Try another fix and add simplier test.

Author: KochetovNicolai

src/Storages/MergeTree/IMergeTreeReader.cpp

@@ -152,14 +152,7 @@ void IMergeTreeReader::evaluateMissingDefaults(Block additional_columns, Columns
             if (res_columns[pos] == nullptr)
                 continue;
 
-            /// We must take column type from part if it exists. Because at the end of defaults
-            /// calculations we will materialize ALL the columns, not only missing.
-            /// If column doesn't exist in part than it will be substituted with default expression
-            const auto * column_in_part = part_columns.tryGet(name_and_type->name);
-            if (column_in_part != nullptr)
-                additional_columns.insert({res_columns[pos], column_in_part->type, name_and_type->name});
-            else
-                additional_columns.insert({res_columns[pos], name_and_type->type, name_and_type->name});
+            additional_columns.insert({res_columns[pos], name_and_type->type, name_and_type->name});
         }
 
         auto dag = DB::evaluateMissingDefaults(
@@ -271,7 +264,8 @@ void IMergeTreeReader::performRequiredConversions(Columns & res_columns) const
         /// Move columns from block.
         name_and_type = requested_columns.begin();
         for (size_t pos = 0; pos < num_columns; ++pos, ++name_and_type)
-            res_columns[pos] = std::move(copy_block.getByName(name_and_type->name).column);
+            if (copy_block.has(name_and_type->name))
+                res_columns[pos] = std::move(copy_block.getByName(name_and_type->name).column);
     }
     catch (Exception & e)
     {

src/Storages/MergeTree/MergeTreeRangeReader.cpp

@@ -1007,6 +1007,10 @@ MergeTreeRangeReader::ReadResult MergeTreeRangeReader::read(size_t max_rows, Mar
                 filterColumns(columns, read_result.final_filter);
             }
 
+            /// If columns not empty, then apply on-fly alter conversions if any required
+            if (!prewhere_info || prewhere_info->perform_alter_conversions)
+                merge_tree_reader->performRequiredConversions(columns);
+
             /// If some columns absent in part, then evaluate default values
             if (should_evaluate_missing_defaults)
             {
@@ -1017,10 +1021,6 @@ MergeTreeRangeReader::ReadResult MergeTreeRangeReader::read(size_t max_rows, Mar
                 addDummyColumnWithRowCount(additional_columns, read_result.num_rows);
                 merge_tree_reader->evaluateMissingDefaults(additional_columns, columns);
             }
-
-            /// If columns not empty, then apply on-fly alter conversions if any required
-            if (!prewhere_info || prewhere_info->perform_alter_conversions)
-                merge_tree_reader->performRequiredConversions(columns);
         }
 
         read_result.columns.reserve(read_result.columns.size() + columns.size());
@@ -1046,14 +1046,14 @@ MergeTreeRangeReader::ReadResult MergeTreeRangeReader::read(size_t max_rows, Mar
             bool should_evaluate_missing_defaults;
             merge_tree_reader->fillMissingColumns(columns, should_evaluate_missing_defaults, read_result.num_rows);
 
-            /// If some columns absent in part, then evaluate default values
-            if (should_evaluate_missing_defaults)
-                merge_tree_reader->evaluateMissingDefaults({}, columns);
-
             /// If result not empty, then apply on-fly alter conversions if any required
             if (!prewhere_info || prewhere_info->perform_alter_conversions)
                 merge_tree_reader->performRequiredConversions(columns);
 
+            /// If some columns absent in part, then evaluate default values
+            if (should_evaluate_missing_defaults)
+                merge_tree_reader->evaluateMissingDefaults({}, columns);
+
             for (size_t i = 0; i < columns.size(); ++i)
                 read_result.columns[i] = std::move(columns[i]);
         }

src/Storages/MergeTree/MergeTreeSequentialSource.cpp

@@ -251,11 +251,11 @@ try
             bool should_evaluate_missing_defaults = false;
             reader->fillMissingColumns(columns, should_evaluate_missing_defaults, rows_read);
 
+            reader->performRequiredConversions(columns);
+
             if (should_evaluate_missing_defaults)
                 reader->evaluateMissingDefaults({}, columns);
 
-            reader->performRequiredConversions(columns);
-
             /// Reorder columns and fill result block.
             size_t num_columns = sample.size();
             Columns res_columns;

tests/queries/0_stateless/03144_alter_column_and_read.reference

@@ -0,0 +1,10 @@
+0	0_42
+1	1_42
+2	2_42
+3	3_42
+4	4_42
+5	5_42
+6	6_42
+7	7_42
+8	8_42
+9	9_42

tests/queries/0_stateless/03144_alter_column_and_read.sql

@@ -0,0 +1,11 @@
+drop table if exists tab;
+create table tab (x UInt32) engine = MergeTree order by tuple();
+
+insert into tab select number from numbers(10);
+
+set alter_sync = 0;
+alter table tab update x = x + sleepEachRow(0.1) where 1;
+alter table tab modify column x String;
+alter table tab add column y String default x || '_42';
+
+select x, y from tab order by x;