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Do Not Merge! Fix for Aliases and Lazy Materialization #2560

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Do Not Merge! Fix for Aliases and Lazy Materialization #2560

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e47d7e8
Fix for Aliases and Lazy Materialization
ilejn Oct 13, 2026
a2ea700
test added
ilejn Oct 13, 2026
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11 changes: 11 additions & 0 deletions src/Processors/QueryPlan/Optimizations/optimizeLazyMaterialization.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -38,6 +38,7 @@ static void removeUsedColumnNames(
String filter_name = {})
{
const auto & actions_outputs = actions.getOutputs();
NameSet passthrough_inputs;

for (const auto * output_node : actions_outputs)
{
Expand All @@ -52,6 +53,16 @@ static void removeUsedColumnNames(
if (!node)
continue;

/// The lazy reader maps each source column to a single output name. If an input
/// is forwarded more than once (for example, both a column and its alias), read
/// it eagerly so all outputs contain real data and do not duplicate lazy row coordinates.
if (node->type == ActionsDAG::ActionType::INPUT && !passthrough_inputs.insert(node->result_name).second)
{
const auto it = alias_index.find(node->result_name);
if (it != alias_index.end())
lazily_read_column_name_set.erase(it->second);
}

if (node->type == ActionsDAG::ActionType::FUNCTION || node->type == ActionsDAG::ActionType::ARRAY_JOIN
|| (!filter_name.empty() && filter_name == output_node->result_name))
{
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10 changes: 10 additions & 0 deletions tests/queries/0_stateless/05317_aliases_and_lazy_materialization.reference
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@@ -0,0 +1,10 @@
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15 changes: 15 additions & 0 deletions tests/queries/0_stateless/05317_aliases_and_lazy_materialization.sql
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@@ -0,0 +1,15 @@
-- Tags: no-random-settings

SET query_plan_optimize_lazy_materialization = 1;
SET query_plan_max_limit_for_lazy_materialization = 10;
SET enable_analyzer = 1;

DROP TABLE IF EXISTS t_str;
CREATE TABLE t_str (id UInt64, ts DateTime, s String, s_alias String ALIAS s)
ENGINE = MergeTree ORDER BY id;
INSERT INTO t_str SELECT number, toDateTime('2026-01-01 00:00:00') + number, toString(number) FROM numbers(100);

-- Selecting both the source column and its alias must materialize both outputs.
SELECT s, s_alias FROM t_str ORDER BY ts DESC LIMIT 10;

DROP TABLE t_str;
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