Paul McCullagh writes:

On Mar 25, 2010, at 8:39 AM, Kristian Nielsen wrote:

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Yes. This is simple enough to do with DBUG. Just insert code that makes each engine fail in their prepare() when the appropriate DBUG flag is set.

Another test we need is to have similar code to crash the server at the same points. Then on server restart check that the other engine does rollback.

It would be great to have these tests. As Arjen says, this code is not well traveled.

An example for how to do this (from a random dive into the source tree) is in mysql-test/suite/maria/t/maria-recovery.test, which uses mysql-test/include/maria_verify_recovery.inc to crash the server at specific point and verify that crash recovery works. It shouldn't be hard to do something similar for this case (also with just commit fail instead of crash). Hopefully this could be useful if someone wants to implement such test.

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(For example current code has no protection against another thransaction seeing a transient state with one engine committed and another not, even using START TRANSACTION WITH CONSISTENT SNAPSHOT. And there are fundamentally unsolvable problems with transactions that span both MVCC- and lock- based engines).

I would be interested in an actual example of something does not work. Right now I have a problem imagining why something would not work.

I just happened to run the following test two days ago: Consider the following tables create table t1 (a int primary key) engine=innodb create table t2 (b int primary key) engine=pbxt insert into t1 values (1) insert into t2 values (1) I run the following statement repeatedly in one thread: UPDATE t1,t2 SET a=a+1,b=b+1 It would be natural to assume that other threads will never be able to see different values for a and b in a single transaction, but that assumption would be wrong. I run the following statement repeatedly in a different thread: SELECT a,b FROM t1,t2 After just a few iterations, this will return a row with a=b+1. (The reason I did this test was that I was looking at the XA multi-engine code, and not seeing any code to enforce cross-engine consistency. I guess this test shows there just is no such code ...) I didn't report it as a bug, as I was not sure if it is a bug or not ... maybe it is? I'd want a better fix than just taking a global lock over every commit (which could hurt performance a lot), and such fix may be non-trivial... Here are the Perl long-liners I used to see this, just run them in parallel to see the failure: perl -MDBI -le 'use strict; my $dbh=DBI->connect("dbi:mysql:database=test", "<user>", "<password>", {RaiseError => 1}); $dbh->do("SET binlog_format=row"); $dbh->do($_) for ("drop table if exists t1,t2", "create table t1 (a int primary key) engine=innodb", "create table t2 (b int primary key) engine=pbxt", "begin", "insert into t1 values (1)", "insert into t2 values (1)", "commit"); for (;;) { $dbh->do("UPDATE t1,t2 SET a=a+1,b=b+1");}' perl -MDBI -le 'use strict; my $dbh=DBI->connect("dbi:mysql:database=test", "<user>", "<password>", {RaiseError => 1}); $dbh->do("SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ"); for(;;) {my $a=$dbh->selectrow_arrayref("SELECT a,b FROM t1,t2"); print join(" ", @$a); die if $a->[0] != $a->[1];}' (CONSISTENT SNAPSHOT does not make a difference, as is seen by replacing the second command with this: perl -MDBI -le 'use strict; my $dbh=DBI->connect("dbi:mysql:database=test", "<user>", "<password>", {RaiseError => 1}); $dbh->do("SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ"); for(;;) {$dbh->do("START TRANSACTION WITH CONSISTENT SNAPSHOT");my $a=$dbh->selectrow_arrayref("SELECT a,b FROM t1,t2"); print join(" ", @$a); die if $a->[0] != $a->[1];$dbh->do("COMMIT");}' ) With respect to the fundamental problems with combining MVCC and locking engines; I know I reported a documentation bug for this long ago, though google failed to find it for me. And I'm not sure how to repeat it now, as I don't have any non-mvcc engines easily available (MariaDB has no NDB, and BDB is gone also). But it goes something like this: In a locking engine, a transaction sees the (consistent) state of the database as it is at the *end* of the transactions. So the transaction can see all other transactions that committed before it did. In an mvcc engine, a transaction sees the (consistent) state of the database as it is at the *start* of the transaction. So it sees no transactions that started after it did. So to get an inconsistency, something like this should work: TRN1: SET TRANSACTION ISOLATION LEVEL REPEATABLE READ; TRN1: BEGIN; TRN1: SELECT * FROM mvcc_table; TRN2: BEGIN; TRN2: UPDATE mvcc_table SET amount - amount - 100; TRN2: UPDATE locking_table SET amount = amount + 100; TRN2: COMMIT; TRN1: SELECT * from locking_table; TRN1: COMMIT; In such a case, TRN1 will see the update of the locking_table, but not the update of the mvcc_table, which gives an inconsistent view of the database. I don't really see any way to solve this. (Of course one could add LOCK IN SHARE MODE to every select, in effect turning the mvcc engine into a locking engine). - Kristian.