Tom Lane wrote:
In short, parallel pg_restore is guaranteed to fail on any input file made with a pre-8.4 pg_dump on Windows. It may be that there's some other mechanism involved in the reports we've gotten of parallel restore failing only some of the time, but I'm thinking that the heretofore unrecognized dependency on pg_dump-time seekability could well explain those too.
IIRC, you can reproduce this on Unix too by sending the output of pg_dump into a pipe. So it's not uniquely a Windows problem.
As Greg suggests, the solution would be to have a second TOC at the end of the file with the offsets. But I think that's way beyond what we should do on the back branches, and really beyond what we should do for 9.0. We should document the limitation.
I see several action items here: 1. The error message emitted by _PrintTocData is incredibly misleading. It needs to be fixed to tell people if the problem is lack of data offsets rather than lack of seek capability.
Agreed.
Another possibility is to just remove the inside-the-loop error test altogether: make it just skip till it finds the desired item, and only throw an error if it hits EOF without finding it. In the case that the error test is trying to catch, this would mean significantly more work done before reporting the error, but do we really care? I'm leaning to this solution because it would not require exporting state from the parallel restore control logic.
Would exporting a bit of state be so bad? It seems like it would be a bit cleaner, and I'll be surprised if it's terribly difficult. It can be set at the top of parallel_restore().
3. Perhaps pg_dump ought to emit a warning when it can't seek, instead of just silently not writing the data offsets. That behavior was okay before when lack of data offsets didn't really matter that much, but lack of data offsets is a serious performance handicap for parallel restore even after we fix the outright failure condition (because each worker is going to read through a lot of data to find what it needs).
For now, yes. But in 9.1 we should write out a second TOC and teach pg_restore to look for it.
4. Is there any value in back-porting the Windows FSEEKO support into 8.3 and 8.2? Arguably, not writing the data offsets is a performance bug. However a back-port won't do anything for people who are dumping with less than the latest minor release of pg_dump, so doing this might be largely wasted effort.
I doubt it's worth it, but I could be persuaded otherwise. cheers andrew -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
