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From: Robert Haas on 6 Jul 2010 16:29
On Tue, Jul 6, 2010 at 2:49 PM, Alvaro Herrera
<alvherre(a)commandprompt.com> wrote:
> Excerpts from Robert Haas's message of mar jul 06 10:08:21 -0400 2010:
>
>> Treating OID as a user-defined column seems reasonable, and probably
>> not even that messy if we put some appropriate macros in place. �I'm
>> guessing the messy part would be finding all the places that expect to
>> be consulting a real pg_attribute row and supplying them with a
>> faked-up one in its place.
>
> Agreed.
>
> I'm intending to work on logical column identifiers for 9.1. �Perhaps I
> could try to have a look at this, too, while at it.

I have a strong suspicion that's going to be a, ahem, challenging
project. But it would be great to have. Getting rid of the system
column entries from pg_attribute is probably easy by comparison.

When we discussed this previously, Tom suggested that we might want to
have a three-tiered structure: (1) permanent identifier (never
changes, used by other system catalogs to reference the attribute in
question), (2) display position, and (3) physical storage position.
I'm not sure if it's feasible to think about splitting out (2) and (3)
in a single patch, but either one would be useful by itself. Which
are you planning to work on?

One other thought for you to mull over. Currently, we can never
really totally get rid of an attribute because it would leave us at a
loss as to how to interpret the tuples already on disk - we can't cope
with HeapTupleHeaderGetNatts ever decreasing. But maybe instead of
storing natts in the tuple header, we could store a "tuple version
number". Whenever a column is added or dropped, physical storage
layout is changed, etc., we bump the tuple version number but retain
the information necessary to interpret old tuple versions. After
CLUSTER or VACUUM FULL, we can forget about all the old tuple
versions. A regular VACUUM can, if it visits the entire table, forget
about all tuple versions other than the latest which are observed not
to be in use. It's a bit awkward if you go to make a change and
discover that all 2047 possible tuple versions are in use, because now
you have to force a table rewrite for an operation that doesn't
normally require one. But in the immortal words of Bill Gates, 640K
ought to be enough for anybody.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise Postgres Company

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From: Andrew Dunstan on 6 Jul 2010 17:18


Robert Haas wrote:
> On Tue, Jul 6, 2010 at 2:49 PM, Alvaro Herrera
> <alvherre(a)commandprompt.com> wrote:
>
>>
>> I'm intending to work on logical column identifiers for 9.1. Perhaps I
>> could try to have a look at this, too, while at it.
>>
>
> I have a strong suspicion that's going to be a, ahem, challenging
> project. But it would be great to have. Getting rid of the system
> column entries from pg_attribute is probably easy by comparison.
>

It will be a bit invasive, but I'm not so sure that it's difficult, just
a mass of details to take care of. Like you I'd be very glad to see it done.

> When we discussed this previously, Tom suggested that we might want to
> have a three-tiered structure: (1) permanent identifier (never
> changes, used by other system catalogs to reference the attribute in
> question), (2) display position, and (3) physical storage position.
> I'm not sure if it's feasible to think about splitting out (2) and (3)
> in a single patch, but either one would be useful by itself. Which
> are you planning to work on?
>


Why wouldn't it be feasible? In any case, having a mutable logical
column position is the feature that's been most requested.

cheers

andrew




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From: Robert Haas on 6 Jul 2010 17:24
On Tue, Jul 6, 2010 at 5:18 PM, Andrew Dunstan <andrew(a)dunslane.net> wrote:
>> I have a strong suspicion that's going to be a, ahem, challenging
>> project. �But it would be great to have. �Getting rid of the system
>> column entries from pg_attribute is probably easy by comparison.
>
> It will be a bit invasive, but I'm not so sure that it's difficult, just a
> mass of details to take care of. Like you I'd be very glad to see it done.

I guess we'll find out...!

>> When we discussed this previously, Tom suggested that we might want to
>> have a three-tiered structure: (1) permanent identifier (never
>> changes, used by other system catalogs to reference the attribute in
>> question), (2) display position, and (3) physical storage position.
>> I'm not sure if it's feasible to think about splitting out (2) and (3)
>> in a single patch, but either one would be useful by itself. �Which
>> are you planning to work on?
>
> Why wouldn't it be feasible?

Just because it might be too much to do all at once.

> In any case, having a mutable logical column
> position is the feature that's been most requested.

I think that's true. But the physical storage position would give us
a performance benefit, by allowing us to try to avoid useless
alignment padding.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise Postgres Company

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From: Tom Lane on 6 Jul 2010 17:32
Robert Haas <robertmhaas(a)gmail.com> writes:
> On Tue, Jul 6, 2010 at 5:18 PM, Andrew Dunstan <andrew(a)dunslane.net> wrote:
>> Why wouldn't it be feasible?

> Just because it might be too much to do all at once.

My thought is that the hardest part of this is going to be making sure
that every "column index" usage in the code is properly categorized as
to whether it's physical, logical, or identifier index. If we try to
divide the problem into sub-patches, that will probably just increase
the amount of effort because all that code will have to be looked at
twice.

Think of it as Polya's paradox in action.

regards, tom lane

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From: Alvaro Herrera on 7 Jul 2010 13:29
Excerpts from Robert Haas's message of mar jul 06 17:24:21 -0400 2010:
> On Tue, Jul 6, 2010 at 5:18 PM, Andrew Dunstan <andrew(a)dunslane.net> wrote:

> > In any case, having a mutable logical column
> > position is the feature that's been most requested.
>
> I think that's true. But the physical storage position would give us
> a performance benefit, by allowing us to try to avoid useless
> alignment padding.

That's true too. I intend to look at both problems simultaneously, i.e.
decoupling the current attnum in three columns as previously discussed;
as Tom says, I think it'll end up being less work than attacking them
separately. However, I will not attempt to include optimizations such
as avoiding padding, in the first patch, just the possibility that it is
added later.

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