BY: LLOYD ALBIN
8/6/2013

This presentation will describe the finding of a bug in the multithreaded restores and how the fix was implemented.

pg_restore -C -j 3 -d postgres -Fc test_db.dump
When doing a multithreaded restore, you specify the –j option and specify the number of jobs / threads / cores.
In this example there will be three threads spawned by the parent process.
You may only use the –j option with custom or directory backups.

CREATE DATABASE test_db
WITH OWNER = postgres
ENCODING = 'UTF8'
TEMPLATE = template0;
Here we create a test database for our test.

CREATE TABLE public.tbl_test ( pkey TEXT NOT NULL,
);
CONSTRAINT tbl_test_pkey PRIMARY KEY(pkey)
We will create a basic table with just a primary key. We don’t need any other fields for this example.

COMMENT ON INDEX public.tbl_test_pkey
IS 'Index Comment';
We can now add a comment to the index.
This is the whole root of the problem for the multi-threaded restore.
If you don’t have any comment on automatically created index’s, then you won’t have any issues with the multi-threaded restore.

pg_dump -Fc test_db > test_db.dump
dropdb test_db
We don’t need any data for our example, so we are now ready to backup the database. Once the database if backed up we can go ahead and drop it.

pg_restore -C -j 3 -d postgres -Fc test_db.dump
pg_restore: [archiver (db)] Error while PROCESSING TOC: pg_restore: [archiver (db)] Error from TOC entry 2525; 0 0
COMMENT INDEX tbl_test_pkey postgres pg_restore: [archiver (db)] could not execute query:
ERROR: relation
"tbl_test_pkey" does not exist
Command was: COMMENT ON INDEX tbl_test_pkey IS
'Index Comment'; pg_restore: [archiver] worker process failed: exit code 1
What happed is the restore tried to add the comment before the index was created.
I have tested this with
Postgres 9.0.12 & 9.2.4

The first thing you should do is to search the pgsql-bugs and pgsqlhackers mailing lists for your problem. If you don’t find it, then go ahead and submit a bug ticket.
http://www.postgresql.org/support/submitbug
The form will ask you for the following information:
Name
Email
PostgreSQL version
Operating System
Short Description
Long Description

From: lalbin(at)fhcrc(dot)org
To: pgsql-bugs(at)postgresql(dot)org
Subject: BUG #8257: Multi-Core Restore fails when containing index comments
Date: 2013-06-26 23:43:00
The following bug has been logged on the website:
Bug reference: 8257
Logged by: Lloyd Albin
Email address: lalbin(at)fhcrc(dot)org
PostgreSQL version: 9.2.4
Operating system: SUSE Linux (64-bit)
To see the full bug submission: http://www.postgresql.
org/messageid/E1UrzMa-0007Ns-
KX@wrigleys.postgresql
.org

; Selected TOC Entries:
...
170; 1259 69261 TABLE public tbl_test andres
; depends on: 6
1941; 0 69261 TABLE DATA public tbl_test andres
; depends on: 170
1833; 2606 69268 CONSTRAINT public tbl_test_pkey andres
; depends on: 170 170
1950; 0 0 COMMENT public INDEX tbl_test_pkey andres
; depends on: 1832
The problem is that pg_dump makes the comment depend on the index instead of the constraint:
There is no object 1832 in the dump since that was ommitted in favor of the constraint 1833 which internally creates the index.
Andres Freund
PostgreSQL Development
2 nd Quadrant

170; 1259 69261 TABLE public tbl_test andres
; depends on: 6
1941; 0 69261 TABLE DATA public tbl_test andres
; depends on: 170
1833; 2606 69268 CONSTRAINT public tbl_test_pkey andres
; depends on: 170 170
1950; 0 0 COMMENT public INDEX tbl_test_pkey andres
; depends on: 1833
So what we need to do is to make the comment depend on the constraint instead.
Unsurprisingly after that restore completes.
Andres Freund
PostgreSQL Development
2 nd Quadrant http://www.postgresql.
org/messageid/20130627080135.GA12
870@awork2.anarazel.d
e

