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From:Bastian Balthazar Bux Date:October 3 2005 2:24pm
Subject:Re: Double indexes on one field
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Yannick Warnier wrote:
> Le lundi 03 octobre 2005 à 12:29 +0200, Martijn Tonies a écrit :
> 
>>>Using PhpMyAdmin, I seldom get the warning message:
>>>PRIMARY and INDEX keys should not both be set for column `ID`
>>>
>>>I understand its meaning, but I was wondering to what extent having a
>>>field indexed AND being a primary key might slow down/speed up my
>>>queries.
>>>
>>>Is that gonna take twice the time if I am searching on the ID field,
>>>just because there are two indexes?
>>>
>>>I'd like to have a rough idea of how serioulsy I need to avoid these.
>>
>>Creating a PRIMARY KEY will automatically create an index.
>>
>>Why would you create a second index for that field?
> 
> 
> I'm asking myself the same question. I am working on someone else's
> database.
> 
> Thanks both,
> 
> Yannick
> 
> 


Is it a multi-field index ? some versions of phpmyadmin show that
messages also if only one field is duplicated.

example:


CREATE TABLE `tab_sint` (
  `id_cns` tinyint(3) unsigned NOT NULL default '0',
  `anno_dep` smallint(4) unsigned zerofill NOT NULL default '0000',
  `data_dep` smallint(4) unsigned zerofill NOT NULL default '0000',
  `particolare` mediumint(8) unsigned NOT NULL default '0',
  `generale` mediumint(8) unsigned NOT NULL default '0',
  UNIQUE KEY `idx_cns_gen_anno`
(`anno_dep`,`id_cns`,`generale`,`particolare`),
  UNIQUE KEY `idx_cns_par_anno`
(`id_cns`,`anno_dep`,`tipo`,`particolare`,`bis`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8 PACK_KEYS=1 AUTO_INCREMENT=87189031 ;


This schema issue the warning on 'id_cns' but really make sense having 2
indices here since they serves different kind of querys and constraints.

Regards,
Francesco
Thread
Double indexes on one fieldYannick Warnier3 Oct
  • Re: Double indexes on one fieldMartijn Tonies3 Oct
    • Re: Double indexes on one fieldYannick Warnier3 Oct
      • Re: Double indexes on one fieldBastian Balthazar Bux3 Oct
        • Re: Double indexes on one fieldYannick Warnier3 Oct
  • Re: Double indexes on one fieldAlec.Cawley3 Oct