Обсуждение: ALTER TABLE ... SET STORAGE does not propagate to indexes
ALTER TABLE ... SET STORAGE does not propagate to indexes, even though
indexes created afterwards get the new storage setting. So depending on
the order of commands, you can get inconsistent storage settings between
indexes and tables. For example:
create table foo1 (a text);
alter table foo1 alter column a set storage external;
create index foo1i on foo1(a);
insert into foo1 values(repeat('a', 10000));
ERROR: index row requires 10016 bytes, maximum size is 8191
(Storage "external" disables compression.)
but
create table foo1 (a text);
create index foo1i on foo1(a);
alter table foo1 alter column a set storage external;
insert into foo1 values(repeat('a', 10000));
-- no error
Also, this second state cannot be reproduced by pg_dump, so a possible
effect is that such a database would fail to restore.
Attached is a patch that attempts to fix this by propagating the storage
change to existing indexes. This triggers a few regression test
failures (going from no error to error), which I attempted to fix up,
but I haven't analyzed what the tests were trying to do, so it might
need another look.
--
Peter Eisentraut http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
Вложения
On 2020-01-06 13:32, Peter Eisentraut wrote: > Attached is a patch that attempts to fix this by propagating the storage > change to existing indexes. This triggers a few regression test > failures (going from no error to error), which I attempted to fix up, > but I haven't analyzed what the tests were trying to do, so it might > need another look. Attached is a more polished patch, with tests. -- Peter Eisentraut http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
Вложения
Hello Peter,
On Mon, Feb 24, 2020 at 12:59 PM Peter Eisentraut
<peter.eisentraut@2ndquadrant.com> wrote:
>
> On 2020-01-06 13:32, Peter Eisentraut wrote:
> > Attached is a patch that attempts to fix this by propagating the storage
> > change to existing indexes. This triggers a few regression test
> > failures (going from no error to error), which I attempted to fix up,
> > but I haven't analyzed what the tests were trying to do, so it might
> > need another look.
>
> Attached is a more polished patch, with tests.
I've reproduced the issue on head. And, the patch seems to solve the
problem. The patch looks good to me. But, I've a small doubt regarding
the changes in test_decoding regression file.
diff --git a/contrib/test_decoding/sql/toast.sql
b/contrib/test_decoding/sql/toast.sql
..
-INSERT INTO toasted_several(toasted_key) VALUES(repeat('9876543210', 10000));
-SELECT pg_column_size(toasted_key) > 2^16 FROM toasted_several;
+INSERT INTO toasted_several(toasted_key) VALUES(repeat('9876543210', 269));
+SELECT pg_column_size(toasted_key) > 2^11 FROM toasted_several;
This actually tests whether we can decode "old" tuples bigger than the
max heap tuple size correctly which is around 8KB. But, the above
changes will make the tuple size around 3KB. So, it'll not be able to
test that particular scenario.Thoughts?
--
Thanks & Regards,
Kuntal Ghosh
EnterpriseDB: http://www.enterprisedb.com
On 2020-02-24 12:21, Kuntal Ghosh wrote:
> I've reproduced the issue on head. And, the patch seems to solve the
> problem. The patch looks good to me. But, I've a small doubt regarding
> the changes in test_decoding regression file.
>
> diff --git a/contrib/test_decoding/sql/toast.sql
> b/contrib/test_decoding/sql/toast.sql
> ..
> -INSERT INTO toasted_several(toasted_key) VALUES(repeat('9876543210', 10000));
> -SELECT pg_column_size(toasted_key) > 2^16 FROM toasted_several;
> +INSERT INTO toasted_several(toasted_key) VALUES(repeat('9876543210', 269));
> +SELECT pg_column_size(toasted_key) > 2^11 FROM toasted_several;
>
> This actually tests whether we can decode "old" tuples bigger than the
> max heap tuple size correctly which is around 8KB. But, the above
> changes will make the tuple size around 3KB. So, it'll not be able to
> test that particular scenario.Thoughts?
OK, this is interesting. The details of this are somewhat unfamiliar to
me, but it appears that due to TOAST_INDEX_HACK in indextuple.c, an
index tuple cannot be larger than 8191 bytes when untoasted (but not
uncompressed).
