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Should I use select for update?
The SELECT FOR UPDATE statement is used to order transactions by controlling concurrent access to one or more rows of a table. It works by locking the rows returned by a selection query, such that other transactions trying to access those rows are forced to wait for the transaction that locked the rows to finish.
Does select for update block read?
A SELECT FOR UPDATE reads the latest available data, setting exclusive locks on each row it reads.
Does update lock table mysql?
A locking read, an UPDATE , or a DELETE generally set record locks on every index record that is scanned in the processing of an SQL statement. It does not matter whether there are WHERE conditions in the statement that would exclude the row.
What is Gap lock in mysql?
Gap Locks. A gap lock is a lock on a gap between index records, or a lock on the gap before the first or after the last index record. For example, SELECT c1 FROM t WHERE c1 BETWEEN 10 and 20 FOR UPDATE; prevents other transactions from inserting a value of 15 into column t.
Why select for UPDATE is bad?
Answer: The select for update has many issues, and select for update is especially dangerous when a transaction aborts and a “zombie” process continues to hold rows locks. The select for update is not a good locking strategy because there are many things that can go wrong.
Is select for UPDATE bad?
So, nothing bad will happen if you run SELECT FOR UPDATE , but overall performance may be lower due to possible lock waits. Deadlocks are possible, too. If another transaction holds a shared lock and then wants to update the records while SELECT FOR UPDATE is waiting.
Why SELECT for update is bad?
Does select lock a table MySQL?
SELECTs do not normally do any locking that you care about on InnoDB tables. The default transaction isolation level means that selects don’t lock stuff.