postgres how to debug why planning time is too long?

Question

version - postgres 9.6.

I were not so clear in question i asked in past and someone already answer there, so i thought best will be to post new question with more clear info and be more specific about my question.

Trying to join event table with dimension table. event table is a daily partition (3k children) table with check constraints.The event table has 72 columns (i suspect that this is the issue).

I simplify the query in order to demonstrate the question (in practice range is wider and i query field from both tables).

You can see that for this simple query - the plan take almost 10 seconds (my question is about plan time and not execution time). If i query direct on the child table ( please dont advice to use union on all child in range ) query plan is few ms.

explain analyze select campaign_id ,  spent   as spent from events_daily r left join report_campaigns c on r.campaign_id = c.c_id  where  date >= '20170720' and date < '20170721' ;
                                                                      QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------------------------
 Nested Loop Left Join  (cost=0.29..28.88 rows=2 width=26) (actual time=0.021..0.021 rows=0 loops=1)
   ->  Append  (cost=0.00..12.25 rows=2 width=26) (actual time=0.003..0.003 rows=0 loops=1)
         ->  Seq Scan on events_daily r  (cost=0.00..0.00 rows=1 width=26) (actual time=0.002..0.002 rows=0 loops=1)
               Filter: ((date >= '2017-07-20 00:00:00'::timestamp without time zone) AND (date < '2017-07-21 00:00:00'::timestamp without time zone))
         ->  Seq Scan on events_daily_20170720 r_1  (cost=0.00..12.25 rows=1 width=26) (actual time=0.000..0.000 rows=0 loops=1)
               Filter: ((date >= '2017-07-20 00:00:00'::timestamp without time zone) AND (date < '2017-07-21 00:00:00'::timestamp without time zone))
   ->  Index Only Scan using report_campaigns_campaign_idx on report_campaigns c  (cost=0.29..8.31 rows=1 width=8) (never executed)
         Index Cond: (c_id = r.campaign_id)
         Heap Fetches: 0
 Planning time: 8393.337 ms
 Execution time: 0.132 ms
(11 rows)

explain analyze select campaign_id ,  spent   as spent from events_daily_20170720 r left join report_campaigns c on r.campaign_id = c.c_id  where  date >= '20170720' and date < '20170721' ;
                                                                   QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------------------
 Nested Loop Left Join  (cost=0.29..20.57 rows=1 width=26) (actual time=0.008..0.008 rows=0 loops=1)
   ->  Seq Scan on events_daily_20170720 r  (cost=0.00..12.25 rows=1 width=26) (actual time=0.007..0.007 rows=0 loops=1)
         Filter: ((date >= '2017-07-20 00:00:00'::timestamp without time zone) AND (date < '2017-07-21 00:00:00'::timestamp without time zone))
   ->  Index Only Scan using report_campaigns_campaign_idx on report_campaigns c  (cost=0.29..8.31 rows=1 width=8) (never executed)
         Index Cond: (c_id = r.campaign_id)
         Heap Fetches: 0
 Planning time: 0.242 ms
 Execution time: 0.059 ms


\d events_daily_20170720
 date                      | timestamp without time zone |
Check constraints:
    "events_daily_20170720_date_check" CHECK (date >= '2017-07-20 00:00:00'::timestamp without time zone AND date < '2017-07-21 00:00:00'::timestamp without time zone)
Inherits: events_daily

show constraint_exclusion;
 constraint_exclusion
----------------------
 on

When running ltrace it seems that it run this thousands of time on each field (hint that it run on all patitions tables for the plan) :

strlen("process")                                                                                                                                                    = 7
memcpy(0x0b7aac10, "process", 8)                                                                                                                                     = 0x0b7aac10
strlen("channel")                                                                                                                                                    = 7
memcpy(0x0b7aac68, "channel", 8)                                                                                                                                     = 0x0b7aac68
strlen("deleted")                                                                                                                                                    = 7
memcpy(0x0b7aacc0, "deleted", 8)                                                                                                                                     = 0x0b7aacc0
strlen("old_spent")                                                                                                                                                  = 9
memcpy(0x0b7aad18, "old_spent", 10)

Answer 1

The problem is that you have too many partitions.

As the documentation warns:

All constraints on all partitions of the master table are examined during constraint exclusion, so large numbers of partitions are likely to increase query planning time considerably. Partitioning using these techniques will work well with up to perhaps a hundred partitions; don't try to use many thousands of partitions.

You should try to reduce the number of partitions by using a longer time interval for each partition.

Alternatively, you could try to change the application code to directly access the correct partition if possible, but that might prove difficult and it removes many advantages that partitioning should bring.