Backport #57196 to 23.8: Fix incorrect JOIN plan optimization with pa…

…rtially materialized normal projection

src/Processors/QueryPlan/Optimizations/optimizeUseNormalProjection.cpp

@@ -236,7 +236,7 @@ bool optimizeUseNormalProjections(Stack & stack, QueryPlan::Nodes & nodes)
     }
     else
     {
-        const auto & main_stream = iter->node->children.front()->step->getOutputStream();
+        const auto & main_stream = iter->node->children[iter->next_child - 1]->step->getOutputStream();
         const auto * proj_stream = &next_node->step->getOutputStream();
 
         if (auto materializing = makeMaterializingDAG(proj_stream->header, main_stream.header))
@@ -252,7 +252,7 @@ bool optimizeUseNormalProjections(Stack & stack, QueryPlan::Nodes & nodes)
         auto & union_node = nodes.emplace_back();
         DataStreams input_streams = {main_stream, *proj_stream};
         union_node.step = std::make_unique<UnionStep>(std::move(input_streams));
-        union_node.children = {iter->node->children.front(), next_node};
+        union_node.children = {iter->node->children[iter->next_child - 1], next_node};
         iter->node->children[iter->next_child - 1] = &union_node;
     }
 

tests/queries/0_stateless/01710_normal_projection_join_plan_fix.sql

@@ -0,0 +1,13 @@
+DROP TABLE IF EXISTS t1;
+DROP TABLE IF EXISTS t2;
+
+CREATE TABLE t1 (id UInt32, s String) Engine = MergeTree ORDER BY id;
+CREATE TABLE t2 (id1 UInt32, id2 UInt32) Engine = MergeTree ORDER BY id1 SETTINGS index_granularity = 1;
+INSERT INTO t2 SELECT number, number from numbers(100);
+ALTER TABLE t2 ADD PROJECTION proj (SELECT id2 ORDER BY id2);
+INSERT INTO t2 SELECT number, number from numbers(100);
+
+SELECT s FROM t1 as lhs LEFT JOIN (SELECT * FROM t2 WHERE id2 = 2) as rhs ON lhs.id = rhs.id2;
+
+DROP TABLE t1;
+DROP TABLE t2;