Re: Column type modification in big tables

On Fri, Aug 9, 2024 at 2:06 AM Greg Sabino Mullane <htamfids(at)gmail(dot)com>
wrote:

> On Thu, Aug 8, 2024 at 2:39 PM Lok P <loknath(dot)73(at)gmail(dot)com> wrote:
>
>> Can anybody suggest any other possible way here.
>>
>
> Sure - how about not changing the column type at all?
>
> > one of the columns from varchar(20) to varchar(2)
>
> ALTER TABLE foobar ADD CONSTRAINT twocharplease CHECK (length(mycol) <= 2)
> NOT VALID;
>
> > one of the columns from Number(10,2) to Numeric(8,2)
>
> ALTER TABLE foobar ADD CONSTRAINT eightprecision CHECK (mycol <= 10^8) NOT
> VALID;
>
> > two of the columns from varchar(20) to numeric(3)
>
> This one is trickier, as we don't know the contents, nor why it is going
> to numeric(3) - not a terribly useful data type, but let's roll with it and
> assume the stuff in the varchar is a number of some sort, and that we don't
> allow nulls:
>
> ALTER TABLE foobar ADD CONSTRAINT onekorless CHECK (mycol::numeric(3) is
> not null) NOT VALID;
>
> You probably want to check on the validity of the existing rows: see the
> docs on VALIDATE CONSTRAINT here:
>
> https://www.postgresql.org/docs/current/sql-altertable.html
>
>
>
Thank you so much. Will definitely try to evaluate this approach. The Only
concern I have is , as this data is moving downstream with exactly the same
data type and length , so will it cause the downstream code to break while
using this column in the join or filter criteria. Also I believe the
optimizer won't be able to utilize this information while preparing the
execution plan.

Another thing , correct me if wrong, My understanding is , if we want to
run the "validate constraint" command after running this "check constraint
with not valid" command, this will do a full table scan across all the
partitions , but it's still beneficial as compared to updating the columns
values for each rows. Correct me if I'm wrong.