[BugFix] fix cloud native pk index memory statistic leak

[luohaha]

Why I'm doing:

In the current cloud-native PK index memtable implementation, we only account for the memory usage of keys and values. However, since we're using a B-tree for storage, the memory consumption of the B-tree's internal nodes is not being accurately tracked.
The B-tree provides a bytes_used interface to retrieve its total memory usage. However, since keys use std::string, the heap memory allocated by these strings isn't included in the bytes_used count. We need to implement additional tracking for this memory.

What I'm doing:

This pull request refactors the PersistentIndexMemtable implementation in be/src/storage/lake/persistent_index_memtable.cpp to improve memory tracking and efficiency. The changes include replacing std::string with std::string_view for key handling, introducing a new method to check heap allocation for strings, and updating memory usage calculations.

Refactoring for Memory Efficiency:

• Replaced std::string with std::string_view for keys in methods like upsert, insert, erase, erase_with_filter, replace, and get to reduce unnecessary string allocations. ([[1]](https://github.com/StarRocks/starrocks/pull/60566/files#diff-2a0d7738bf81a53d3c0561225e77a7ddfb6f5aec5f831a03c4b0371715c04728L33-R38), [[2]](https://github.com/StarRocks/starrocks/pull/60566/files#diff-2a0d7738bf81a53d3c0561225e77a7ddfb6f5aec5f831a03c4b0371715c04728L54-R65), [[3]](https://github.com/StarRocks/starrocks/pull/60566/files#diff-2a0d7738bf81a53d3c0561225e77a7ddfb6f5aec5f831a03c4b0371715c04728L87-R91), [[4]](https://github.com/StarRocks/starrocks/pull/60566/files#diff-2a0d7738bf81a53d3c0561225e77a7ddfb6f5aec5f831a03c4b0371715c04728L113-R115), [[5]](https://github.com/StarRocks/starrocks/pull/60566/files#diff-2a0d7738bf81a53d3c0561225e77a7ddfb6f5aec5f831a03c4b0371715c04728L130-R135), [[6]](https://github.com/StarRocks/starrocks/pull/60566/files#diff-2a0d7738bf81a53d3c0561225e77a7ddfb6f5aec5f831a03c4b0371715c04728L179-R180))
• Added a new function is_string_heap_allocated in be/src/util/string_util.cpp to determine whether a std::string uses heap memory, and updated _keys_heap_size calculations accordingly. ([[1]](https://github.com/StarRocks/starrocks/pull/60566/files#diff-58984ee0cc1ce8771537ac8af37a9bb3447abc880f413364470f5bbfc989a265R51-R58), [[2]](https://github.com/StarRocks/starrocks/pull/60566/files#diff-987ba58807d358f61be1b8ea0c619122113532bdb5b574f922986a6a79eada42R79))
• Modified PersistentIndexMemtable::memory_usage() to use _keys_heap_size and _map.bytes_used() for more accurate memory usage tracking. ([be/src/storage/lake/persistent_index_memtable.cppL179-R180](https://github.com/StarRocks/starrocks/pull/60566/files#diff-2a0d7738bf81a53d3c0561225e77a7ddfb6f5aec5f831a03c4b0371715c04728L179-R180))
• Renamed _keys_size to _keys_heap_size and reset it in the clear() method to ensure proper memory management. ([[1]](https://github.com/StarRocks/starrocks/pull/60566/files#diff-0fa7a9fa792819b27a25ea176c8c8154af0ed664f12dfd466c8ed781e304b2aeL81-R81), [[2]](https://github.com/StarRocks/starrocks/pull/60566/files#diff-2a0d7738bf81a53d3c0561225e77a7ddfb6f5aec5f831a03c4b0371715c04728R189))

Testing Enhancements:

• Added a unit test test_is_string_heap_allocated in be/test/util/string_util_test.cpp to verify the behavior of the is_string_heap_allocated function for both small and large strings. ([be/test/util/string_util_test.cppR107-R115](https://github.com/StarRocks/starrocks/pull/60566/files#diff-ca9144c8e70f0db720811da5c0d9413dc7a706795fc7ae268380b3cd55f669b0R107-R115))

What type of PR is this:

• BugFix
• Feature
• Enhancement
• Refactor
• UT
• Doc
• Tool

Does this PR entail a change in behavior?

• Yes, this PR will result in a change in behavior.
• No, this PR will not result in a change in behavior.

If yes, please specify the type of change:

• Interface/UI changes: syntax, type conversion, expression evaluation, display information
• Parameter changes: default values, similar parameters but with different default values
• Policy changes: use new policy to replace old one, functionality automatically enabled
• Feature removed
• Miscellaneous: upgrade & downgrade compatibility, etc.

Checklist:

• I have added test cases for my bug fix or my new feature
• This pr needs user documentation (for new or modified features or behaviors)
  • I have added documentation for my new feature or new function
• This is a backport pr

Bugfix cherry-pick branch check:

• I have checked the version labels which the pr will be auto-backported to the target branch
  • 3.5
  • 3.4
  • 3.3

[Copilot]

Pull Request Overview

This PR refactors PersistentIndexMemtable to accurately track heap memory usage for keys by switching from std::string to std::string_view, adding a helper to detect heap-allocated strings, and updating memory accounting and tests.

• Replace std::string conversions with std::string_view in all mutating and lookup methods to avoid unnecessary allocations.
• Introduce is_string_heap_allocated to detect when a std::string uses heap memory and update _keys_heap_size accordingly.
• Update memory_usage() and clear() to include heap key sizes, and add unit tests for is_string_heap_allocated.

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
be/src/util/string_util.h	Declare is_string_heap_allocated helper
be/src/util/string_util.cpp	Define heap-allocation detection logic
be/test/util/string_util_test.cpp	Add unit test for is_string_heap_allocated
be/src/storage/lake/persistent_index_memtable.h	Rename _keys_size to _keys_heap_size
be/src/storage/lake/persistent_index_memtable.cpp	Use string_view, update heap-size tracking, memory_usage(), and clear()

[github-actions]

[FE Incremental Coverage Report]

✅ pass : 0 / 0 (0%)

[github-actions]

[Java-Extensions Incremental Coverage Report]

✅ pass : 0 / 0 (0%)

[github-actions]

[BE Incremental Coverage Report]

✅ pass : 19 / 20 (95.00%)

file detail

	path	covered_line	new_line	coverage	not_covered_line_detail
🔵	be/src/storage/lake/persistent_index_memtable.cpp	15	16	93.75%	[165]
🔵	be/src/util/string_util.cpp	4	4	100.00%	[]