[fix](be) struct_element(stru, 'str') offset optimization bug

be/src/core/column/column_string.h

@@ -298,7 +298,10 @@ class ColumnStr final : public COWHelper<IColumn, ColumnStr<T>> {
             running_offset += lengths[i];
             offsets_ptr[i] = static_cast<T>(running_offset);
         }
-        chars.resize(offsets[old_rows + num - 1]);
+        // OFFSET_ONLY columns carry valid offsets but no real string payload. Use non-zero
+        // placeholders so char-padding shrink logic cannot recompute these offsets as zero-length
+        // strings when this column is nested under a struct that also contains CHAR fields.
+        chars.resize_fill(offsets[old_rows + num - 1], 1);
     }
 
     void insert_many_strings(const StringRef* strings, size_t num) override {

be/src/exprs/function/function_struct_element.cpp

@@ -101,9 +101,9 @@ class FunctionStructElement : public IFunction {
         size_t index;
         RETURN_IF_ERROR(get_element_index(*struct_type, index_column, index_type, &index));
         ColumnPtr res_column = struct_col->get_column_ptr(index);
-        ColumnPtr ele_column = res_column->clone_resized(res_column->size());
-        //This function must return a ColumnNullable column, so it is necessary to convert the result column into ColumnNullable.
-        block.replace_by_position(result, make_nullable(ele_column));
+        // This function must return a ColumnNullable column, so it is necessary to convert the
+        // result column into ColumnNullable.
+        block.replace_by_position(result, make_nullable(res_column));
         return Status::OK();
     }
 

regression-test/data/nereids_rules_p0/column_pruning/string_length_column_pruning.out

@@ -1,4 +1,22 @@
 -- This file is automatically generated. You should know what you did if you want to edit this
+-- !struct_offset_group_min_by --
+\N	v6
+1	v0
+2	v1
+3	v2
+4	v3
+5	v4
+6	v5
+
+-- !struct_offset_group_count --
+\N	1
+1	1
+2	2
+3	2
+4	2
+5	1
+6	1
+
 -- !array_full_access_strips_offset --
 1	3	[1, 2, 3]
 

regression-test/suites/nereids_rules_p0/column_pruning/string_length_column_pruning.groovy

@@ -90,6 +90,53 @@ suite("string_length_column_pruning") {
         notContains "type=bigint"
     }
     sql "select length(struct_element(struct_col, 'f3')) from slcp_str_tbl"
+
+    sql """ DROP TABLE IF EXISTS slcp_struct_offset_group_tbl """
+    sql """
+        CREATE TABLE slcp_struct_offset_group_tbl (
+            id INT,
+            val STRING,
+            s STRUCT<c: CHAR(10), b: STRING>
+        ) ENGINE = OLAP
+        DUPLICATE KEY(id)
+        DISTRIBUTED BY HASH(id) BUCKETS 3
+        PROPERTIES ("replication_allocation" = "tag.location.default: 1")
+    """
+    sql """
+        INSERT INTO slcp_struct_offset_group_tbl VALUES
+            (0, 'v0', named_struct('c', 'x', 'b', 'a')),
+            (1, 'v1', named_struct('c', 'x', 'b', 'bb')),
+            (2, 'v2', named_struct('c', 'x', 'b', 'ccc')),
+            (3, 'v3', named_struct('c', 'x', 'b', 'dddd')),
+            (4, 'v4', named_struct('c', 'x', 'b', 'eeeee')),
+            (5, 'v5', named_struct('c', 'x', 'b', 'ffffff')),
+            (6, 'v6', named_struct('c', 'x', 'b', NULL)),
+            (7, 'v7', named_struct('c', 'x', 'b', 'gg')),
+            (8, 'v8', named_struct('c', 'x', 'b', 'hhhh')),
+            (9, 'v9', named_struct('c', 'x', 'b', 'iii'))
+    """
+    explain {
+        sql """
+            select length(struct_element(s, 'b')), min_by(val, id)
+            from slcp_struct_offset_group_tbl
+            group by 1
+        """
+        contains "nested columns"
+        contains "s.b.OFFSET"
+    }
+    order_qt_struct_offset_group_min_by """
+        select length(struct_element(s, 'b')), min_by(val, id)
+        from slcp_struct_offset_group_tbl
+        group by 1
+        order by 1, 2
+    """
+    order_qt_struct_offset_group_count """
+        select length(struct_element(s, 'b')), count(*)
+        from slcp_struct_offset_group_tbl
+        group by 1
+        order by 1
+    """
+
     // length() in both SELECT and WHERE: predicate must remain length(str_col) > 1,
     // never be rewritten to CAST(str_col AS int) > 1. Slot type must stay varchar.
     explain {