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Remove unnecesary SELECT * FROM () #3

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7ffe1ae
Remove unnecesary SELECT * FROM ()
palegre-tiny May 11, 2022
1a78b7f
Add setting remove_redundant_select_asterisk_from
May 12, 2022
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2 changes: 2 additions & 0 deletions src/Core/Settings.h
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Original file line number Diff line number Diff line change
Expand Up @@ -578,6 +578,8 @@ static constexpr UInt64 operator""_Gb(unsigned long long value)
\
M(Bool, allow_unrestricted_reads_from_keeper, false, "Allow unrestricted (without condition on path) reads from system.zookeeper table, can be handy, but is not safe for zookeeper", 0) \
\
M(Bool, remove_redundant_select_asterisk_from, true, "Remove reduntant SELECT * FROM () in the query/subqueries", 0) \
\
/** Experimental functions */ \
M(Bool, allow_experimental_funnel_functions, false, "Enable experimental functions for funnel analysis.", 0) \
M(Bool, allow_experimental_nlp_functions, false, "Enable experimental functions for natural language processing.", 0) \
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160 changes: 160 additions & 0 deletions src/Interpreters/UnionSelectOptimizerVisitor.cpp
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Original file line number Diff line number Diff line change
@@ -0,0 +1,160 @@
#include <Interpreters/UnionSelectOptimizerVisitor.h>
#include <Parsers/ASTAsterisk.h>
#include <Parsers/ASTExpressionList.h>
#include <Parsers/ASTSelectQuery.h>
#include <Parsers/ASTSelectWithUnionQuery.h>
#include <Parsers/ASTSubquery.h>
#include <Parsers/ASTTablesInSelectQuery.h>

/*
option a)
┌─explain─────────────────────────────────────┐
│ SelectWithUnionQuery (children 1) │ <- Replace this node
│ ExpressionList (children 1) │
│ SelectQuery (children 2) │
│ ExpressionList (children 1) │
│ Asterisk │
│ TablesInSelectQuery (children 1) │
│ TablesInSelectQueryElement (children 1) │
│ TableExpression (children 1) │
│ Subquery (children 1) │
│ SelectWithUnionQuery (children 1) │ <- with this one
│ (whatever) │
└─────────────────────────────────────────────┘

option b)
┌─explain─────────────────────────────────────┐
│ SelectWithUnionQuery (children 1) │
│ ExpressionList (children 2) │
│ SelectQuery (children 2) │ <- Replace this node
│ ExpressionList (children 1) │
│ Asterisk │
│ TablesInSelectQuery (children 1) │
│ TablesInSelectQueryElement (children 1) │
│ TableExpression (children 1) │
│ Subquery (children 1) │
│ SelectWithUnionQuery (children 1) │
│ ExpressionList (children 1) │
│ SelectQuery (children 1) │ <- with this one
│ ExpressionList (children 1) │
│ Literal UInt64_1 (alias n) │
│ SelectQuery (children 2) │
│ ExpressionList (children 1) │
│ Asterisk │
│ TablesInSelectQuery (children 1) │
│ TablesInSelectQueryElement (children 1) │
│ TableExpression (children 1) │
│ Subquery (children 1) │
│ SelectWithUnionQuery (children 1) │
│ ExpressionList (children 1) │
│ SelectQuery (children 1) │
│ ExpressionList (children 1) │
│ Literal UInt64_2 (alias n) │
└─────────────────────────────────────────────┘


*/
namespace DB
{

void UnionSelectOptimizerVisitor::visit(ASTPtr & ast)
{
if (ast->as<ASTSelectWithUnionQuery>() || ast->as<ASTSelectQuery>())
while (visit_replace(ast))
;

for (ASTPtr & child : ast->children)
visit(child);
}

bool UnionSelectOptimizerVisitor::visit_replace(DB::ASTPtr & node)
{
DB::ASTPtr query;
DB::ASTPtr next = node;
if (next->as<DB::ASTSelectWithUnionQuery>())
{
if (!move_next<DB::ASTExpressionList>(next))
return false;

if (!move_next<DB::ASTSelectQuery>(next))
return false;

for (DB::ASTPtr & child : next->children)
{
if (child->as<DB::ASTExpressionList>())
{
next = child;
if (!move_next<DB::ASTAsterisk>(next))
return false;
continue;
}
if (child->as<DB::ASTTablesInSelectQuery>())
{
next = child;
if (!move_next<DB::ASTTablesInSelectQueryElement>(next))
return false;

if (!move_next<DB::ASTTableExpression>(next))
return false;

if (!move_next<DB::ASTSubquery>(next))
return false;

if (!move_next<DB::ASTSelectWithUnionQuery>(next))
return false;

query = next;
continue;
}
else
return false;
}
}
else if (next->as<DB::ASTSelectQuery>())
{
if (next->children.size() != 2)
return false;

for (DB::ASTPtr & child : next->children)
{
if (child->as<DB::ASTExpressionList>())
{
next = child;
if (!move_next<DB::ASTAsterisk>(next))
return false;
continue;
}
if (child->as<DB::ASTTablesInSelectQuery>())
{
next = child;
if (!move_next<DB::ASTTablesInSelectQueryElement>(next))
return false;

if (!move_next<DB::ASTTableExpression>(next))
return false;

if (!move_next<DB::ASTSubquery>(next))
return false;

if (!move_next<DB::ASTSelectWithUnionQuery>(next))
return false;

if (!move_next<DB::ASTExpressionList>(next))
return false;

if (!move_next<DB::ASTSelectQuery>(next))
return false;

query = next;
continue;
}
else
return false;
}
}

node.swap(query);
return true;
}

}
28 changes: 28 additions & 0 deletions src/Interpreters/UnionSelectOptimizerVisitor.h
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Original file line number Diff line number Diff line change
@@ -0,0 +1,28 @@
#pragma once

#include <Parsers/IAST.h>

namespace DB
{

struct UnionSelectOptimizerVisitor
{
public:
static void visit(ASTPtr & ast);

private:
template <typename T>
static bool move_next(DB::ASTPtr & item)
{
if (item->children.size() != 1 || !item->children.at(0)->as<T>())
{
return false;
}
item = item->children.at(0);
return true;
}


static bool visit_replace(ASTPtr & node);
};
}
7 changes: 7 additions & 0 deletions src/Interpreters/executeQuery.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -68,6 +68,7 @@
#include <base/demangle.h>

#include <random>
#include <Interpreters/UnionSelectOptimizerVisitor.h>


namespace ProfileEvents
Expand Down Expand Up @@ -567,6 +568,12 @@ static std::tuple<ASTPtr, BlockIO> executeQueryImpl(
NormalizeSelectWithUnionQueryVisitor{data}.visit(ast);
}

/// Remove redundant queries
if (settings.remove_redundant_select_asterisk_from)
{
UnionSelectOptimizerVisitor::visit(ast);
}

/// Check the limits.
checkASTSizeLimits(*ast, settings);

Expand Down