Combination of partial and full indexes

Hi.

I had a fight with a query planner because it doesn’t listen.

There are two indexes:

- with expression in descending order:
"offers_offer_next_update_idx" btree (offer_next_update(update_ts, update_freq) DESC) WHERE o_archived = false
- unique with two columns:
"offers_source_id_o_key_idx" UNIQUE, btree (source_id, o_key)

Here's the query with filter for offers.source_id columns which
is pretty slow because "offers_source_id_o_key_idx" is not used:

EXPLAIN ANALYZE
SELECT offers.o_url AS offers_o_url
FROM offers
WHERE offers.source_id = 1 AND offers.o_archived = false AND now() > offer_next_update(offers.update_ts, offers.update_freq)
ORDER BY offer_next_update(offers.update_ts, offers.update_freq) DESC
LIMIT 1000;

Limit (cost=0.68..23403.77 rows=1000 width=116) (actual time=143.544..147.870 rows=1000 loops=1)
-> Index Scan using offers_offer_next_update_idx on offers (cost=0.68..1017824.69 rows=43491 width=116) (actual time=143.542..147.615 rows=1000 loops=1)
Index Cond: (now() > offer_next_update(update_ts, update_freq))
Filter: (source_id = 1)
Rows Removed by Filter: 121376
Total runtime: 148.023 ms

When I remove filter on offers.source_id, query plan looks like this:

EXPLAIN ANALYZE
SELECT offers.o_url AS offers_o_url
FROM offers
WHERE offers.o_archived = false AND now() > offer_next_update(offers.update_ts, offers.update_freq)
ORDER BY offer_next_update(offers.update_ts, offers.update_freq) DESC
LIMIT 1000;

Limit (cost=0.68..4238.27 rows=1000 width=116) (actual time=0.060..3.877 rows=1000 loops=1)
-> Index Scan using offers_offer_next_update_idx on offers (cost=0.68..1069411.78 rows=252363 width=116) (actual time=0.058..3.577 rows=1000 loops=1)
Index Cond: (now() > offer_next_update(update_ts, update_freq))
Total runtime: 4.031 ms

I even tried to change orders of conditions in second query but it doesn't seem
to make a difference for a planner.

Shouldn't query planner use offers_source_id_o_key_idx to speed up query above?

PostgreSQL version: PostgreSQL 9.3.12 on x86_64-unknown-linux-gnu, compiled by gcc (Ubuntu 4.8.4-2ubuntu1~14.04.1) 4.8.4, 64-bit

Configuration:
name | current_setting | source
------------------------------+----------------------------------------+----------------------
application_name | psql | client
checkpoint_completion_target | 0.9 | configuration file
checkpoint_segments | 3 | configuration file
client_encoding | UTF8 | client
DateStyle | ISO, MDY | configuration file
default_text_search_config | pg_catalog.english | configuration file
effective_cache_size | 128MB | configuration file
external_pid_file | /var/run/postgresql/9.3-main.pid | configuration file
lc_messages | en_US.UTF-8 | configuration file
lc_monetary | en_US.UTF-8 | configuration file
lc_numeric | en_US.UTF-8 | configuration file
lc_time | en_US.UTF-8 | configuration file
max_connections | 100 | configuration file
max_locks_per_transaction | 168 | configuration file
max_stack_depth | 2MB | environment variable
port | 5432 | configuration file
shared_buffers | 4GB | configuration file
temp_buffers | 12MB | configuration file
unix_socket_directories | /var/run/postgresql | configuration file
work_mem | 16MB | configuration file

Definitions:

CREATE OR REPLACE FUNCTION public.offer_next_update(last timestamp without time zone, minutes smallint)
RETURNS timestamp without time zone
LANGUAGE plpgsql
IMMUTABLE
AS $function$
BEGIN
RETURN last + (minutes || ' min')::interval;
END
$function$