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Top Forums Programming Getting Rows from a MySQL Table with max values? Post 302633841 by Corona688 on Wednesday 2nd of May 2012 02:03:10 PM
Old 05-02-2012
I'm not saying a full table-scan will happen as much as, the result of an indexed query can't be different than what a table scan could do. (Except in order, which isn't guaranteed anyway, ORDER BY sorts after the fact.) Therefore, it won't use information from the index to know the maximum in advance. Your second query, where you just 'select max(..) from ...' would be able to use the index.

I'm not sure index is required either, as much as the performance could be quite poor if it isn't.
 

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MYSQLDUMPSLOW(1)					       MySQL Database System						  MYSQLDUMPSLOW(1)

NAME
mysqldumpslow - Summarize slow query log files SYNOPSIS
mysqldumpslow [options] [log_file ...] DESCRIPTION
The MySQL slow query log contains information about queries that take a long time to execute (see Section 5.2.5, "The Slow Query Log"). mysqldumpslow parses MySQL slow query log files and prints a summary of their contents. Normally, mysqldumpslow groups queries that are similar except for the particular values of number and string data values. It "abstracts" these values to N and 'S' when displaying summary output. The -a and -n options can be used to modify value abstracting behavior. Invoke mysqldumpslow like this: shell> mysqldumpslow [options] [log_file ...] mysqldumpslow supports the following options. o --help Display a help message and exit. o -a Do not abstract all numbers to N and strings to 'S'. o --debug, -d Run in debug mode. o -g pattern Consider only queries that match the (grep-style) pattern. o -h host_name Host name of MySQL server for *-slow.log file name. The value can contain a wildcare. The default is * (match all). o -i name Name of server instance (if using mysql.server startup script). o -l Do not subtract lock time from total time. o -n N Abstract numbers with at least N digits within names. o -r Reverse the sort order. o -s sort_type How to sort the output. The value of sort_type should be chosen from the following list: o t, at: Sort by query time or average query time o l, al: Sort by lock time or average lock time o s, as: Sort by rows sent or average rows sent o c: Sort by count o -t N Display only the first N queries in the output. o --verbose, -v Verbose mode. Print more information about what the program does. Example of usage: shell> mysqldumpslow Reading mysql slow query log from /usr/local/mysql/data/mysqld51-apple-slow.log Count: 1 Time=4.32s (4s) Lock=0.00s (0s) Rows=0.0 (0), root[root]@localhost insert into t2 select * from t1 Count: 3 Time=2.53s (7s) Lock=0.00s (0s) Rows=0.0 (0), root[root]@localhost insert into t2 select * from t1 limit N Count: 3 Time=2.13s (6s) Lock=0.00s (0s) Rows=0.0 (0), root[root]@localhost insert into t1 select * from t1 COPYRIGHT
Copyright 2007-2008 MySQL AB, 2008-2010 Sun Microsystems, Inc. This documentation is free software; you can redistribute it and/or modify it only under the terms of the GNU General Public License as published by the Free Software Foundation; version 2 of the License. This documentation is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with the program; if not, write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA or see http://www.gnu.org/licenses/. SEE ALSO
For more information, please refer to the MySQL Reference Manual, which may already be installed locally and which is also available online at http://dev.mysql.com/doc/. AUTHOR
Sun Microsystems, Inc. (http://www.mysql.com/). MySQL 5.1 04/06/2010 MYSQLDUMPSLOW(1)
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