Partitioned table - scans through every partitions

Hi,

We have an issue with one of our partitioned tables. It has a column with timestamp without time zone type, and we had to partition it daily. To do that, we created the following constraints like this example:
CHECK (to_char(impression_time, 'YYYYMMDD'::text) = '20170202'::text)

The problem we’re facing is no matter how we’re trying to select from it, it scans through every partitions.

Parent table:
Table "public.dfp_in_network_impressions"
Column | Type | Modifiers
-----------------+-----------------------------+-----------
impression_time | timestamp without time zone |
nexus_id | character varying |
line_item_id | bigint |
creative_id | bigint |
ad_unit_id | bigint |
Triggers:
insert_dfp_in_network_impressions_trigger BEFORE INSERT ON dfp_in_network_impressions FOR EACH ROW EXECUTE PROCEDURE dfp_in_network_impressions_insert_function()
Number of child tables: 214 (Use \d+ to list them.)

One example of the child tables:
Table "dfp_in_network_impressions.dfp_in_network_impressions_20170202"
Column | Type | Modifiers
-----------------+-----------------------------+-----------
impression_time | timestamp without time zone |
nexus_id | character varying |
line_item_id | bigint |
creative_id | bigint |
ad_unit_id | bigint |
Indexes:
"idx_dfp_in_network_impressions_20170202_creative_id" btree (creative_id)
"idx_dfp_in_network_impressions_20170202_line_item_id" btree (line_item_id)
Check constraints:
"dfp_in_network_impressions_20170202_impression_time_check" CHECK (to_char(impression_time, 'YYYYMMDD'::text) = '20170202'::text)
Inherits: dfp_in_network_impressions

Confirmed that the records are in the correct partitions.

We even tried to query with the exact same condition as it is defined in the check constraint:
explain select * from dfp_in_network_impressions where to_char(impression_time, 'YYYYMMDD'::text) = '20170202'::text;
QUERY PLAN
---------------------------------------------------------------------------------------------------
Append (cost=0.00..18655467.21 rows=3831328 width=45)
-> Seq Scan on dfp_in_network_impressions (cost=0.00..0.00 rows=1 width=64)
Filter: (to_char(impression_time, 'YYYYMMDD'::text) = '20170202'::text)
-> Seq Scan on dfp_in_network_impressions_20170101 (cost=0.00..7261.48 rows=1491 width=45)
Filter: (to_char(impression_time, 'YYYYMMDD'::text) = '20170202'::text)
-> Seq Scan on dfp_in_network_impressions_20170219 (cost=0.00..20824.01 rows=4277 width=45)
Filter: (to_char(impression_time, 'YYYYMMDD'::text) = '20170202'::text)
-> Seq Scan on dfp_in_network_impressions_20170102 (cost=0.00..28899.83 rows=5935 width=45)
Filter: (to_char(impression_time, 'YYYYMMDD'::text) = '20170202'::text)
-> Seq Scan on dfp_in_network_impressions_20170220 (cost=0.00..95576.80 rows=19629 width=45)
Filter: (to_char(impression_time, 'YYYYMMDD'::text) = '20170202'::text)
-> Seq Scan on dfp_in_network_impressions_20170103 (cost=0.00..88588.22 rows=18194 width=45)
Filter: (to_char(impression_time, 'YYYYMMDD'::text) = '20170202'::text)
-> Seq Scan on dfp_in_network_impressions_20170221 (cost=0.00..116203.54 rows=23865 width=45)
Filter: (to_char(impression_time, 'YYYYMMDD'::text) = '20170202'::text)
-> Seq Scan on dfp_in_network_impressions_20170410 (cost=0.00..158102.98 rows=32470 width=45)
Filter: (to_char(impression_time, 'YYYYMMDD'::text) = '20170202'::text)
-> Seq Scan on dfp_in_network_impressions_20170531 (cost=0.00..116373.83 rows=23900 width=45)
Filter: (to_char(impression_time, 'YYYYMMDD'::text) = '20170202'::text)
-> Seq Scan on dfp_in_network_impressions_20170104 (cost=0.00..91502.48 rows=18792 width=45)
Filter: (to_char(impression_time, 'YYYYMMDD'::text) = '20170202'::text)
-> Seq Scan on dfp_in_network_impressions_20170222 (cost=0.00..106469.76 rows=21866 width=45)
Filter: (to_char(impression_time, 'YYYYMMDD'::text) = '20170202'::text)
-> Seq Scan on dfp_in_network_impressions_20170411 (cost=0.00..152244.92 rows=31267 width=45)
Filter: (to_char(impression_time, 'YYYYMMDD'::text) = '20170202'::text)
-> Seq Scan on dfp_in_network_impressions_20170601 (cost=0.00..117742.66 rows=24181 width=45)
Filter: (to_char(impression_time, 'YYYYMMDD'::text) = '20170202'::text)
-> Seq Scan on dfp_in_network_impressions_20170105 (cost=0.00..87029.80 rows=17874 width=45)
Filter: (to_char(impression_time, 'YYYYMMDD'::text) = '20170202'::text)
-> Seq Scan on dfp_in_network_impressions_20170223 (cost=0.00..105371.79 rows=21641 width=45)
Filter: (to_char(impression_time, 'YYYYMMDD'::text) = '20170202'::text)
-> Seq Scan on dfp_in_network_impressions_20170412 (cost=0.00..143897.43 rows=29553 width=45)
Filter: (to_char(impression_time, 'YYYYMMDD'::text) = '20170202'::text)
… Etc.

