It seems this is a pgsql api issue not an eT dbtools issue. 

When a field's type, name, or length is changed, on a postgresl system, the sequence and index values are dropped.

This is because it creates a new table, copies the original tables content, then deletes the original table and renames the new table (with the new schema)

The index and it's sequence needs to be recreated

Perhaps just using a dummy column instead of a dummy table would work .. but it would prevent the index field name or type from being changed.

This is an example for postgresql to get the maximum value for a field and another example to set the sequance value to that value plus one

SELECT max(detail_id) from phpgw_ttrack_job_details
select setval('phpgw_ttrack_job__detail_id_seq',441);
alter table phpgw_ttrack_job_details alter detail_id set default nextval('phpgw_ttrack_job__detail_id_seq');

I thought this would be handy for dbtools