Postgresql – Planning of Select on PostgreSQL 9.6 partitioned table very slow

I have a rather massive table of financial data in Postgres 9.6. It is partitioned by trading day (DATE type) and security ID (BIGINT). Overall the table comprises about 9300 partitions, takeing up 1765 GB in total. I use the conventional method of partitioning using CHECK constraints. The partitions are unlogged tables which I load in-bulk via COPY.

When I perform a query which can be satisfied from a single partition, the planner applies constraint exclusion and determines correctly which table should be consulted. However this takes about 12 seconds, and sometimes a lot more the first time. Could you suggest ways to help the planner work faster?

Example EXPLAIN ANALYZE output:

trading_offline=# EXPLAIN ANALYZE--                        
select * from public."ticks"
where trading_day ='2016-06-20'::date and inst_id= 1535681 and is_trade = true
and qty<-200;
                                                                        QUERY PLAN                                                                           
----------------------------------------------------------------------------------------------------------------------------------------------------------------
Append  (cost=0.00..16751.50 rows=16 width=177) (actual time=299.068..415.080 rows=5 loops=1)
->  Seq Scan on ticks  (cost=0.00..0.00 rows=1 width=161) (actual time=0.008..0.008 rows=0 loops=1)
        Filter: (is_trade AND (qty < '-200'::integer) AND (trading_day = '2016-06-20'::date) AND (inst_id = 1535681))
->  Bitmap Heap Scan on "ticks$20160620$1535681"  (cost=766.35..16751.50 rows=15 width=177) (actual time=299.059..415.068 rows=5 loops=1)
        Filter: (is_trade AND (qty < '-200'::integer) AND (trading_day = '2016-06-20'::date) AND (inst_id = 1535681))
        Rows Removed by Filter: 41833
        Heap Blocks: exact=10002
        ->  Bitmap Index Scan on "ticks$20160620$1535681_is_trade_idx"  (cost=0.00..766.35 rows=41323 width=0) (actual time=45.212..45.212 rows=41838 loops=1)
            Index Cond: (is_trade = true)
Planning time: 12233.724 ms
Execution time: 415.411 ms