>-----Original Message----- >From: Tompkins Neil [mailto:neil.tompk...@googlemail.com] >Sent: Thursday, July 22, 2010 3:39 PM >To: Shawn Green (MySQL) >Cc: mysql@lists.mysql.com >Subject: Re: combined or single indexes? > >Thanks for your reply, and sorry for not verifying in the manual. Another >couple of questions I have : > >If I run a EXPLAIN query and SELECT against a primary key and SELECT fields >which are not indexed, I assume that returned EXPLAIN statement as below, >means I don't need to index additional fields providing the PRIMARY KEY is >included in the SELECT statement ? > >table,type,possible_keys,key,key_len,ref,rows,Extra, >Products,const,PRIMARY,PRIMARY,8,const,1,, > [JS] Your posts will be more legible if you use "\G" instead of ";" at the end of an EXPLAIN.
As for the indexing, if you only need one key then you only need one key. Just remember that when you test things with sample data, MySQL might make surprising decisions based upon the amount of data. You'll only really know what will happen if you have a substantial data set. >Also, if I want to add a index to an existing table containing 9000 records, >how long should I expect this to take ? Is it instant ? > [JS] Faster than you can type, I should think. Regards, Jerry Schwartz Global Information Incorporated 195 Farmington Ave. Farmington, CT 06032 860.674.8796 / FAX: 860.674.8341 E-mail: je...@gii.co.jp Web site: www.the-infoshop.com >Cheers >Neil > > >On Thu, Jul 22, 2010 at 5:20 PM, Shawn Green (MySQL) < >shawn.l.gr...@oracle.com> wrote: > >> On 7/21/2010 1:02 PM, Tompkins Neil wrote: >> >>> Hi >>> >>> So Just running a basic query I get returned the following : >>> >>> table,type,possible_keys,key,key_len,ref,rows,Extra, >>> Products,ALL,,,,,9884,where used, >>> >>> Therefore, I assume "*ALL*" is the worst possible type and should look at >>> adding a an index to this particular field ? >>> >>> >> Why assume when the manual is right there to remove all doubt? >> >> http://dev.mysql.com/doc/refman/5.1/en/explain-output.html >> #### >> ALL >> >> A full table scan is done for each combination of rows from the previous >> tables. This is normally not good if the table is the first table not >> marked >> const, and usually very bad in all other cases. Normally, you can avoid ALL >> by adding indexes that enable row retrieval from the table based on >> constant >> values or column values from earlier tables. >> #### >> >> >> >> >> -- >> Shawn Green >> MySQL Principle Technical Support Engineer >> Oracle USA, Inc. >> Office: Blountville, TN >> -- MySQL General Mailing List For list archives: http://lists.mysql.com/mysql To unsubscribe: http://lists.mysql.com/mysql?unsub=arch...@jab.org
