List:General Discussion« Previous MessageNext Message »
From:Dan Nelson Date:November 27 2006 2:37am
Subject:Re: Performance Problems With Two Tables With Over 500K Rows
View as plain text  
In the last episode (Nov 26), John Kopanas said:
> On 11/26/06, Dan Nelson <dnelson@stripped> wrote:
> >In the last episode (Nov 26), John Kopanas said:
> >> Thanks a lot for your help.
> >>
> >> The query should and only does return 1-6 rows depending on the id.
> >> Never more then that.  Here are the comperative EXPLAINs:
> >>
> >> mysql> EXPLAIN SELECT * FROM purchased_services WHERE id = 1000;
> >> 
>
> >>+----+-------------+--------------------+-------+---------------+---------+---------+-------+------+-------+
> >>| id | select_type | table              | type  | possible_keys | key   |
> key_len | ref   | rows | Extra |
>
> >>+----+-------------+--------------------+-------+---------------+---------+---------+-------+------+-------+
> >>|  1 | SIMPLE      | purchased_services | const | PRIMARY       | PRIMARY |  
>     4 | const |    1 |       |
>
> >>+----+-------------+--------------------+-------+---------------+---------+---------+-------+------+-------+
> >> 1 row in set (0.00 sec)
> >
> >This query definitly should run almost instantly, since it looks like a
> >direct lookup on the primary key.
> >
> >> mysql> EXPLAIN SELECT * FROM purchased_services WHERE company_id = 1000;
> >> 
>
> >>+----+-------------+--------------------+------+-------------------------------------+------+---------+------+--------+-------------+
> >>| id | select_type | table              | type | possible_keys           |
> key  | key_len | ref  | rows   | Extra       |
>
> >>+----+-------------+--------------------+------+-------------------------------------+------+---------+------+--------+-------------+
> >>|  1 | SIMPLE      | purchased_services | ALL  |
> purchased_services_company_id_index | NULL |    NULL | NULL | 626188 | Using where |
>
> >>+----+-------------+--------------------+------+-------------------------------------+------+---------+------+--------+-------------+
> >> 1 row in set (0.00 sec)
> >
> >This EXPLAIN indicates that mysql thinks that the query would match
> >~600k rows and will do a full table scan.  Mysql only keeps a single
> >"cardinality" value for each index that estimates how many records have
> >a unique value in the index.  This can cause problems for the optimizer
> >if you have one value for say 60% of the table, and unique values for
> >the rest.  You can try adding a FORCE INDEX clause to the query and see
> >if that helps.
>
> When I did a:
> 
> SELECT * FROM purchased_services WHERE company_id = 1000;
> 
> It took me 7 seconds.  This is driving me crazy!
>
> I am going to have to try this on another computer and see if I am
> going to get the same results on another system.  Argh...

Yes, a full table scan on 500k rows may very well take 7 seconds.  Did
you try with a FORCE INDEX (purchased_services_company_id) clause?

-- 
	Dan Nelson
	dnelson@stripped
Thread
Performance Problems With Two Tables With Over 500K RowsJohn Kopanas25 Nov
  • Re: Performance Problems With Two Tables With Over 500K RowsJohn Kopanas26 Nov
    • Re: Re: Performance Problems With Two Tables With Over 500K RowsJohn Kopanas26 Nov
      • Re: Re: Re: Performance Problems With Two Tables With Over 500K RowsDan Buettner26 Nov
        • Re: Re: Re: Re: Performance Problems With Two Tables With Over 500K RowsJohn Kopanas27 Nov
  • Re: Performance Problems With Two Tables With Over 500K RowsDan Nelson26 Nov
    • Re: Re: Performance Problems With Two Tables With Over 500K RowsJohn Kopanas27 Nov
      • Re: Performance Problems With Two Tables With Over 500K RowsDan Nelson27 Nov
        • Re: Re: Performance Problems With Two Tables With Over 500K RowsJohn Kopanas27 Nov
          • Re: Performance Problems With Two Tables With Over 500K RowsDan Nelson27 Nov
            • Re: Re: Performance Problems With Two Tables With Over 500K RowsJohn Kopanas27 Nov
          • Re: Re: Performance Problems With Two Tables With Over 500KRowsmos27 Nov
        • Re: Performance Problems With Two Tables With Over 500K RowsDominik Klein27 Nov
  • Re: Performance Problems With Two Tables With Over 500K RowsDuncan Hill27 Nov