On Sat, Jul 4, 2015 at 2:26 AM, Andres Freund <and...@anarazel.de> wrote: > On 2015-07-03 19:02:29 +0200, Andres Freund wrote: >> Maybe I'm just daft right now (35C outside, 32 inside, so ...), but I'm >> right now missing how the whole "skip wal logging if relation has just >> been truncated" optimization can ever actually be crashsafe unless we >> use a new relfilenode (which we don't!).
Agreed... When I ran the following test scenario, I found that the loaded data disappeared after the crash recovery. 1. start PostgreSQL server with wal_level = minimal 2. execute the following SQL statements \copy (SELECT num FROM generate_series(1,10) num) to /tmp/num.csv with csv BEGIN; CREATE TABLE test (i int primary key); TRUNCATE TABLE test; \copy test from /tmp/num.csv with csv COMMIT; SELECT COUNT(*) FROM test; -- returns 10 3. shutdown the server with immediate mode 4. restart the server 5. execute the following SQL statement after crash recovery ends SELECT COUNT(*) FROM test; -- returns 0.. In #2, 10 rows were copied and the transaction was committed. The subsequent statement of "select count(*)" obviously returned 10. However, after crash recovery, in #5, the same statement returned 0. That is, the loaded (+ committed) 10 data was lost after the crash. > We actually used to use a different relfilenode, but optimized that > away: cab9a0656c36739f59277b34fea8ab9438395869 > > commit cab9a0656c36739f59277b34fea8ab9438395869 > Author: Tom Lane <t...@sss.pgh.pa.us> > Date: Sun Aug 23 19:23:41 2009 +0000 > > Make TRUNCATE do truncate-in-place when processing a relation that was > created > or previously truncated in the current (sub)transaction. This is safe > since > if the (sub)transaction later rolls back, we'd just discard the rel's > current > physical file anyway. This avoids unreasonable growth in the number of > transient files when a relation is repeatedly truncated. Per a > performance > gripe a couple weeks ago from Todd Cook. > > to me the reasoning here looks flawed. Before this commit, when I ran the above test scenario, no data loss happened. Regards, -- Fujii Masao -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers