This patch is for a bug in backend/utils/time/combocid.c.
In HeapTupleHeaderAdjustCmax, the code makes the incorrect assumption that the raw command id corresponds to cmin, when the raw command id can in fact be a combo cid. It needs to be fixed to call HeapTupleHeaderGetCmin to get cmin. That's all there is to my patch, but read on for more details.
One thing that can happen is that multiple subtransactions may delete a tuple and each abort. Each subtransaction (except the first one) will store a combo cid that uses another combo cid as cmin. Then when cmin is accessed by a later operation, HeapTupleHeaderGetCmin will return the previous combo cid. In unlucky situations, the bogus cmin will be so high that it looks like the tuple was inserted in the future.
I have pasted an example below to show how things can go wrong. The second SELECT leaves out some tuples because their stored cmin is actually a large combo cid. The third SELECT gives correct results because the inserting transaction committed, so the command id is ignored. The script file with these commands in in the attachment, and I think it could make for a valuable regression test.
Side note: Another idea for a fix would be to have tqual.c change a combocid to a simple cmin when it does a time qual check and sees that the deleting subtransaction aborted. This is no good because it requires a write lock to change two header fields atomically, namely the HEAP_COMBOCID hint and t_cid. There are probably other issues, but this seems like a show-stopper.
The one-line patch is in the attachment. It gives correct behavior on my test case. This is my first patch; I hope I did it right!
Thanks! Karl Schnaitter test=# CREATE TABLE tab(a INT); CREATE TABLE test=# BEGIN; BEGIN test=# INSERT INTO tab VALUES(1); INSERT 0 1 test=# INSERT INTO tab VALUES(2); INSERT 0 1 test=# INSERT INTO tab VALUES(3); INSERT 0 1 test=# SAVEPOINT s; DELETE FROM tab; ROLLBACK TO SAVEPOINT s; SAVEPOINT DELETE 3 ROLLBACK test=# SELECT a FROM tab; a --- 1 2 3 (3 rows) test=# SAVEPOINT s; DELETE FROM tab; ROLLBACK TO SAVEPOINT s; SAVEPOINT DELETE 3 ROLLBACK test=# SAVEPOINT s; DELETE FROM tab; ROLLBACK TO SAVEPOINT s; SAVEPOINT DELETE 3 ROLLBACK test=# SAVEPOINT s; DELETE FROM tab; ROLLBACK TO SAVEPOINT s; SAVEPOINT DELETE 3 ROLLBACK test=# SELECT a FROM tab; a --- 1 (1 row) test=# COMMIT; COMMIT test=# SELECT a FROM tab; a --- 1 2 3 (3 rows) test=# DROP TABLE tab; DROP TABLE
combocid.tgz
Description: GNU Zip compressed data
-- Sent via pgsql-patches mailing list (pgsql-patches@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-patches
