[GitHub] [doris] BiteTheDDDDt commented on a diff in pull request #13273: [improvement](schema change) avoid using column ptr swap

[GitBox <gi...@apache.org>]

BiteTheDDDDt commented on code in PR #13273:
URL: https://github.com/apache/doris/pull/13273#discussion_r995308928


##########
be/src/olap/schema_change.cpp:
##########
@@ -834,8 +834,34 @@ Status RowBlockChanger::change_block(vectorized::Block* ref_block,
     }
 
     for (auto it : swap_idx_map) {
-        new_block->get_by_position(it.second).column.swap(
-                ref_block->get_by_position(it.first).column);
+        auto& ref_col = ref_block->get_by_position(it.first);
+        auto& new_col = new_block->get_by_position(it.second);
+
+        bool ref_col_nullable = ref_col.column->is_nullable();
+        bool new_col_nullable = new_col.column->is_nullable();
+
+        if (ref_col_nullable != new_col_nullable) {
+            // not nullable to nullable
+            if (new_col_nullable) {
+                auto* new_nullable_col = assert_cast<vectorized::ColumnNullable*>(
+                        std::move(*new_col.column).mutate().get());
+
+                new_nullable_col->swap_nested_column(ref_col.column);
+                new_nullable_col->get_null_map_data().resize_fill(new_nullable_col->size());
+            } else {
+                // nullable to not nullable:

Review Comment:
   It is best to add a logic to check that the nullmap does not contain null. If there is null data, the schema change will fail to prevent accidental data loss.



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