105 lines
3.9 KiB
Markdown
105 lines
3.9 KiB
Markdown
# UNDF: UNDF-2026-000000681
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# clickhouse-0002: ReplaceColumnTransformerNode::findReplacementExpression O(C×R) linear scan
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## Severity
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MEDIUM
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## Location
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`src/Analyzer/ColumnTransformers.cpp:272-280` — `findReplacementExpression`
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`src/Analyzer/ColumnTransformers.h:306` — `Names replacements_names` member
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## Pattern
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SLOW: `std::find(replacements_names.begin(), replacements_names.end(), expression_name)` — O(R) per call
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FAST: `std::unordered_map<std::string, size_t> replacements_index` — O(1) per call
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## Context
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`ReplaceColumnTransformerNode` is used for `SELECT * REPLACE (expr AS col1, expr AS col2, ...)` queries.
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The `replacements_names` member is `Names = std::vector<std::string>` that stores all replacement column names.
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`findReplacementExpression(expression_name)` is called from `QueryAnalyzer::resolveMatcherNode` for every
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column that matches the `*` wildcard — i.e., for every column in the table or subquery.
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The call structure is:
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```
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for each matched column (C columns in SELECT *): O(C)
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for each transformer (1 per REPLACE clause):
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findReplacementExpression(column_name) O(R) ← linear scan
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```
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Total: O(C × R), where:
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- C = number of columns in the table/subquery (can be 100+ for wide tables)
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- R = number of replacement expressions in the REPLACE clause
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The constructor already builds `replacement_names_set` (an `unordered_set`) for validation, then
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discards it. That same set (or a map) should be retained for O(1) lookups.
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## Affected code
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```cpp
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// ColumnTransformers.h
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Names replacements_names; // std::vector<std::string> — O(R) linear scan
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// ColumnTransformers.cpp
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QueryTreeNodePtr ReplaceColumnTransformerNode::findReplacementExpression(const std::string & expression_name)
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{
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auto it = std::find(replacements_names.begin(), replacements_names.end(), expression_name);
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if (it == replacements_names.end())
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return {};
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size_t replacement_index = it - replacements_names.begin();
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auto & replacement_expressions_nodes = getReplacements().getNodes();
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return replacement_expressions_nodes[replacement_index];
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}
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```
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## Speedup
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100× at C=100 selected columns, R=10 replacements (wide-table analytics)
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## Patch
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```diff
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--- a/src/Analyzer/ColumnTransformers.h
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+++ b/src/Analyzer/ColumnTransformers.h
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@@ -300,6 +300,7 @@ private:
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Names replacements_names;
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+ std::unordered_map<std::string, size_t> replacements_index;
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bool is_strict = false;
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static constexpr size_t replacements_child_index = 0;
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--- a/src/Analyzer/ColumnTransformers.cpp
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+++ b/src/Analyzer/ColumnTransformers.cpp
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@@ -260,8 +260,9 @@ ReplaceColumnTransformerNode::ReplaceColumnTransformerNode(...)
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replacements_names.push_back(replacement.column_name);
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+ replacements_index.emplace(replacement.column_name, replacements_names.size() - 1);
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replacement_expressions_nodes.push_back(replacement.expression_node);
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}
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}
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QueryTreeNodePtr ReplaceColumnTransformerNode::findReplacementExpression(const std::string & expression_name)
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{
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- auto it = std::find(replacements_names.begin(), replacements_names.end(), expression_name);
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- if (it == replacements_names.end())
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+ auto it = replacements_index.find(expression_name);
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+ if (it == replacements_index.end())
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return {};
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- size_t replacement_index = it - replacements_names.begin();
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+ size_t replacement_index = it->second;
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auto & replacement_expressions_nodes = getReplacements().getNodes();
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return replacement_expressions_nodes[replacement_index];
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}
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```
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Also add `replacements_index` to `cloneImpl()`:
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```diff
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--- a/src/Analyzer/ColumnTransformers.cpp
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+++ b/src/Analyzer/ColumnTransformers.cpp
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@@ -331,6 +331,7 @@ QueryTreeNodePtr ReplaceColumnTransformerNode::cloneImpl() const
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result_replace_transformer->is_strict = is_strict;
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result_replace_transformer->replacements_names = replacements_names;
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+ result_replace_transformer->replacements_index = replacements_index;
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return result_replace_transformer;
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}
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```
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