When working with large databases, ALTER TABLE operations can take significant time to complete. A critical scenario occurs when these operations are interrupted, particularly during index creation. MySQL handles this situation through a specific kill flag mechanism.

The database engine checks the kill flag before processing each block of rows during ALTER TABLE operations. This design allows for relatively safe interruption of long-running schema modifications. Here's the internal workflow:

while (processing_rows) {
    check_kill_flag();
    if (kill_flag_set) {
        rollback_operation();
        drop_temp_table();
        return ERROR;
    }
    process_row_block();
}

When you terminate an ALTER TABLE operation mid-execution:

• The temporary table created during the operation is automatically deleted
• The original table remains unchanged and intact
• No partial schema modifications persist
• The operation must be restarted from scratch

Consider this common scenario where we add an index to a large table:

-- Original command
ALTER TABLE large_transactions ADD INDEX idx_customer_id (customer_id);

-- In another session
SHOW PROCESSLIST;
KILL [process_id];

After killing the process, you can verify the table status:

SHOW INDEX FROM large_transactions;
SELECT COUNT(*) FROM information_schema.innodb_trx;

If you accidentally kill an important ALTER TABLE operation:

1. Check for any lingering temporary tables with SHOW TABLES LIKE '#sql%'
2. Verify table integrity with CHECK TABLE tablename
3. Monitor disk space as interrupted operations may leave temporary files
4. Consider using online DDL (ALTER TABLE ... ALGORITHM=INPLACE) for safer modifications

For large tables, these best practices minimize interruption risks:

• Use pt-online-schema-change for zero-downtime alterations
• Schedule DDL during low-traffic periods
• Monitor long-running transactions with performance_schema
• Consider using gh-ost for very large tables

When executing ALTER TABLE operations in MySQL, particularly index creation, the database engine periodically checks for kill flags between processing blocks of rows. This safety mechanism ensures that interrupting such operations doesn't immediately corrupt your data.

-- Example of a long-running ALTER TABLE operation
ALTER TABLE large_dataset ADD INDEX idx_customer_name (customer_name);
-- If killed during execution, MySQL will clean up temporary resources

MySQL's documentation clearly states the behavior when killing ALTER TABLE:

• The operation stops at the next safe checkpoint
• Any temporary table created for the operation is deleted
• The original table remains intact in its pre-ALTER state
• No partial indexes or corrupted metadata remains

Consider these practical recovery steps if you've killed an index operation:

-- 1. Verify table status
SHOW TABLE STATUS LIKE 'your_table';

-- 2. Check for ongoing operations
SHOW PROCESSLIST;

-- 3. Re-run the ALTER statement if needed
ALTER TABLE your_table ADD INDEX idx_recovery (column_name);

While killing the operation won't corrupt data, be aware of:

• Disk space usage during temporary table creation
• Performance impact from rollback operations
• Table locks that may persist until cleanup completes

For production environments with large tables:

-- Use pt-online-schema-change for minimal downtime
pt-online-schema-change --alter "ADD INDEX idx_name (name)" D=database,t=table

-- Or use MySQL 8.0+ instant ADD COLUMN feature
ALTER TABLE orders ADD COLUMN priority TINYINT DEFAULT 5, ALGORITHM=INSTANT;

Implement proper monitoring to avoid unexpected kills:

-- Check progress in MySQL 5.7+
SELECT * FROM performance_schema.events_stages_current
WHERE EVENT_NAME LIKE 'stage/innodb/alter%';

-- For MariaDB users
SHOW PROFILE;

Remember that while MySQL handles killed ALTER operations gracefully, preventing unnecessary interruptions should be part of your database maintenance strategy.