12

The following query on a foreign takes about 5 seconds to execute on 3.2 million rows:

SELECT x."IncidentTypeCode", COUNT(x."IncidentTypeCode") 
FROM "IntterraNearRealTimeUnitReflexes300sForeign" x 
WHERE x."IncidentDateTime" >= '05/01/2016' 
GROUP BY x."IncidentTypeCode" 
ORDER BY 1;

When I execute the same query on normal table, it returns in .6 seconds. The execution plans are quite different:

Normal Table

Sort  (cost=226861.20..226861.21 rows=4 width=4) (actual time=646.447..646.448 rows=7 loops=1) 
  Sort Key: "IncidentTypeCode" 
  Sort Method: quicksort  Memory: 25kB 
  -> HashAggregate (cost=226861.12..226861.16 rows=4 width=4) (actual  time=646.433..646.434 rows=7 loops=1)
     Group Key: "IncidentTypeCode"
     -> Bitmap Heap Scan on "IntterraNearRealTimeUnitReflexes300s" x  (cost=10597.63..223318.41 rows=708542 width=4) (actual time=74.593..342.110 rows=709376 loops=1) 
        Recheck Cond: ("IncidentDateTime" >= '2016-05-01 00:00:00'::timestamp without time zone) 
        Rows Removed by Index Recheck: 12259 
        Heap Blocks: exact=27052 lossy=26888
        -> Bitmap Index Scan on idx_incident_date_time_300  (cost=0.00..10420.49 rows=708542 width=0) (actual time=69.722..69.722 rows=709376 loops=1) 
           Index Cond: ("IncidentDateTime" >= '2016-05-01 00:00:00'::timestamp without time zone) 

Planning time: 0.165 ms 
Execution time: 646.512 ms

Foreign Table

Sort  (cost=241132.04..241132.05 rows=4 width=4) (actual time=4782.110..4782.112 rows=7 loops=1)   
  Sort Key: "IncidentTypeCode" 
  Sort Method: quicksort  Memory: 25kB
  -> HashAggregate  (cost=241131.96..241132.00 rows=4 width=4) (actual time=4782.097..4782.100 rows=7 loops=1)
     Group Key: "IncidentTypeCode"
     -> Foreign Scan on "IntterraNearRealTimeUnitReflexes300sForeign" x  (cost=10697.63..237589.25 rows=708542 width=4) (actual time=1.916..4476.946 rows=709376 loops=1) 

Planning time: 1.413 ms 
Execution time: 4782.660 ms

I think I'm paying a high price for the GROUP BY clause, which isn't passed to the foreign server when I EXPLAIN VERBOSE:

SELECT
    "IncidentTypeCode"
FROM
    PUBLIC ."IntterraNearRealTimeUnitReflexes300s"
WHERE
    (
        (
            "IncidentDateTime" >= '2016-05-01 00:00:00' :: TIMESTAMP WITHOUT TIME ZONE
        )
    )

This returns 700k rows. Is there a way around this?

I spent a lot of time reading this documentation page yesterday, and thought I had found my answer with setting use_remote_estimate to true, but it had no effect.

I do have access to the foreign server to create objects if necessary. The timestamp value in the WHERE clause can be anything; it does not come from a list of predefined values.

  • 3
    There are some pushdown improvements in 9.6 that might be of interest: wiki.postgresql.org/wiki/NewIn96#postgres_fdw – Jack Douglas Sep 19 '16 at 7:50
  • When you say normal vs foreign table are you running across the same table (locally and remotely) or actually different tables(it reads as if they are), if they are different check the indexing on the remote server make sure they're the same as you appear to be reading completely different information sources IntterraNearRealTimeUnitReflexes300sForeign vs IntterraNearRealTimeUnitReflexes300s and idx_incident_date_time_300 I presume the 300s ones are the same, but it might be worth checking if the idx_incident_date_time_300 index exists on the foreign server – Ste Bov Sep 28 '16 at 9:34
  • 2
    From what I understand, aggregates (COUNT) are not pushed to the remote server, which would explain the long request time. It seems this feature will appear in pg 10 - depesz.com/2016/10/25/… – Jerome WAGNER Jan 30 '17 at 15:04
  • @JeromeWAGNER - Awesome – J-DawG Jan 30 '17 at 17:53
7
+200

If you use use_remote_estimate be sure to run ANALYZE the foreign table (I see estimations pretty close with the returned, you'd probably did it). Also, the pushdown improvements are not available in <9.5 version. I also assume that you have the same table structure on the remote server either (including indexes). If a bitmap is needed due to the low cardinality, it won't use the index due to the limitations on the pushdown mechanism. You may want to reduce the amount of returned rows to force a BTREE index scan (timestamp ranges). Unfortunately, there is no clean way to avoid the SeqScan on the remote server if the filter returns +10% of the rows of the table (may vary this percentage if the planner considers that scanning the whole table is cheaper than seek reads). If you are using SSD, you will probably find useful to tweak random_page_cost).

You can use CTE to isolate the GROUP BY behavior:

WITH atable AS (
    SELECT "IncidentTypeCode"
    FROM PUBLIC ."IntterraNearRealTimeUnitReflexes300s"
    WHERE 
       ("IncidentDateTime" 
              BETWEEN '2016-05-01 00:00:00'::TIMESTAMP WITHOUT TIME ZONE 
                  AND '2016-05-02 00:00:00'::TIMESTAMP WITHOUT TIME ZONE)
)
SELECT atable."IncidentTypeCode", COUNT(atable.IncidentTypeCode) 
FROM atable
GROUP BY atable."IncidentTypeCode" 
ORDER BY atable."IncidentTypeCode";
  • 1
    Performance was the same using the CTE. Will try the random_page_cost settings though. Thanks! – J-DawG Sep 28 '16 at 16:45

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