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Until now, our Serializable mode has in fact been what's called Snapshot Isolation, which allows some anomalies that could not occur in any serialized ordering of the transactions. This patch fixes that using a method called Serializable Snapshot Isolation, based on research papers by Michael J. Cahill (see README-SSI for full references). In Serializable Snapshot Isolation, transactions run like they do in Snapshot Isolation, but a predicate lock manager observes the reads and writes performed and aborts transactions if it detects that an anomaly might occur. This method produces some false positives, ie. it sometimes aborts transactions even though there is no anomaly. To track reads we implement predicate locking, see storage/lmgr/predicate.c. Whenever a tuple is read, a predicate lock is acquired on the tuple. Shared memory is finite, so when a transaction takes many tuple-level locks on a page, the locks are promoted to a single page-level lock, and further to a single relation level lock if necessary. To lock key values with no matching tuple, a sequential scan always takes a relation-level lock, and an index scan acquires a page-level lock that covers the search key, whether or not there are any matching keys at the moment. A predicate lock doesn't conflict with any regular locks or with another predicate locks in the normal sense. They're only used by the predicate lock manager to detect the danger of anomalies. Only serializable transactions participate in predicate locking, so there should be no extra overhead for for other transactions. Predicate locks can't be released at commit, but must be remembered until all the transactions that overlapped with it have completed. That means that we need to remember an unbounded amount of predicate locks, so we apply a lossy but conservative method of tracking locks for committed transactions. If we run short of shared memory, we overflow to a new "pg_serial" SLRU pool. We don't currently allow Serializable transactions in Hot Standby mode. That would be hard, because even read-only transactions can cause anomalies that wouldn't otherwise occur. Serializable isolation mode now means the new fully serializable level. Repeatable Read gives you the old Snapshot Isolation level that we have always had. Kevin Grittner and Dan Ports, reviewed by Jeff Davis, Heikki Linnakangas and Anssi Kääriäinen
138 lines
3.3 KiB
SQL
138 lines
3.3 KiB
SQL
--
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-- PREPARED TRANSACTIONS (two-phase commit)
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--
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-- We can't readily test persistence of prepared xacts within the
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-- regression script framework, unfortunately. Note that a crash
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-- isn't really needed ... stopping and starting the postmaster would
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-- be enough, but we can't even do that here.
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-- create a simple table that we'll use in the tests
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CREATE TABLE pxtest1 (foobar VARCHAR(10));
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INSERT INTO pxtest1 VALUES ('aaa');
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-- Test PREPARE TRANSACTION
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BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE;
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UPDATE pxtest1 SET foobar = 'bbb' WHERE foobar = 'aaa';
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SELECT * FROM pxtest1;
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PREPARE TRANSACTION 'foo1';
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SELECT * FROM pxtest1;
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-- Test pg_prepared_xacts system view
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SELECT gid FROM pg_prepared_xacts;
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-- Test ROLLBACK PREPARED
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ROLLBACK PREPARED 'foo1';
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SELECT * FROM pxtest1;
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SELECT gid FROM pg_prepared_xacts;
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-- Test COMMIT PREPARED
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BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE;
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INSERT INTO pxtest1 VALUES ('ddd');
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SELECT * FROM pxtest1;
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PREPARE TRANSACTION 'foo2';
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SELECT * FROM pxtest1;
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COMMIT PREPARED 'foo2';
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SELECT * FROM pxtest1;
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-- Test duplicate gids
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BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE;
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UPDATE pxtest1 SET foobar = 'eee' WHERE foobar = 'ddd';
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SELECT * FROM pxtest1;
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PREPARE TRANSACTION 'foo3';
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SELECT gid FROM pg_prepared_xacts;
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BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE;
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INSERT INTO pxtest1 VALUES ('fff');
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SELECT * FROM pxtest1;
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-- This should fail, because the gid foo3 is already in use
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PREPARE TRANSACTION 'foo3';
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SELECT * FROM pxtest1;
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ROLLBACK PREPARED 'foo3';
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SELECT * FROM pxtest1;
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-- Clean up
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DROP TABLE pxtest1;
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-- Test subtransactions
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BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE;
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CREATE TABLE pxtest2 (a int);
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INSERT INTO pxtest2 VALUES (1);
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SAVEPOINT a;
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INSERT INTO pxtest2 VALUES (2);
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ROLLBACK TO a;
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SAVEPOINT b;
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INSERT INTO pxtest2 VALUES (3);
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PREPARE TRANSACTION 'regress-one';
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CREATE TABLE pxtest3(fff int);
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-- Test shared invalidation
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BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE;
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DROP TABLE pxtest3;
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CREATE TABLE pxtest4 (a int);
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INSERT INTO pxtest4 VALUES (1);
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INSERT INTO pxtest4 VALUES (2);
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DECLARE foo CURSOR FOR SELECT * FROM pxtest4;
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-- Fetch 1 tuple, keeping the cursor open
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FETCH 1 FROM foo;
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PREPARE TRANSACTION 'regress-two';
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-- No such cursor
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FETCH 1 FROM foo;
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-- Table doesn't exist, the creation hasn't been committed yet
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SELECT * FROM pxtest2;
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-- There should be two prepared transactions
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SELECT gid FROM pg_prepared_xacts;
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-- pxtest3 should be locked because of the pending DROP
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set statement_timeout to 2000;
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SELECT * FROM pxtest3;
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reset statement_timeout;
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-- Disconnect, we will continue testing in a different backend
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\c -
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-- There should still be two prepared transactions
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SELECT gid FROM pg_prepared_xacts;
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-- pxtest3 should still be locked because of the pending DROP
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set statement_timeout to 2000;
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SELECT * FROM pxtest3;
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reset statement_timeout;
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-- Commit table creation
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COMMIT PREPARED 'regress-one';
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\d pxtest2
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SELECT * FROM pxtest2;
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-- There should be one prepared transaction
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SELECT gid FROM pg_prepared_xacts;
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-- Commit table drop
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COMMIT PREPARED 'regress-two';
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SELECT * FROM pxtest3;
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-- There should be no prepared transactions
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SELECT gid FROM pg_prepared_xacts;
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-- Clean up
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DROP TABLE pxtest2;
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DROP TABLE pxtest3; -- will still be there if prepared xacts are disabled
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DROP TABLE pxtest4;
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