Assertion marked_for_read() failed in virtual my_time_t Field_timesta…

[pranavktiwari]

fixes MDEV-34992

Problem:

SELECT on a table with a TIMESTAMP column and a virtual column
expression containing a field reference crashes with assertion
marked_for_read() in Field_timestamp0::get_timestamp.

Cause:

convert_item_for_comparison() physically evaluates the subject item
via Datetime dt(thd, subject, ...) to attempt a TIMESTAMP vs DATETIME
optimization. During vcol_fix_expr() at open_table() time,
used_tables_cache is not yet propagated up the item tree, causing
const_item() to return true incorrectly for a non-constant expression
containing a field reference. The evaluation then reads Field_timestamp0
before read bitmaps are initialized, violating marked_for_read().

Fix:

Add find_field_processor() to the Item base class, overridden in
Item_field to return true. Guard convert_item_for_comparison() with
a walk() check to skip physical evaluation if the subject subtree
contains any field reference.

[gemini-code-assist]

Code Review

This pull request introduces a new processor check_field_ref_processor to check for field references in Item and Item_field classes, and updates Type_handler_datetime_common::convert_item_for_comparison to utilize it. The review feedback suggests breaking down a long, complex conditional statement in sql/sql_type.cc into multiple lines to improve readability.

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