contrib/clickhouse/src/Interpreters/RewriteCountDistinctVisitor.h


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#pragma once

#include <Parsers/IAST.h>
#include <Interpreters/InDepthNodeVisitor.h>
#include "Interpreters/TreeRewriter.h"

namespace DB
{

class ASTFunction;

/// Really simple rewrite 'select countDistinct(a) from t' to 'select count(1) from (select a from t groupBy a)'
class RewriteCountDistinctFunctionMatcher
{
public:
    struct Data {};
    static void visit(ASTPtr & ast, Data &);
    static bool needChildVisit(const ASTPtr &, const ASTPtr &) { return true; }
};

using RewriteCountDistinctFunctionVisitor = InDepthNodeVisitor<RewriteCountDistinctFunctionMatcher, true>;
}