[incorrecthorse]

Excellent advice.

On the technical side, I believed waiting was due to the lock queue rather that having acquired an ACCESS EXCLUSIVE lock. The ALTER is specifically _waiting_ for any lock lower than ACCESS EXCLUSIVE to be release.

[kroolik]

It also makes all new readers/writers to wait for that lock, essentially leading to downtime until the lock is eventually acquired and released. This is the classic readers/writers library example, and you want to avoid starving the writers.

Thats why size of data is the least of your issues - its the access patterns/hotness that are the issue.