MDEV-36896 - Assertion `marked_for_read()' failed in virtual String *Field_varstring::val_str(String *, String *)

[pranavktiwari]

fixes MDEV-36896

Problem:

Executing queries that require virtual/generated column evaluation during filesort trigger sdebug assertions due to missing columns in read_set.

Cause:

find_all_keys() temporarily assigns TABLE::tmp_set as both read_set and write_set. Later, TABLE::update_virtual_field() clears tmp_set before evaluating virtual column dependencies.

Since all three pointers reference the same bitmap, clearing tmp_set also clears the active column maps, causing required columns to be missing during execution.

Fix:

Remove the bitmap_clear_all(&tmp_set) call from TABLE::update_virtual_field(). The dependency walk populates the required bits for virtual column evaluation, and clearing the shared bitmap can unintentionally invalidate the active read_set/write_set.

[gemini-code-assist]

Code Review

This pull request comments out the column_bitmaps_set call in sql/filesort.cc to address an assertion failure. However, the reviewer notes that this is incorrect because it bypasses the temporary read map optimization and pollutes the original read set. The recommended approach is to identify the missing field and ensure it is properly registered instead of disabling the bitmap switch.