MDEV-39620: Incorrect error message when dropping a column with a combined unique index

[pranavktiwari]

fixes MDEV-39620

Problem:

Dropping a column that is part of a composite unique index returns the
misleading error ER_KEY_COLUMN_DOES_NOT_EXIST, suggesting that the
column is missing even though it exists in the table.

Cause:

During ALTER TABLE ... DROP COLUMN, the server validates the resulting
index definitions after removing the column. If an index still references
the dropped column, the validation reports ER_KEY_COLUMN_DOES_NOT_EXIST,
which reflects the intermediate table definition rather than the actual
reason for the failure.

Fix:

Replace ER_KEY_COLUMN_DOES_NOT_EXIST with
ER_CANT_DROP_FIELD_OR_KEY and include a message indicating that the
column cannot be dropped because it is part of an index, making the
reported error more descriptive for users.

[gemini-code-assist]

Code Review

This pull request updates the error handling when attempting to drop a column or key that is part of an index. In sql/sql_table.cc, the error code has been changed from ER_KEY_COLUMN_DOES_NOT_EXIST to ER_CANT_DROP_FIELD_OR_KEY, providing a more descriptive message. The corresponding test files and expected results in mysql-test/main/ have been updated to match this new behavior. There are no review comments to evaluate, and I have no additional feedback to provide.

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[Copilot]

Pull request overview

This PR addresses MDEV-39620 by changing the server error surfaced during ALTER TABLE ... DROP COLUMN when the column is still referenced by a (composite) index, and updates MTR expectations accordingly.

Changes:

• Updates mysql_prepare_alter_table() to emit ER_CANT_DROP_FIELD_OR_KEY instead of ER_KEY_COLUMN_DOES_NOT_EXIST for a dropped column that is still referenced by a unique/primary index.
• Adjusts multiple mysql-test suites to expect the new error code and message output.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
sql/sql_table.cc	Changes the emitted error when an index still references a dropped column.
mysql-test/suite/versioning/t/alter.test	Updates expected error code for DROP COLUMN cases involving indexes.
mysql-test/suite/versioning/r/alter.result	Updates expected error output text for those cases.
mysql-test/suite/innodb/t/innodb-table-online.test	Updates expected error code for an online ALTER scenario.
mysql-test/suite/innodb/t/innodb-index.test	Updates expected error code in several DROP COLUMN/index scenarios.
mysql-test/suite/innodb/r/innodb-table-online.result	Updates expected error output text.
mysql-test/suite/innodb/r/innodb-index.result	Updates expected error output text in several scenarios.
mysql-test/main/long_unique_bugs.test	Updates expected error code for a long-unique/index DROP COLUMN case.
mysql-test/main/long_unique_bugs.result	Updates expected error output text.
mysql-test/main/alter_table.test	Updates expected error code for a DROP COLUMN + composite unique index case.
mysql-test/main/alter_table.result	Updates expected error output text.

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[sanja-byelkin]

why existing tests was changed?

Also test fails looks related to the changes of existing tests