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We have two tables, dst_port_hour and dst_port_day, which should be  
very similar, they both have about 50.000.000 rows. In both tables we  
have an index for period_id.

We run postgresql  7.4.5 on a dedicated Debian server, with dual Intel  
Xeon 3GHz and 4GB memory.

The problem is that on the dst_port_day table, postgresql is using  
seqscan, and not the index when it should. Forcing the use of the index  
by setting enable_seqscan to false, makes the query lighthening fast.  
When using seqscan, the query takes several minutes. The planner  
calculates the cost for Index scan to be much more than sequence scan.

Why is our query planner misbehaving?

Here are the exaplain analyze output with and without index-force:


SET enable_seqscan=false;

stager=>  explain analyze SELECT cur.portnr FROM dst_port_day cur WHERE  
cur.period_id='2779' GROUP BY cur.portnr  ORDER BY SUM(cur.octets) DESC  
  LIMIT 5;
                                                                          
            QUERY PLAN
- ------------------------------------------------------------------------ 
- ------------------------------------------------------------------------ 
- ---------------------------------
  Limit  (cost=2022664.62..2022664.63 rows=5 width=12) (actual  
time=831.772..831.816 rows=5 loops=1)
    ->  Sort  (cost=2022664.62..2022664.82 rows=80 width=12) (actual  
time=831.761..831.774 rows=5 loops=1)
          Sort Key: sum(octets)
          ->  HashAggregate  (cost=2022661.89..2022662.09 rows=80  
width=12) (actual time=587.036..663.991 rows=16396 loops=1)
                ->  Index Scan using dst_port_day_period_id_key on  
dst_port_day cur  (cost=0.00..2019931.14 rows=546150 width=12) (actual  
time=0.038..303.801 rows=48072 loops=1)
                      Index Cond: (period_id = 2779)
  Total runtime: 836.362 ms
(7 rows)



SET enable_seqscan=true;

stager=>   explain analyze SELECT cur.portnr FROM dst_port_day cur  
WHERE cur.period_id='2779' GROUP BY cur.portnr  ORDER BY  
SUM(cur.octets) DESC  LIMIT 5;
                                                                        
QUERY PLAN
- ------------------------------------------------------------------------ 
- ------------------------------------------------------------------------ 
- ------
  Limit  (cost=1209426.88..1209426.89 rows=5 width=12) (actual  
time=299053.006..299053.053 rows=5 loops=1)
    ->  Sort  (cost=1209426.88..1209427.08 rows=80 width=12) (actual  
time=299052.995..299053.008 rows=5 loops=1)
          Sort Key: sum(octets)
          ->  HashAggregate  (cost=1209424.15..1209424.35 rows=80  
width=12) (actual time=298803.273..298881.020 rows=16396 loops=1)
                ->  Seq Scan on dst_port_day cur  (cost=0.00..1206693.40  
rows=546150 width=12) (actual time=298299.508..298526.544 rows=48072  
loops=1)
                      Filter: (period_id = 2779)
  Total runtime: 299057.643 ms
(7 rows)

- -- 
Andreas Ãkre Solberg, UNINETT AS Testnett
Contact info and Public PGP Key available on:
http://andreas.solweb.no/?account=Work

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=bYtH
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TIP 9: the planner will ignore your desire to choose an index scan if your
      joining column's datatypes do not match

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