diff --git a/be/src/vec/functions/function_string.cpp b/be/src/vec/functions/function_string.cpp
index f98589b1965ccc..9f37d9d8d05d09 100644
--- a/be/src/vec/functions/function_string.cpp
+++ b/be/src/vec/functions/function_string.cpp
@@ -740,16 +740,17 @@ struct StringSpace {
                          ColumnString::Offsets& res_offsets) {
         res_offsets.resize(data.size());
         size_t input_size = res_offsets.size();
-        std::vector<char, Allocator_<char>> buffer;
+        // sample to get approximate best reserve size
+        if (input_size > 4) {
+            res_data.reserve(((data[0] + data[input_size >> 1] + data[input_size >> 2] +
+                               data[input_size - 1]) >>
+                              2) *
+                             input_size);
+        }
         for (size_t i = 0; i < input_size; ++i) {
-            buffer.clear();
-            if (data[i] > 0) {
-                buffer.resize(data[i]);
-                for (size_t j = 0; j < data[i]; ++j) {
-                    buffer[j] = ' ';
-                }
-                StringOP::push_value_string(std::string_view(buffer.data(), buffer.size()), i,
-                                            res_data, res_offsets);
+            if (data[i] > 0) [[likely]] {
+                res_data.resize_fill(res_data.size() + data[i], ' ');
+                res_offsets[i] = res_data.size();
             } else {
                 StringOP::push_empty_string(i, res_data, res_offsets);
             }
diff --git a/regression-test/suites/nereids_function_p0/scalar_function/S.groovy b/regression-test/suites/nereids_function_p0/scalar_function/S.groovy
index 98a8685a88840b..ea0d80d8adce85 100644
--- a/regression-test/suites/nereids_function_p0/scalar_function/S.groovy
+++ b/regression-test/suites/nereids_function_p0/scalar_function/S.groovy
@@ -126,6 +126,7 @@ suite("nereids_scalar_fn_S") {
 
 	sql "select space(10) from fn_test order by kint"
 	sql "select space(10) from fn_test_not_nullable order by kint"
+	sql """select k from (select length(space(number)) k from numbers("number" = "10"))t;""" // before #44919 will crash
 	qt_sql_split_part_Varchar_Varchar_Integer "select split_part(kvchrs1, ' ', 1) from fn_test order by kvchrs1"
 	qt_sql_split_part_Varchar_Varchar_Integer_notnull "select split_part(kvchrs1, ' ', 1) from fn_test_not_nullable order by kvchrs1"
 	qt_sql_split_part_String_String_Integer "select split_part(kstr, ' ', 1) from fn_test order by kstr"