Deadlocks in MySQL

A deadlock occurs when two or more transactions are waiting for each other
to release locks, creating a cycle that prevents any of them from proceeding.
This situation can severely impact performance and data consistency
in high-concurrency environments.

Basic Scenario

  • Transaction A locks row 1 and waits for row 2.
  • Transaction B locks row 2 and waits for row 1.
  • Both transactions are now stuck, waiting for the other to release its lock.

MySQL’s InnoDB engine automatically detects deadlocks and resolves them by
rolling back one of the transactions, allowing the other to proceed.

Common Causes of Deadlocks

  • Inconsistent Locking Order: Transactions acquire locks in different sequences across tables or rows.
  • High Concurrency: Multiple sessions accessing shared resources simultaneously increases collision risk.
  • Long-Running Transactions: Holding locks for extended periods blocks other transactions.
  • Foreign Key Constraints: Cascading updates or deletes across related tables can trigger deadlocks.
  • Complex Joins or Subqueries: Queries involving multiple tables with overlapping locks can create circular dependencies.
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