What the test case above is testing is a situation where the heap tuple
is stored toasted uncompressed (storage external) but the index tuple is
not (probably compressed inline). This is exactly the situation that I
was contending should not be possible, because it cannot be dumped or
restored.
An alternative would be that we make this situation fully supported.
Then we'd probably need at least ALTER INDEX ... ALTER COLUMN ... SET
STORAGE, and some pg_dump support.
Thoughts?
--
Peter Eisentraut http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
On Tue, Feb 25, 2020 at 1:09 PM Peter Eisentraut
<peter.eisentraut@2ndquadrant.com> wrote:
>
> On 2020-02-24 12:21, Kuntal Ghosh wrote:
> > I've reproduced the issue on head. And, the patch seems to solve the
> > problem. The patch looks good to me. But, I've a small doubt regarding
> > the changes in test_decoding regression file.
> >
> > diff --git a/contrib/test_decoding/sql/toast.sql
> > b/contrib/test_decoding/sql/toast.sql
> > ..
> > -INSERT INTO toasted_several(toasted_key) VALUES(repeat('9876543210', 10000));
> > -SELECT pg_column_size(toasted_key) > 2^16 FROM toasted_several;
> > +INSERT INTO toasted_several(toasted_key) VALUES(repeat('9876543210', 269));
> > +SELECT pg_column_size(toasted_key) > 2^11 FROM toasted_several;
> >
> > This actually tests whether we can decode "old" tuples bigger than the
> > max heap tuple size correctly which is around 8KB. But, the above
> > changes will make the tuple size around 3KB. So, it'll not be able to
> > test that particular scenario.Thoughts?
>
> OK, this is interesting. The details of this are somewhat unfamiliar to
> me, but it appears that due to TOAST_INDEX_HACK in indextuple.c, an
> index tuple cannot be larger than 8191 bytes when untoasted (but not
> uncompressed).
>
> What the test case above is testing is a situation where the heap tuple
> is stored toasted uncompressed (storage external) but the index tuple is
> not (probably compressed inline). This is exactly the situation that I
> was contending should not be possible, because it cannot be dumped or
> restored.
>
Yeah. If we only commit this patch to fix the issue, we're going to
put some restriction for the above situation, i.e., the index for an
external attribute has to be stored as an external (i.e. uncompressed)
value. So, a lot of existing workload might start failing after an
upgrade. I think there should be an option to store the index of an
external attribute as a compressed inline value.
> An alternative would be that we make this situation fully supported.
> Then we'd probably need at least ALTER INDEX ... ALTER COLUMN ... SET
> STORAGE, and some pg_dump support.
>
> Thoughts?
Yes. We need the support for this syntax along with the bug fix patch.
--
Thanks & Regards,
Kuntal Ghosh
EnterpriseDB: http://www.enterprisedb.com
On 2020-Feb-25, Peter Eisentraut wrote: > An alternative would be that we make this situation fully supported. Then > we'd probably need at least ALTER INDEX ... ALTER COLUMN ... SET STORAGE, > and some pg_dump support. I think this is a more promising direction. -- Álvaro Herrera https://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
> ALTER TABLE ... SET STORAGE does not propagate to indexes, even though
> indexes created afterwards get the new storage setting. So depending on
> the order of commands, you can get inconsistent storage settings between
> indexes and tables.
> indexes created afterwards get the new storage setting. So depending on
> the order of commands, you can get inconsistent storage settings between
> indexes and tables.
I've absolutely noticed this behavior, I just thought it was intentional for some reason.
Having this behavior change as stated above would be very welcome in my opinion. It's always something i've had to manually think about in my migration scripts, so it would be welcome from my view.
