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

Thu, 22 Sep 2005 16:11:34 -0700 

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" <[EMAIL PROTECTED]> 
To: "Josh Berkus" <josh@agliodbs.com>
Cc: <pgsql-performance@postgresql.org>; "Jean-Pierre Pelletier" <[EMAIL PROTECTED]>; "John Arbash Meinel" <[EMAIL PROTECTED]> 
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 


---------------------------(end of broadcast)---------------------------
TIP 9: In versions below 8.0, the planner will ignore your desire to
      choose an index scan if your joining column's datatypes do not
      match

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