archiver startup is slow

The archiver takes a long time to start, evidently because of checking/crawling all the existing rows. At LAM, the raw tables have ~170M rows:

2017-01-05 17:31:36+0000 Log opened.
2017-01-05 17:31:36+0000 importing postgres dbapi module psycopg2
2017-01-05 17:31:36+0000 database: table enu__slit contains 1 rows
2017-01-05 17:33:50+0000 database: table reply_raw contains 170736146 rows
2017-01-05 17:35:06+0000 database: table reply_hdr contains 170686655 rows
2017-01-05 17:35:06+0000 database: table hub__commanders contains 104527 rows
2017-01-05 17:35:06+0000 database: table hub__users contains 104526 rows
2017-01-05 17:35:06+0000 database: table actors contains 44 rows

It is not clear whether that scan is necessary. If not perhaps we can remove it. If it is, perhaps we need to use a union database or something.