Re: Partitioning and unique key - Mailing list pgsql-general

On Sun, 1 Sept 2024 at 10:03, veem v <veema0000@gmail.com> wrote:

On Sun, 1 Sept 2024 at 09:13, David G. Johnston <david.g.johnston@gmail.com> wrote:

On Saturday, August 31, 2024, veem v <veema0000@gmail.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.