It scans through every partitions. Shouldn’t it only scan the dfp_in_network_impressions.dfp_in_network_impressions_20170202 child table? Or we missing something?
Any advice/help would highly appreciated.

System details:
Postgres version: PostgreSQL 9.4.4 on x86_64-unknown-linux-gnu, compiled by gcc (GCC) 4.4.7 20120313 (Red Hat 4.4.7-11), 64-bit
The constraint_exclusion parameter is set to partition, but same behavior when I set it to “on”.

SELECT name, current_setting(name), SOURCE
FROM pg_settings
WHERE SOURCE NOT IN ('default', 'override’);

name | current_setting | source
------------------------------+-----------------------------------------+----------------------
application_name | psql | client
archive_command | /var/db/wal_archive.sh %p %f | configuration file
archive_mode | on | configuration file
autovacuum_naptime | 1min | configuration file
checkpoint_completion_target | 0.9 | configuration file
checkpoint_segments | 32 | configuration file
client_encoding | UTF8 | client
DateStyle | ISO, MDY | configuration file
default_text_search_config | pg_catalog.english | configuration file
effective_cache_size | 96GB | configuration file
huge_pages | try | 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
listen_addresses | * | configuration file
log_autovacuum_min_duration | 0 | configuration file
log_checkpoints | on | configuration file
log_connections | on | configuration file
log_destination | stderr | configuration file
log_directory | /var/log/postgresql | configuration file
log_duration | on | configuration file
log_file_mode | 0640 | configuration file
log_filename | postgresql-%Y%m%d.log | configuration file
log_line_prefix | %t [%p]: [%l-1] %h %d %u | configuration file
log_lock_waits | on | configuration file
log_min_duration_statement | 100ms | configuration file
log_min_error_statement | warning | configuration file
log_min_messages | warning | configuration file
log_rotation_age | 1d | configuration file
log_rotation_size | 0 | configuration file
log_statement | ddl | configuration file
log_timezone | US/Central | configuration file
log_truncate_on_rotation | on | configuration file
logging_collector | on | configuration file
maintenance_work_mem | 1GB | configuration file
max_connections | 110 | configuration file
max_locks_per_transaction | 256 | configuration file
max_stack_depth | 2MB | environment variable
max_wal_senders | 3 | configuration file
port | 5432 | configuration file
shared_buffers | 64GB | configuration file
TimeZone | US/Central | configuration file
track_activities | on | configuration file
track_counts | on | configuration file
track_functions | none | configuration file
track_io_timing | off | configuration file
wal_keep_segments | 2000 | configuration file
wal_level | hot_standby | configuration file
work_mem | 768MB | configuration file

Linux 2.6.32-504.30.3.el6.x86_64 #1 SMP Wed Jul 15 10:13:09 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux

Thank you!
Aniko