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?
