Re: slow joins?

Joe --

>________________________________
> From: Joe Van Dyk <joe(at)tanga(dot)com>
>To: pgsql-performance(at)postgresql(dot)org
>Sent: Friday, April 5, 2013 6:42 PM
>Subject: Re: [PERFORM] slow joins?
>
>
>(https://gist.github.com/joevandyk/df0df703f3fda6d14ae1/raw/c15cae813913b7f8c35b24b467a0c732c0100d79/gistfile1.txt shows a non-wrapped version of the queries and plan)
>
>
>
>
>On Fri, Apr 5, 2013 at 6:38 PM, Joe Van Dyk <joe(at)tanga(dot)com> wrote:
>
>On 9.2.4, running two identical queries except for the value of a column in the WHERE clause. Postgres is picking very different query plans, the first is much slower than the second.
>>
>>
>>Any ideas on how I can speed this up?  I have btree indexes for all the columns used in the query.
>>
>>explain analyze                                                                                    
>>SELECT COUNT(*)                                                                                    
>>FROM purchased_items pi                                                                            
>>inner join line_items li on li.id = pi.line_item_id                                                
>>inner join products      on products.id = li.product_id                                            
>>WHERE products.drop_shipper_id = 221;
>>
>> Aggregate  (cost=193356.31..193356.32 rows=1 width=0) (actual time=2425.225..2425.225 rows=1 loops=1)
>>   ->  Hash Join  (cost=78864.43..193160.41 rows=78360 width=0) (actual time=726.612..2424.206 rows=8413 loops=1)
>>         Hash Cond: (pi.line_item_id = li.id)
>>         ->  Seq Scan on purchased_items pi  (cost=0.00..60912.39 rows=3724639 width=4) (actual time=0.008..616.812 rows=3724639 loops=1)
>>         ->  Hash  (cost=77937.19..77937.19 rows=56499 width=4) (actual time=726.231..726.231 rows=8178 loops=1)
>>               Buckets: 4096  Batches: 4  Memory Usage: 73kB
>>               ->  Hash Join  (cost=1684.33..77937.19 rows=56499 width=4) (actual time=1.270..723.222 rows=8178 loops=1)
>>                     Hash Cond: (li.product_id = products.id)
>>                     ->  Seq Scan on line_items li  (cost=0.00..65617.18 rows=2685518 width=8) (actual time=0.081..392.926 rows=2685499 loops=1)
>>                     ->  Hash  (cost=1676.60..1676.60 rows=618 width=4) (actual time=0.835..0.835 rows=618 loops=1)
>>                           Buckets: 1024  Batches: 1  Memory Usage: 22kB
>>                           ->  Bitmap Heap Scan on products  (cost=13.07..1676.60 rows=618 width=4) (actual time=0.185..0.752 rows=618 loops=1)
>>                                 Recheck Cond: (drop_shipper_id = 221)
>>                                 ->  Bitmap Index Scan on index_products_on_drop_shipper_id  (cost=0.00..12.92 rows=618 width=0) (actual time=0.125..0.125 rows=618 loops=1)
>>                                       Index Cond: (drop_shipper_id = 221)
>> Total runtime: 2425.302 ms
>>
>>
>>explain analyze                                                                                    
>>SELECT COUNT(*)                                                                                    
>>FROM purchased_items pi                                                                            
>>inner join line_items li on li.id = pi.line_item_id                                                
>>inner join products      on products.id = li.product_id                                            
>>WHERE products.drop_shipper_id = 2;                                                                
>>                                                                                                                     
>>
>> Aggregate  (cost=29260.40..29260.41 rows=1 width=0) (actual time=0.906..0.906 rows=1 loops=1)
>>   ->  Nested Loop  (cost=0.00..29254.38 rows=2409 width=0) (actual time=0.029..0.877 rows=172 loops=1)
>>         ->  Nested Loop  (cost=0.00..16011.70 rows=1737 width=4) (actual time=0.021..0.383 rows=167 loops=1)
>>               ->  Index Scan using index_products_on_drop_shipper_id on products  (cost=0.00..80.41 rows=19 width=4) (actual time=0.010..0.074 rows=70 loops=1)
>>                     Index Cond: (drop_shipper_id = 2)
>>               ->  Index Scan using index_line_items_on_product_id on line_items li  (cost=0.00..835.70 rows=279 width=8) (actual time=0.002..0.004 rows=2 loops=70)
>>                     Index Cond: (product_id = products.id)
>>         ->  Index Only Scan using purchased_items_line_item_id_idx on purchased_items pi  (cost=0.00..7.60 rows=2 width=4) (actual time=0.002..0.003 rows=1 loops=167)
>>               Index Cond: (line_item_id = li.id)
>>               Heap Fetches: 5
>> Total runtime: 0.955 ms
>>(11 rows)
>>
>

Does drop_shipper+id have a much larger number of rows which is making the scanner want to avoid an indexed scan or otherwise prefer a sequential scan on products and on line_items ?

What are the stats settings for these tables ?

HTH,

Greg WIlliamson