Horton

Re: WAL logging problem in 9.4.3?

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Fujii Masao <masao.fujii@gmail.com>, Martijn van Oosterhout <kleptog@svana.org>, PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2015-07-03T23:25:23Z
Lists: pgsql-hackers 
On 2015-07-03 18:38:37 -0400, Tom Lane wrote:
> > Why exactly?  The first truncation in the (sub)xact would have assigned a
> new relfilenode, why do we need another one?  The file in question will
> go away on crash/rollback in any case, and no other transaction can see
> it yet.

Consider:

BEGIN;
CREATE TABLE;
INSERT largeval;
TRUNCATE;
INSERT 1;
COPY;
INSERT 2;
COMMIT;

INSERT 1 is going to be WAL logged. For that to work correctly TRUNCATE
has to be WAL logged, as otherwise there'll be conflicting/overlapping
tuples on the target page.

But:

The truncation itself is not fully wal logged, neither is the COPY. Both
rely on heap_sync()/immedsync(). For that to be correct the current
relfilenode's truncation may *not* be wal-logged, because the contents
of the COPY or the truncation itself will only be on-disk, not in the
WAL.

Only being on-disk but not in the WAL is a problem if we crash and
replay the truncate record.

> I'm prepared to believe that some bit of logic is doing the wrong
> thing in this state, but I do not agree that truncate-in-place is
> unworkable.

Unless we're prepared to make everything that potentially WAL logs
something do the rel->rd_createSubid == mySubid && dance, I can't see
that working.

Commits

  1. Add perl2host call missing from a new test file.

  2. Skip WAL for new relfilenodes, under wal_level=minimal.

  3. Revert "Skip WAL for new relfilenodes, under wal_level=minimal."

  4. Back-patch log_newpage_range().

  5. During heap rebuild, lock any TOAST index until end of transaction.

  6. In log_newpage_range(), heed forkNum and page_std arguments.

  7. Back-patch src/test/recovery and PostgresNode from 9.6 to 9.5.

  8. Reduce pg_ctl's reaction time when waiting for postmaster start/stop.

  9. Accelerate end-of-transaction dropping of relations

  10. Redesign the planner's handling of index-descent cost estimation.

  11. Make TRUNCATE do truncate-in-place when processing a relation that was created