[snip]
>  - data migration: set default values, migrate old data to new form:
> this we usually do in chunks, as it takes a long time but takes only
> shared locks;
  ^^^^^^

This should read row locks, compared to table locks for structural
changes.

BTW, only the structural changes have to go through slony EXECUTE
SCRIPT, the data changes are applied online.

Cheers,
Csaba.