MDEV-36990 : SIGFPE in get_max_range_rowid_filter_elems_for_table

[pranavktiwari]

fixes MDEV-36990

Problem:

ALTER TABLE allowing a column defined as BINARY(0) to be used as part of a PRIMARY KEY or UNIQUE KEY can lead to a zero-length index column. This results in tab->file->ref_length becoming 0 inside the storage engine, which later causes a division-by-zero crash in get_max_range_rowid_filter_elems_for_table() during optimizer cost estimation for range queries.

Cause:

During key initialization in init_key_part_spec(), validation relied on column->flags & NOT_NULL_FLAG to detect non-nullable key columns. However, in ALTER TABLE ... CHANGE COLUMN, the Create_field is only partially initialized and does not yet propagate implicit constraints such as PRIMARY KEY ⇒ NOT NULL. As a result, NOT_NULL_FLAG may be 0 even for primary key columns, allowing key_part_length == 0 cases to bypass validation.

This leads to invalid index definitions where a primary key column contributes zero bytes to the index.

Fix:

Strengthen key validation in init_key_part_spec() by rejecting zero-length
key parts for columns that are effectively non-nullable in key context,
including PRIMARY KEY columns whose implicit NOT NULL property may not yet
be reflected in Create_field::flags during ALTER TABLE processing.

This prevents creation of invalid zero-length index definitions and avoids
propagation of ref_length=0 metadata that can later trigger optimizer
crashes.

[gemini-code-assist]

Code Review

This pull request modifies the key part specification initialization in sql/sql_table.cc to include primary keys in the condition check when key_part_length is zero. The feedback suggests adding explicit parentheses around the bitwise AND operation to clarify operator precedence and prevent potential compiler warnings when combined with the logical OR operator.

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