Обсуждение: What to watch out for when ALTERing NUMERIC(38,0) to BIGINT?

AWS RDS Postgresql 12.10


We've got tables with columns of data type NUMERIC(38,0) which are a legacy from an Oracle migration.

Besides what's mentioned in https://www.postgresql.org/docs/12/ddl-alter.html#id-1.5.4.8.10, what happens internally when I run:
ALTER TABLE foo ALTER COLUMN bar TYPE BIGINT;

I'm thinking mostly of record fragmentation.

"David G. Johnston" <david.g.johnston@gmail.com> writes:
> On Thu, Jul 28, 2022 at 8:13 AM Ron <ronljohnsonjr@gmail.com> wrote:
>> Besides what's mentioned in
>> https://www.postgresql.org/docs/12/ddl-alter.html#id-1.5.4.8.10, what
>> happens *internally* when I run:
>> ALTER TABLE foo ALTER COLUMN bar TYPE BIGINT;

> IIUC, that would be the silver lining in all of this - the rewritten table
> would have zero fragmentation and bloat.

Yeah.  What happens internally is a table rewrite: the entire content
of the table (and its indexes) is written into a new set of files.
At commit, the old files are deleted.  The main gotchas, for a large
table, are the transient disk space consumption and the fact that the
table stays exclusively locked the whole time.

            regards, tom lane

On 7/28/22 10:26, Tom Lane wrote:
> "David G. Johnston" <david.g.johnston@gmail.com> writes:
>> On Thu, Jul 28, 2022 at 8:13 AM Ron <ronljohnsonjr@gmail.com> wrote:
>>> Besides what's mentioned in
>>> https://www.postgresql.org/docs/12/ddl-alter.html#id-1.5.4.8.10, what
>>> happens *internally* when I run:
>>> ALTER TABLE foo ALTER COLUMN bar TYPE BIGINT;
>> IIUC, that would be the silver lining in all of this - the rewritten table
>> would have zero fragmentation and bloat.
> Yeah.  What happens internally is a table rewrite: the entire content
> of the table (and its indexes) is written into a new set of files.
> At commit, the old files are deleted.  The main gotchas, for a large
> table, are the transient disk space consumption and the fact that the
> table stays exclusively locked the whole time.

Rewriting a 3TB table doesn't seem appetizing.  Fortunately, there's only 
one table like that...

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