Why does SELECT FOR UPDATE works only within a transaction?
According to the documentation:
Locking of rows for update using
SELECT FOR UPDATE
only applies when autocommit is disabled (either by beginning transaction withSTART TRANSACTION
or by setting autocommit to 0. If autocommit is enabled, the rows matching the specification are not locked.
In other words, if you don't execute your first SELECT FOR UPDATE
inside a transaction, no rows are locked.
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Cratylus
Updated on June 04, 2022Comments
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Cratylus about 2 years
I think I am confused with the SELECT FOR UPDATE construct.
Example:mysql> select * from employees2; +-------+----------+--------+-----------+ | EmpId | EmpName | DeptId | EmpSalary | +-------+----------+--------+-----------+ | 1 | John | 1 | 5000.00 | | 2 | Albert | 1 | 4500.00 | | 3 | Crain | 2 | 6000.00 | | 4 | Micheal | 2 | 5000.00 | | 5 | David | NULL | 34.00 | | 6 | Kelly | NULL | 457.00 | | 7 | Rudy | 1 | 879.00 | | 8 | Smith | 2 | 7878.00 | | 9 | Karsen | 5 | 878.00 | | 10 | Stringer | 5 | 345.00 | | 11 | Cheryl | NULL | NULL | +-------+----------+--------+-----------+ 11 rows in set (0.00 sec)
I do the following in a script:
#!/usr/bin/perl use strict; use warnings; use DBI; my $dbh = DBI->connect('dbi:mysql:testdb','root','1234', {'RaiseError' => 1, 'AutoCommit' => 0}) or die "Connection Error: $DBI::errstr\n"; my $sql = "select * from employees2 where EmpId IN (2,10) for update"; my $sth = $dbh->prepare($sql); $sth->execute or die "SQL Error: $DBI::errstr\n"; while (my @row = $sth->fetchrow_array) { print "@row\n"; } sleep(9000); $dbh->commit;
I also in parallel a console and connect to the database.
So I run the script first and then in another session I do:mysql> select * from employees2 where EmpId IN (10) for update;
The second select blocks as it refers to the same row.
This blocks either I do:mysql> set autocommit = 0; mysql> begin; mysql> select * from employees2 where EmpId IN (10) for update; mysql> commit;
or just
mysql> select * from employees2 where EmpId IN (10) for update;
So it blocks irrelevant if it is in a transaction or not.
Now if I change the script as:my $dbh = DBI->connect('dbi:mysql:practice','root','') or die "Connection Error: $DBI::errstr\n";
I.e the script does not run within a transaction the second session does not block!
Why does it block only if the script runs within a transaction?-
ThisSuitIsBlackNot about 10 yearsTo answer your new question:
SELECT ... FOR UPDATE
only locks rows until you commit the transaction in which it is executed.autocommit
is enabled by default, so when you don't explicitly disable it, every SQL statement you execute forms a separate transaction (with InnoDB, that is). In other words, your firstSELECT ... FOR UPDATE
is executed and then immediately committed, releasing the locks.
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91DarioDev over 4 yearsWhat about if instead it’s a single query but the for update is part of a subquery of an update query? In that case the query is only one and not in a transaction but I guess it should work because the commit is done only after the update statement. And it is the parent of the select subquery having the for update