fix

be/src/vec/columns/column_string.cpp

@@ -90,12 +90,18 @@ void ColumnStr<T>::shrink_padding_chars() {
     auto* offset = offsets.data();
     size_t size = offsets.size();
 
+    // deal the 0-th element. no need to move.
+    auto next_start = offset[0];
     offset[0] = strnlen(data, size_at(0));
     for (size_t i = 1; i < size; i++) {
-        auto length = strnlen(get_data_at(i).data, size_at(i));
-        memmove(data + offset[i-1], get_data_at(i).data, length);
-        offset[i] = length;
-    }
+        // get the i-th length and whole move it to cover the last's trailing void
+        auto length = strnlen(data + next_start, offset[i] - next_start);
+        memmove(data + offset[i - 1], data + next_start, length);
+        // offset i will be changed. so save the old value for (i+1)-th to get its length.
+        next_start = offset[i];
+        offset[i] = offset[i - 1] + length;
+    }
+    chars.resize_fill(offsets.back()); // just call it to shrink memory here. no possible to expand.
 }
 
 // This method is only called by MutableBlock::merge_ignore_overflow

be/src/vec/columns/column_string.h

@@ -85,6 +85,7 @@ class ColumnStr final : public COWHelper<IColumn, ColumnStr<T>> {
     /// For convenience, every string ends with terminating zero byte. Note that strings could contain zero bytes in the middle.
     Chars chars;
 
+    // Start position of i-th element.
     size_t ALWAYS_INLINE offset_at(ssize_t i) const { return offsets[i - 1]; }
 
     /// Size of i-th element, including terminating zero.

be/test/vec/columns/column_string_test.cpp

@@ -0,0 +1,48 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+#include "vec/columns/column_string.h"
+
+#include <gmock/gmock-more-matchers.h>
+#include <gtest/gtest.h>
+
+#include "vec/common/string_ref.h"
+#include "vec/core/types.h"
+
+using namespace doris;
+using namespace doris::vectorized;
+
+TEST(ColumnStringTest, shrink_padding_chars) {
+    ColumnString::MutablePtr col = ColumnString::create();
+    col->insert_data("123\0   ", 7);
+    col->insert_data("456\0xx", 6);
+    col->insert_data("78", 2);
+    col->shrink_padding_chars();
+
+    EXPECT_EQ(col->size(), 3);
+    EXPECT_EQ(col->get_data_at(0), StringRef("123"));
+    EXPECT_EQ(col->get_data_at(0).size, 3);
+    EXPECT_EQ(col->get_data_at(1), StringRef("456"));
+    EXPECT_EQ(col->get_data_at(1).size, 3);
+    EXPECT_EQ(col->get_data_at(2), StringRef("78"));
+    EXPECT_EQ(col->get_data_at(2).size, 2);
+
+    col->insert_data("xyz", 2); // only xy
+
+    EXPECT_EQ(col->size(), 4);
+    EXPECT_EQ(col->get_data_at(3), StringRef("xy"));
+}