
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*/
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struct OptimizeGroupOrderLimitBy
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{
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void process(ASTPtr & ast, TypeAndConstantInference & expression_info);
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};
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}
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