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Re: Queries 15 times slower on 8.1 beta 2 than on 8.0

From: "Jean-Pierre Pelletier" <pelletier_32(at)sympatico(dot)ca>
To: "Michael Fuhr" <mike(at)fuhr(dot)org>, "Josh Berkus" <josh(at)agliodbs(dot)com>
Cc: <pgsql-performance(at)postgresql(dot)org>,"John Arbash Meinel" <john(at)arbash-meinel(dot)com>
Subject: Re: Queries 15 times slower on 8.1 beta 2 than on 8.0
Date: 2005-09-22 23:07:41
Message-ID: BAYC1-PASMTP04E49D978E6756650A97F695970@CEZ.ICE (view raw or flat)
Thread:
Lists: pgsql-performance
I don't know if it makes a difference but in my tables,
content.supplierid and content.priceid were nullable.

----- Original Message ----- 
From: "Michael Fuhr" <mike(at)fuhr(dot)org>
To: "Josh Berkus" <josh(at)agliodbs(dot)com>
Cc: <pgsql-performance(at)postgresql(dot)org>; "Jean-Pierre Pelletier" 
<pelletier_32(at)sympatico(dot)ca>; "John Arbash Meinel" <john(at)arbash-meinel(dot)com>
Sent: Thursday, September 22, 2005 6:54 PM
Subject: Re: [PERFORM] Queries 15 times slower on 8.1 beta 2 than on 8.0


> On Thu, Sep 22, 2005 at 03:19:05PM -0700, Josh Berkus wrote:
>> > "  ->  Seq Scan on price p  (cost=0.00..11317.75 rows=581475 width=4)
>> > (actual time=0.004..1143.720 rows=581475 loops=1)"
>>
>> Well, this is your pain point.   Can we see the index scan plan on 8.1?
>> Given that it's *expecting* only one row, I can't understand why it's
>> using a seq scan ...
>
> I've created a simplified, self-contained test case for this:
>
> CREATE TABLE price (
>    priceid  integer PRIMARY KEY
> );
>
> CREATE TABLE supplier (
>    supplierid  integer PRIMARY KEY
> );
>
> CREATE TABLE content (
>    contentid   integer PRIMARY KEY,
>    supplierid  integer NOT NULL REFERENCES supplier,
>    priceid     integer NOT NULL REFERENCES price
> );
>
> INSERT INTO price (priceid) SELECT * FROM generate_series(1, 50000);
> INSERT INTO supplier (supplierid) SELECT * FROM generate_series(1, 10000);
> INSERT INTO content (contentid, supplierid, priceid) VALUES (1, 1, 50000);
>
> ANALYZE price;
> ANALYZE supplier;
> ANALYZE content;
>
> EXPLAIN ANALYZE
> SELECT 0
> FROM content c
> LEFT OUTER JOIN supplier s ON c.supplierid = s.supplierid
> LEFT OUTER JOIN price p    ON c.priceid = p.priceid;
>
> Here's the EXPLAIN ANALYZE from 8.0.3:
>
> Nested Loop Left Join  (cost=0.00..7.06 rows=1 width=0) (actual 
> time=0.180..0.232 rows=1 loops=1)
>   ->  Nested Loop Left Join  (cost=0.00..4.04 rows=1 width=4) (actual 
> time=0.105..0.133 rows=1 loops=1)
>         ->  Seq Scan on content c  (cost=0.00..1.01 rows=1 width=8) 
> (actual time=0.021..0.029 rows=1 loops=1)
>         ->  Index Scan using supplier_pkey on supplier s  (cost=0.00..3.01 
> rows=1 width=4) (actual time=0.052..0.059 rows=1 loops=1)
>               Index Cond: ("outer".supplierid = s.supplierid)
>   ->  Index Scan using price_pkey on price p  (cost=0.00..3.01 rows=1 
> width=4) (actual time=0.046..0.055 rows=1 loops=1)
>         Index Cond: ("outer".priceid = p.priceid)
> Total runtime: 0.582 ms
>
> Here it is from 8.1beta2:
>
> Merge Right Join  (cost=4.05..1054.06 rows=1 width=0) (actual 
> time=676.863..676.895 rows=1 loops=1)
>   Merge Cond: ("outer".priceid = "inner".priceid)
>   ->  Index Scan using price_pkey on price p  (cost=0.00..925.00 
> rows=50000 width=4) (actual time=0.035..383.345 rows=50000 loops=1)
>   ->  Sort  (cost=4.05..4.05 rows=1 width=4) (actual time=0.152..0.159 
> rows=1 loops=1)
>         Sort Key: c.priceid
>         ->  Nested Loop Left Join  (cost=0.00..4.04 rows=1 width=4) 
> (actual time=0.082..0.111 rows=1 loops=1)
>               ->  Seq Scan on content c  (cost=0.00..1.01 rows=1 width=8) 
> (actual time=0.016..0.024 rows=1 loops=1)
>               ->  Index Scan using supplier_pkey on supplier s 
> (cost=0.00..3.01 rows=1 width=4) (actual time=0.039..0.047 rows=1 loops=1)
>                     Index Cond: ("outer".supplierid = s.supplierid)
> Total runtime: 677.563 ms
>
> If we change content's priceid then we get the same plan but faster 
> results:
>
> UPDATE content SET priceid = 1;
>
> Merge Right Join  (cost=4.05..1054.06 rows=1 width=0) (actual 
> time=0.268..0.303 rows=1 loops=1)
>   Merge Cond: ("outer".priceid = "inner".priceid)
>   ->  Index Scan using price_pkey on price p  (cost=0.00..925.00 
> rows=50000 width=4) (actual time=0.049..0.061 rows=2 loops=1)
>   ->  Sort  (cost=4.05..4.05 rows=1 width=4) (actual time=0.187..0.192 
> rows=1 loops=1)
>         Sort Key: c.priceid
>         ->  Nested Loop Left Join  (cost=0.00..4.04 rows=1 width=4) 
> (actual time=0.099..0.128 rows=1 loops=1)
>               ->  Seq Scan on content c  (cost=0.00..1.01 rows=1 width=8) 
> (actual time=0.025..0.033 rows=1 loops=1)
>               ->  Index Scan using supplier_pkey on supplier s 
> (cost=0.00..3.01 rows=1 width=4) (actual time=0.046..0.053 rows=1 loops=1)
>                     Index Cond: ("outer".supplierid = s.supplierid)
> Total runtime: 0.703 ms
>
> -- 
> Michael Fuhr 


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