Re: ALTER TEXT field to VARCHAR(1024)

Hello,

i am using Postgres 9.2 and I'd like to perform the following ALTER statement on a database table with about 30M entries :

ALTER TABLE images ALTER COLUMN mask_descriptors TYPE VARCHAR(1024);

The mask_descriptors field is currently having the type TEXT.

I want to perform the ALTER due to the fact that it seems that copying the contents of the table to a BI SQL Server is done in row by row (instead of batch) when handling CLOBs.

From the Postgres documentation I got the following :

http://www.postgresql.org/docs/8.3/static/datatype-character.html

Tip: There are no performance differences between these three types, apart from increased storage size when using the blank-padded type, and a few extra cycles to check the length when storing into a length-constrained column. While character has performance advantages in some other database systems, it has no such advantages in PostgreSQL. In most situations text or character varying should be used instead.

Can anybody explain me what happens in the background when the alter statement is executed? I've tried it out on a small copy of the table (70K) and the operation completed in 0.2 seconds.

Will the table be completely locked during the execution of the ALTER statement?

I am curious as to why you want to change text to VARCHAR(1024), especially as I think that the middleware should be controlling how long a string is saved in the database rather than end user client code (for several reasons., including security concerns).  However, I do not know your use cases, nor your overall situation - so my concerns may not apply to you.


Cheers,
Gavin