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slow plan on join when adding where clause

From: "L(dot) Loewe" <lloewe(at)hotmail(dot)com>
To: pgsql-novice(at)postgresql(dot)org
Subject: slow plan on join when adding where clause
Date: 2010-03-29 00:19:04
Message-ID: op.vaa1x2utb4gxma@oakcreek2 (view raw or flat)
Thread:
Lists: pgsql-novice
Hi all


I have a fairly large table ("main" ~50M rows) containing a timestamp  
(indexed),
  a sensor_id (indexed) and some sensor data ranging over a few years.

The second table ("events" <500 rows) contains a set of interesting events  
with
an events.id and two timestamps: events.start and events.stop plus some
additional data.

Now I want to join these tables to get the sensor data for those events.

The interval between start and stop is quite short for each event (usually  
a
couple of minutes) so that there aren't too many rows from table "main"
matching this criteria (~1K) for each event:

SELECT * FROM events, main WHERE main.ti > events.start and main.ti <  
events.stop;

EXPLAIN ANALYZE gives

Nested Loop  (cost=0.00..27621542.27 rows=524505120 width=68)
                  (actual time=0.038..42.314 rows=69209 loops=1)
    ->  Seq Scan on events (cost=0.00..2.24 rows=124 width=28)
                 (actual time=0.006..0.025 rows=124 loops=1)
    ->  Index Scan using idx_main_ti on main  (cost=0.00..159306.16  
rows=4229880 width=40)
                 (actual time=0.016..0.178 rows=558 loops=124)
          Index Cond: ((main.ti > events.start) AND (main.ti < events.stop))
Total runtime: 47.682 ms

So far so good, however if I add a sensor_id constraint the planner  
chooses a
different approach:


SELECT  * FROM  events, main WHERE  main.ti > events.start and
                                     main.ti < events.stop and sensor_id=1;

Nested Loop  (cost=7309.32..1422246.30 rows=4795865 width=68)
                 (actual time=23427.599..23886.276 rows=772 loops=1)
    Join Filter: ((main.ti > events.start) AND (main.ti < events.stop))
    ->  Bitmap Heap Scan on main (cost=7306.96..343174.23 rows=348087  
width=40)
                 (actual time=3771.719..9508.728 rows=490984 loops=1)
          Recheck Cond: (sensor_id= 1)
          ->  Bitmap Index Scan on idx_main_sens (cost=0.00..7219.94  
rows=348087 width=0)
                 (actual time=3769.075..3769.075 rows=491102 loops=1)
                Index Cond: (sensor_id= 1)
    ->  Materialize  (cost=2.36..3.60 rows=124 width=28)
                 (actual time=0.000..0.010 rows=124 loops=490984)
          ->  Seq Scan on events (cost=0.00..2.24 rows=124 width=28)
                (actual time=0.005..0.021 rows=124 loops=1)
Total runtime: 23886.494 ms


Apparently the planner assumes that sensor_id=1 will return fewer rows than
the time constraint while the opposite is true:
sensor_id=1 -> ~ 500K , time window -> ~ 1K.

Is there a way to hint the planner to use plan 1 even with the sensor_id=1
clause or am I doing something fundamentally wrong here?

Thanks

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