Why is table-level locking better than row-level locking for large tables?

Question

According to the MySQL manual:

For large tables, table locking is often better than row locking,

Why is this? I would presume that row-level locking is better because when you lock on a larger table, you're locking more data.

Answer 1

• Row locking needs more memory than table or page level locking.

• Have to acquire many more locks with row locking, which expends more resources

From http://www.devshed.com/c/a/MySQL/MySQL-Optimization-part-2/

• Advantages of row-level locking:

  • Fewer lock conflicts when accessing different rows in many threads.
  • Fewer changes for rollbacks.
  • Makes it possible to lock a single row a long time.

• Disadvantages of row-level locking:

  • Takes more memory than page-level or table-level locks.
  • Is slower than page-level or table-level locks when used on a large part of the table because you must acquire many more locks.
  • Is definitely much worse than other locks if you often do GROUP BY operations on a large part of the data or if you often must scan the entire table.
  • With higher-level locks, you can also more easily support locks of different types to tune the application, because the lock overhead is less than for row-level locks.

• Table locks are superior to page-level or row-level locks in the following cases:

  • Most statements for the table are reads.
  • Read and updates on strict keys, where you update or delete a row that can be fetched with a single key read: UPDATE tbl_name SET column=value WHERE unique_key_col=key_value; DELETE FROM tbl_name WHERE unique_key_col=key_value;
  • SELECT combined with concurrent INSERT statements, and very few UPDATE and DELETE statements.
  • Many scans or GROUP BY operations on the entire table without any writers.

Answer 2

from the (pre-edit) link

Slower than page-level or table-level locks when used on a large part of the table because you must acquire many more locks

use a row level lock if you are only hitting a row or two. If your code hits many or unknown rows, stick with table lock.