Re: Partitioning and unique key

On Sun, 1 Sept 2024 at 11:38, veem v <veema0000(at)gmail(dot)com> wrote:

>
> On Sun, 1 Sept 2024 at 10:03, veem v <veema0000(at)gmail(dot)com> wrote:
>
>>
>> On Sun, 1 Sept 2024 at 09:13, David G. Johnston <
>> david(dot)g(dot)johnston(at)gmail(dot)com> wrote:
>>
>>> On Saturday, August 31, 2024, veem v <veema0000(at)gmail(dot)com> wrote:
>>>
>>>>
>>>>
>>>> iii)And then alter the datatype of the partition key transaction_date
>>>> to DATE in one shot at the table level(which should be fast as its having
>>>> more granularity as compare to existing timestamptype, so should be catalog
>>>> or dictionary change only), and that will remain the part of composite PK
>>>> (transaction_id,transaction_date).
>>>>
>>>
>>> While this might seem logical, in reality date and timestamptz are
>>> different fixed-width data types and thus any attempt to change from one to
>>> the other will involve a table rewrite. Best you could do is leave the
>>> timestamptz in place and just truncate to day so the time is always
>>> midnight UTC.
>>>
>>>
>>>>
>>>>
>> Here , if we keep the PK column as is i.e. the transaction_timestamp as
>> timestamptz but truncate the time component , in that case again in future
>> if someone tries to insert(using insert on conflict) data into the table
>> with time component , it will get consumed and will not be restricted by
>> the PK constraint. So I was trying to make the data type also as DATE for
>> the transaction_timestap column.
>>
>> As in this case anyway we have to create another column to populate the
>> date+timestamp values as we cant throw those values away per business need,
>> so we will be kind of rewriting the table.So is it okay if if we will
>>
>> 1) Detach all the partitions.
>> 2)Do the alter using "only" key word in table level. (For adding new
>> column transaction_timestamp_new to hold date+timestamp value and also
>> altering the existing transaction_timestamp column to DATE from type
>> timestamptz).
>> 3)Then do the data fix(delete the duplicates) and alter the column, one
>> partition at a time for all of the partitions and once done , attach those
>> partitions one by one.
>> 5)Rename the columns at table level.Hope this won't need any table
>> rewrite.
>>
>> Is there any downside if we go by the above approach?
>>
>
> Or do you mean to say there is no way we can modify the data type of a
> partition key even by detaching the partitions one by one? And thus we may
> have only way left is to create the table from scratch with partitions and
> populate the data to it? I was avoiding this because we have many indexes
> also in it , so creating from scratch means creating those indexes again.
> So I wanted to achieve it by detaching partitions, doing the required
> change and attaching it again.
>
>
I tried by detaching the partitions, but still then it's not allowing me
to alter the DAT TYPE of the partition key and throwing error as below.

ERROR: cannot alter column "<patition_key_column>" because it is part of
the partition key of relation "<table_name>"

Now I am thinking if it's really going to get too complex if we try to
stick with the partition detach and attach strategy. As a few teammates say
, having a new column added with just a date type and then drop the
existing FK and PK first and then detach all the partitions, and attach the
partitions back using the new DATE column. and then recreate the PK again.
Btw we have ~5 partition tables with parent child relationship on which
this fix has to be applied. So I'm still wondering the best way possible
for fixing this issue.