Does update lock the row or the entire table?

Question

Basic question: When I update a row in a database table, does it lock the current row or the entire table?

I am using MySQL. When I update column which is not unique.

Is the locking different from one database engine to another? I have read this article but I am still confused.

Answer 1

The links go into gory details, but this question seems to need a simple yes/no answer.

For ENGINE=MyISAM or MEMORY, the only lock is a table lock.

For ENGINE=InnoDB:

Think of it this way -- It locks every row it had to look at.

• No index on the column -- It had to check every row, so all rows are locked. That effectively locks the entire table.

• UNIQUE index on the column -- Only one row need be touched, hence, locked.

• In between... A non-unique INDEX on the column -- It must lock all the rows with that value. (Possibly the 'next' row will get locked, too.)

PS: the PRIMARY KEY is a UNIQUE index in MySQL.

Some other vendors have different index definitions, and some do have "table locks". Some "escalate" a bunch of row locks to a "table lock".

Bottom line:

• Add an INDEX, either UNIQUE or not, as appropriate.
• If you need to look at the row before potentially UPDATEing it, use a transaction and SELECT ... FOR UPDATE.