Andres Freund <andres(at)2ndquadrant(dot)com> writes:
> The problem is that pg_dump makes the comment depend on the index instead of the constraint:
Yeah, I figured that out yesterday, but hadn't gotten to writing a patch yet.
> ... So what we need to do is to make the comment depend on the constraint instead.
Your proposed patch will only fix the problem for dumps created after it ships. In the past, we've tried to deal with this type of issue by having pg_restore fix up the dependencies when reading a dump, so that it would still work on existing dumps.
I'm afraid there may be no way to do that in this case --- it doesn't look like there's enough info in the dump to tell where the dependency link should have led. But we should think about it a little before taking the easy way out.
Tom Lane points out that the fix will only fix new dumps and not any previous dumps.
http://www.postgresql.
org/messageid/20760.1372343354@ss s.pgh.pa.us

On 2013-06-27 10:29:14 -0400, Tom Lane wrote:
> > ... So what we need to do is to make the comment depend on the constraint instead.
> Your proposed patch will only fix the problem for dumps created after it ships. In the past, we've tried to deal with this type of issue by having pg_restore fix up the dependencies when reading a dump, so that it would still work on existing dumps.
Yes :(. On the other hand, it's probably not too common to create comments on indexes that haven't been created explicitly.
> I'm afraid there may be no way to do that in this case --- it doesn't look like there's enough info in the dump to tell where the dependency link should have led. But we should think about it a little before taking the easy way out.
The only thing I could think of - but which I thought to be too kludgey - was to simply delay the creation of all comments and restore them together with
ACLs. I don't think we can have dependencies towards comments.
Andres Freund points out that unfortunately this is the best fix.
http://www.postgresql.
org/messageid/20130627144315.GL125
4@alap2.anarazel.de

Andres Freund <andres(at)2ndquadrant(dot)com> writes:
> On 2013-06-27 10:29:14 -0400, Tom Lane wrote:
>> Your proposed patch will only fix the problem for dumps created after it ships. In the past, we've tried to deal with this type of issue by having pg_restore fix up the dependencies when reading a dump, so that it would still work on existing dumps.
> Yes :(. On the other hand, it's probably not too common to create comments on indexes that haven't been created explicitly.
Perhaps. The lack of previous complaints does suggest this situation isn't so common.
>> I'm afraid there may be no way to do that in this case --- it doesn't look like there's enough info in the dump to tell where the dependency link should have led.
But we should think about it a little before taking the easy way out.
> The only thing I could think of - but which I thought to be too kludgey - was to simply delay the creation of all comments and restore them together with ACLs.
I don't like that either, though we may be forced into it if we find more bugs in comment dependencies.
Anyway, fixing pg_dump's logic is not wrong; I was just hoping we could also think of a workaround on the pg_restore side.
Tom Lane, says that
Andres was correct with patching pg_dump but wished that pg_restore could also have been patched to support the bad dumps but agrees that the only way to fix it is not a good workaround.
http://www.postgresql.
org/messageid/25223.1372353553@sss
.pgh.pa.us

Andres Freund <andres(at)2ndquadrant(dot)com> writes:
> There is no object 1832 in the dump since that was ommitted in favor of the constraint 1833 which internally creates the index. So what we need to do is to make the comment depend on the constraint instead.
> With the attached patch we get: [ the right thing ]
Applied with minor cosmetic changes.
Tom Lane approved the patch.
http://www.postgresql.
org/messageid/26518.1372355789@ss s.pgh.pa.us

Mark index-constraint comments with correct dependency in pg_dump.
When there's a comment on an index that was created with UNIQUE or PRIMARY KEY constraint syntax, we need to label the comment as depending on the constraint not the index, since only the constraint object actually appears in the dump. This incorrect dependency can lead to parallel pg_restore trying to restore the comment before the index has been created, per bug #8257 from Lloyd Albin.
This patch fixes pg_dump to produce the right dependency in dumps made in the future.
Usually we also try to hack pg_restore to work around bogus dependencies, so that existing (wrong) dumps can still be restored in parallel mode; but that doesn't seem practical here since there's no easy way to relate the constraint dump entry to the comment after the fact.
Andres Freund
Branch
------
REL9_3_STABLE
REL9_2_STABLE
REL9_1_STABLE
REL9_0_STABLE
REL8_4_STABLE master
You can view all the committed patched on the pgsql-committers mailing list.
Each branch patch is a separate email on the pgsql-committers mailing list.

These are the affected versions:
8.4.17
9.0.13
9.1.9
9.2.4
9.3 Beta 1 and possibly Beta 2 since it was released the same day as the patches were committed.
It is affecting all current versions 8.4+ as of 8/6/2013.