From: | Vitaly Burovoy <vitaly(dot)burovoy(at)gmail(dot)com> |
---|---|
To: | Andres Freund <andres(at)anarazel(dot)de> |
Cc: | Serge Rielau <serge(at)rielau(dot)com>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Fast AT ADD COLUMN with DEFAULTs |
Date: | 2016-10-05 22:39:48 |
Message-ID: | CAKOSWN=v4Kv040qQqYvgEXWwfokb-5YpyL-rtxZ3s1VBFZoK8g@mail.gmail.com |
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Lists: | pgsql-hackers |
On 10/5/16, Vitaly Burovoy <vitaly(dot)burovoy(at)gmail(dot)com> wrote:
> On 10/5/16, Andres Freund <andres(at)anarazel(dot)de> wrote:
>> On 2016-10-05 15:23:05 -0700, Vitaly Burovoy wrote:
>>> On 10/5/16, Andres Freund <andres(at)anarazel(dot)de> wrote:
>>> > On 2016-10-05 11:58:33 -0700, Serge Rielau wrote:
>>> >> Dear Hackers,
>>> >> I’m working on a patch that expands PG’s ability to add columns to a
>>> >> table
>>> >> without a table rewrite (i.e. at O(1) cost) from the
>>> >> nullable-without-default to a more general case.
>>> >
>>> > If I understand this proposal correctly, altering a column default
>>> > will
>>> > still have trigger a rewrite unless there's previous default?
>>>
>>> No, "a second “exist default"" was mentioned, i.e. it is an additional
>>> column in a system table (pg_attribute) as default column values of
>>> the "pre-alter" era. It solves changing of the default expression of
>>> the same column later.
>>
>> Don't think that actually solves the issue. The default might be unset
>> for a while, for example. Essentially you'd need to be able to associate
>> arbitrary number of default values with an arbitrary set of rows.
>>
>> ALTER TABLE foo ALTER COLUMN withdefault DROP DEFAULT;
>> INSERT id = 1;
>> ALTER TABLE foo ALTER COLUMN withdefault SET DEFAULT 1;
>> ALTER TABLE foo ALTER COLUMN withdefault DROP DEFAULT;
>> INSERT id = 2;
>> ALTER TABLE foo ALTER COLUMN withdefault SET DEFAULT 2;
>> ALTER TABLE foo ALTER COLUMN withdefault DROP DEFAULT;
>> INSERT id = 3;
>> ALTER TABLE foo ALTER COLUMN withdefault SET DEFAULT 3;
>>
>> The result here would be that there's three rows with a default value
>> for foo that's the same as their id. None of them has that column
>> present in the row.
>
> I'm sorry, while I was writting "pre-alter" I meant
> "pre-alter-add-column" era (not "pre-alter-set-default"), all later
> default changes "current" default, whereas "pre-alter-add-column" adds
> value if current column number < TupleDesc.natts.
>
> All your DDL are in the "post-alter-add-column" era.
I'm so sorry, I was in a hurry. Of course,
- if current column number < TupleDesc.natts.
+ if current column number > TupleDesc.natts.
--
Best regards,
Vitaly Burovoy
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