-Adam
On 2020-03-30 18:17, Alvaro Herrera wrote: > On 2020-Feb-25, Peter Eisentraut wrote: >> An alternative would be that we make this situation fully supported. Then >> we'd probably need at least ALTER INDEX ... ALTER COLUMN ... SET STORAGE, >> and some pg_dump support. > > I think this is a more promising direction. I have started implementing the ALTER INDEX command, which by itself isn't very hard, but it requires significant new infrastructure in pg_dump, and probably also a bit of work in psql, and that's all a bit too much right now. An alternative for the short term is the attached patch. It's the same as before, but I have hacked up the test_decoding test to achieve the effect of ALTER INDEX with direct catalog manipulation. This preserves the spirit of the test case, but allows us to fix everything else about this situation. One thing to remember is that the current situation is broken. While you can set index columns to have different storage than the corresponding table columns, pg_dump does not preserve that, because it dumps indexes after ALTER TABLE commands. So at the moment, having these two things different isn't really supported. The proposed patch just makes this behave consistently and allows adding an ALTER INDEX command later on if desired. -- Peter Eisentraut http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
Вложения
I'm surprised that this hasn't applied yet, because: On 2020-Apr-09, Peter Eisentraut wrote: > One thing to remember is that the current situation is broken. While you > can set index columns to have different storage than the corresponding table > columns, pg_dump does not preserve that, because it dumps indexes after > ALTER TABLE commands. So at the moment, having these two things different > isn't really supported. So I have to ask -- are you planning to get this patch pushed and backpatched? -- Álvaro Herrera https://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
On 2020-04-22 01:56, Alvaro Herrera wrote: > I'm surprised that this hasn't applied yet, because: > > On 2020-Apr-09, Peter Eisentraut wrote: > >> One thing to remember is that the current situation is broken. While you >> can set index columns to have different storage than the corresponding table >> columns, pg_dump does not preserve that, because it dumps indexes after >> ALTER TABLE commands. So at the moment, having these two things different >> isn't really supported. > > So I have to ask -- are you planning to get this patch pushed and > backpatched? I think I should, but I figured I want to give some extra time for people to consider the horror that I created in the test_decoding tests. -- Peter Eisentraut http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
On 2020-04-22 16:26, Peter Eisentraut wrote: > On 2020-04-22 01:56, Alvaro Herrera wrote: >> I'm surprised that this hasn't applied yet, because: >> >> On 2020-Apr-09, Peter Eisentraut wrote: >> >>> One thing to remember is that the current situation is broken. While you >>> can set index columns to have different storage than the corresponding table >>> columns, pg_dump does not preserve that, because it dumps indexes after >>> ALTER TABLE commands. So at the moment, having these two things different >>> isn't really supported. >> >> So I have to ask -- are you planning to get this patch pushed and >> backpatched? > > I think I should, but I figured I want to give some extra time for > people to consider the horror that I created in the test_decoding tests. OK then, if there are no last-minute objects, I'll commit this for the upcoming minor releases. This is the patch summary again: Date: Thu, 9 Apr 2020 14:10:01 +0200 Subject: [PATCH v3] Propagate ALTER TABLE ... SET STORAGE to indexes When creating a new index, the attstorage setting of the table column is copied to regular (non-expression) index columns. But a later ALTER TABLE ... SET STORAGE is not propagated to indexes, thus creating an inconsistent and undumpable state. -- Peter Eisentraut http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
On 2020-05-06 16:37, Peter Eisentraut wrote: > On 2020-04-22 16:26, Peter Eisentraut wrote: >> On 2020-04-22 01:56, Alvaro Herrera wrote: >>> I'm surprised that this hasn't applied yet, because: >>> >>> On 2020-Apr-09, Peter Eisentraut wrote: >>> >>>> One thing to remember is that the current situation is broken. While you >>>> can set index columns to have different storage than the corresponding table >>>> columns, pg_dump does not preserve that, because it dumps indexes after >>>> ALTER TABLE commands. So at the moment, having these two things different >>>> isn't really supported. >>> >>> So I have to ask -- are you planning to get this patch pushed and >>> backpatched? >> >> I think I should, but I figured I want to give some extra time for >> people to consider the horror that I created in the test_decoding tests. > > OK then, if there are no last-minute objects, I'll commit this for the > upcoming minor releases. I have committed this and backpatched to PG12 and PG11. Before that, the catalog manipulation code is factored quite differently and it would be more complicated to backpatch and I didn't find that worth it. -- Peter Eisentraut http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services