Hello everybody!

I have found a performance issue with 2 equivalent queries stably taking different (~x2) time to finish. In just a few words it can be described like this: if you have a lot of values in an IN() statement, you should put most heavy (specifying most number of rows) ids first.
This is mostly just a bug submit, than looking for help.

So this is what I have:

• RHEL
• PostgreSQL 8.3.1
• A table ext_feeder_item with ~4.6M records.
  kia=# \d+ ext_feeder_item;
  Table "public.ext_feeder_item"
  Column | Type | Modifiers | Description
  ----------+--------------------------+--------------------------------------------------------------+-------------
  id | bigint | not null default nextval('ext_feeder_item_id_seq'::regclass) |
  feed_id | bigint | not null |
  pub_date | timestamp with time zone | |
  Indexes:
  "ext_feeder_item_pkey" PRIMARY KEY, btree (id)
  "ext_feeder_item_feed_id_pub_date_idx" btree (feed_id, pub_date)
  "ext_feeder_item_idx" btree (feed_id)
  Triggers:
  ....
  Has OIDs: no
  
• Statistics for the fields feed_id and pub_date are set to 1000;
• The table have just been vacuumed and analyzed.
• A simple query to the table:
  SELECT
  id
  FROM
  ext_feeder_item AS i
  WHERE
  i.feed_id IN (...)
  ORDER BY pub_date DESC, id DESC
  LIMIT 11 OFFSET 0;
  
  with many (~1200) ids in the IN() statement.
• The count of rows distribution for these ids (may be thought of as foreign keys in this table) is the following:
  id = 54461: ~180000 - actually the most heavy id in the whole table.
  other ids: a single id at most specifies 2032 rows; 6036 rows total.
• If I perform a query with
  IN(54461, ...)
  it stably (5 attempts) takes 4.5..4.7 secs. to perform.
  QUERY PLAN
  Limit  (cost=1463104.22..1463104.25 rows=11 width=16) (actual time=4585.420..4585.452 rows=11 loops=1)
    ->  Sort  (cost=1463104.22..1464647.29 rows=617228 width=16) (actual time=4585.415..4585.425 rows=11 loops=1)
          Sort Key: pub_date, id
          Sort Method:  top-N heapsort  Memory: 17kB
          ->  Bitmap Heap Scan on ext_feeder_item i (cost=13832.40..1449341.79 rows=617228 width=16) (actual time=894.622..4260.441 rows=185625 loops=1)
                Recheck Cond: (feed_id = ANY ('{54461, ...}'::bigint[]))
                ->  Bitmap Index Scan on ext_feeder_item_idx (cost=0.00..13678.10 rows=617228 width=0) (actual time=884.686..884.686 rows=185625 loops=1)
                      Index Cond: (feed_id = ANY ('{54461, ...}'::bigint[]))
  Total runtime: 4585.852 ms
  
• If I perform a query with
  IN(..., 54461)
  it stably (5 attempts) takes 9.3..9.5 secs. to perform.
  QUERY PLAN
  Limit  (cost=1463104.22..1463104.25 rows=11 width=16) (actual time=9330.267..9330.298 rows=11 loops=1)
    ->  Sort  (cost=1463104.22..1464647.29 rows=617228 width=16) (actual time=9330.263..9330.273 rows=11 loops=1)
          Sort Key: pub_date, id
          Sort Method:  top-N heapsort  Memory: 17kB
          ->  Bitmap Heap Scan on ext_feeder_item i (cost=13832.40..1449341.79 rows=617228 width=16) (actual time=1018.401..8971.029 rows=185625 loops=1)
                Recheck Cond: (feed_id = ANY ('{... ,54461}'::bigint[]))
                ->  Bitmap Index Scan on ext_feeder_item_idx (cost=0.00..13678.10 rows=617228 width=0) (actual time=1008.791..1008.791 rows=185625 loops=1)
                      Index Cond: (feed_id = ANY ('{... ,54461}'::bigint[]))
  Total runtime: 9330.729 ms
  

I don't know what are the roots of the problem, but I think that some symptomatic healing could be applied: the PostgreSQL could sort the IDs due to the statistics.
So currently I tend to select the IDs from another table ordering them due to their weights: it's easy for me thanks to denormalization.

Also I would expect from PostgreSQL that it sorted the values to make index scan more sequential, but this expectation already conflicts with the bug described above :)