Why isn't PG using an index-only scan?

Hello,

I have this very simple query:

INSERT INTO copyrightad (idoeu, idad, role, mechowned, perfowned, iscontrolled)
SELECT o.idoeu, c.idad, LEFT(sipa_capacity1,3), sipa_mech_owned, sipa_perf_owned, sipa_controlled='Y'
FROM imaestro.msipfl ip
JOIN oeu o ON o.imworkid=ip.sipa_song_code
JOIN ad c ON c.imcompid=ip.sipa_ip_code
WHERE ip.sipa_comp_or_publ='C';

And here are the number of elements in each table:

imaestro.msipfl: 1550019

oeu: 1587533

ad: 304986

The explain plan is saying this:

                                               QUERY PLAN
----------------------------------------------------------------------------------------------------------
Insert on copyrightad  (cost=613790.59..4448045.97 rows=0 width=0)
  ->  Merge Join  (cost=613790.59..4448045.97 rows=84972138 width=328)
        Merge Cond: (((c.imcompid)::numeric) = ip.sipa_ip_code)
        ->  Sort  (cost=35712.97..36475.44 rows=304986 width=8)
              Sort Key: ((c.imcompid)::numeric)
              ->  Index Only Scan using ix_ad_imcompid on ad c  (cost=0.42..7931.21 rows=304986 width=8)
        ->  Materialize  (cost=578077.61..594521.17 rows=3288712 width=23)
              ->  Sort  (cost=578077.61..586299.39 rows=3288712 width=23)
                    Sort Key: ip.sipa_ip_code
                    ->  Hash Join  (cost=56043.04..154644.48 rows=3288712 width=23)
                          Hash Cond: ((o.imworkid)::numeric = ip.sipa_song_code)
                          ->  Seq Scan on oeu o  (cost=0.00..41901.33 rows=1587533 width=8)
                          ->  Hash  (cost=48542.24..48542.24 rows=600064 width=25)
                                ->  Seq Scan on msipfl ip  (cost=0.00..48542.24 rows=600064 width=25)
                                      Filter: ((sipa_comp_or_publ)::text = 'C'::text)

Table ad has this index:

"ix_ad_imcompid" btree (imcompid, idad)

Table oeu has this one:

"ix_oeu_imcompid" btree (imworkid, idoeu)

idad and idoeu are both primary keys of, respectively, ad and oeu.

The resultset should be 591615 rows because:

select sipa_comp_or_publ, count(*) from imaestro.msipfl group by 1;
sipa_comp_or_publ | count   
-------------------+--------
C                 | 591615
P                 | 958404

Is it normal that PG is doing a seq scan on table oeu but an index-only scan on ad? I had to stop the query after 5 hours, how can I make this faster? Of course I ran VACUUM ANALYZE.

These are the memory settings I have, but I have plenty of unused RAM. Should I bump something up?

shared_buffers = 16GB                   # min 128kB                                                                                                  
#huge_pages = try                       # on, off, or try                                                                                            
#huge_page_size = 0                     # zero for system default                                                                                    
#temp_buffers = 8MB                     # min 800kB                                                                                                  
#max_prepared_transactions = 0          # zero disables the feature                                                                                  
work_mem = 128MB                                # min 64kB          

$ free -h
              total        used        free      shared  buff/cache   available
Mem:           125Gi        18Gi       3.1Gi        15Gi       121Gi       106Gi
Swap:          4.0Gi       2.0Gi       2.0Gi

Thanks for your help,

JC