Comments on: EXPLAIN EXTENDED can tell you all kinds of interesting things http://www.mysqlperformanceblog.com/2010/06/15/explain-extended-can-tell-you-all-kinds-of-interesting-things/ Percona's MySQL & InnoDB performance and scalability blog Sat, 11 Feb 2012 16:45:54 +0000 hourly 1 http://wordpress.org/?v=3.2.1 By: Shantanu Oak http://www.mysqlperformanceblog.com/2010/06/15/explain-extended-can-tell-you-all-kinds-of-interesting-things/comment-page-1/#comment-770884 Shantanu Oak Sat, 07 Aug 2010 13:22:49 +0000 http://www.mysqlperformanceblog.com/?p=2508#comment-770884 I use explain extended to know the table name that the column belongs. I lot of programmers do not mention the table alias before the column name in the query, if the column name is unique. I find it easy to use extended to know the table name. I use explain extended to know the table name that the column belongs. I lot of programmers do not mention the table alias before the column name in the query, if the column name is unique. I find it easy to use extended to know the table name.

]]> By: daveyhook http://www.mysqlperformanceblog.com/2010/06/15/explain-extended-can-tell-you-all-kinds-of-interesting-things/comment-page-1/#comment-768141 daveyhook Wed, 30 Jun 2010 07:40:15 +0000 http://www.mysqlperformanceblog.com/?p=2508#comment-768141 Hi Peter, It seems that a 'const' table contains no rows may be used in an OUTER JOIN in newer version. mysql> EXPLAIN extended SELECT j1.c1 FROM j1, j2, j3 WHERE j1.c1 = j2.c1 AND j3.c1 = j1.c1; +----+-------------+-------+------+---------------+------+---------+------+------+-----------------------------------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+------+---------+------+------+-----------------------------------------------------+ | 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Impossible WHERE noticed after reading const tables | +----+-------------+-------+------+---------------+------+---------+------+------+-----------------------------------------------------+ 1 row in set, 1 warning (0.01 sec) mysql> show warnings; +-------+------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | Level | Code | Message | +-------+------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | Note | 1003 | select `test`.`j1`.`c1` AS `c1` from `test`.`j1` join `test`.`j2` join `test`.`j3` where ((`test`.`j2`.`c1` = `test`.`j1`.`c1`) and (`test`.`j3`.`c1` = `test`.`j1`.`c1`)) | +-------+------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ 1 row in set (0.00 sec) mysql> select version(); +----------------+ | version() | +----------------+ | 5.0.26-max-log | +----------------+ 1 row in set (0.00 sec) Hi Peter,
It seems that a ‘const’ table contains no rows may be used in an OUTER JOIN in newer version.

mysql> EXPLAIN extended SELECT j1.c1 FROM j1, j2, j3 WHERE j1.c1 = j2.c1 AND j3.c1 = j1.c1;
+—-+————-+——-+——+—————+——+———+——+——+—————————————————–+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+—-+————-+——-+——+—————+——+———+——+——+—————————————————–+
| 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Impossible WHERE noticed after reading const tables |
+—-+————-+——-+——+—————+——+———+——+——+—————————————————–+
1 row in set, 1 warning (0.01 sec)

mysql> show warnings;
+——-+——+—————————————————————————————————————————————————————————-+
| Level | Code | Message |
+——-+——+—————————————————————————————————————————————————————————-+
| Note | 1003 | select `test`.`j1`.`c1` AS `c1` from `test`.`j1` join `test`.`j2` join `test`.`j3` where ((`test`.`j2`.`c1` = `test`.`j1`.`c1`) and (`test`.`j3`.`c1` = `test`.`j1`.`c1`)) |
+——-+——+—————————————————————————————————————————————————————————-+
1 row in set (0.00 sec)

mysql> select version();
+—————-+
| version() |
+—————-+
| 5.0.26-max-log |
+—————-+
1 row in set (0.00 sec)

]]> By: Shane Bester http://www.mysqlperformanceblog.com/2010/06/15/explain-extended-can-tell-you-all-kinds-of-interesting-things/comment-page-1/#comment-767315 Shane Bester Fri, 18 Jun 2010 06:18:42 +0000 http://www.mysqlperformanceblog.com/?p=2508#comment-767315 Oh yes, I forgot to mention explain extended is very unstable and will crash server for certain queries.. put [ site:bugs.mysql.com "explain extended" crash ] into google for proof :) be careful! Oh yes, I forgot to mention explain extended is very unstable and will crash server for certain queries.. put [ site:bugs.mysql.com "explain extended" crash ] into google for proof :) be careful!

]]> By: Matthew Rudy Jacobs http://www.mysqlperformanceblog.com/2010/06/15/explain-extended-can-tell-you-all-kinds-of-interesting-things/comment-page-1/#comment-767294 Matthew Rudy Jacobs Fri, 18 Jun 2010 03:23:45 +0000 http://www.mysqlperformanceblog.com/?p=2508#comment-767294 I remember when "EXPLAIN EXTENDED" crashed our DB :s I remember when “EXPLAIN EXTENDED” crashed our DB
:s

]]> By: Justin Swanhart http://www.mysqlperformanceblog.com/2010/06/15/explain-extended-can-tell-you-all-kinds-of-interesting-things/comment-page-1/#comment-767194 Justin Swanhart Wed, 16 Jun 2010 21:25:04 +0000 http://www.mysqlperformanceblog.com/?p=2508#comment-767194 Hi Shane, Thanks for the tip. Hi Shane,

Thanks for the tip.

]]> By: Bill http://www.mysqlperformanceblog.com/2010/06/15/explain-extended-can-tell-you-all-kinds-of-interesting-things/comment-page-1/#comment-767175 Bill Wed, 16 Jun 2010 18:08:09 +0000 http://www.mysqlperformanceblog.com/?p=2508#comment-767175 I've been using MySQL for a long time, before 4.1, I'm a little embarrassed I never knew this existed. I’ve been using MySQL for a long time, before 4.1, I’m a little embarrassed I never knew this existed.

]]> By: Shane Bester http://www.mysqlperformanceblog.com/2010/06/15/explain-extended-can-tell-you-all-kinds-of-interesting-things/comment-page-1/#comment-767141 Shane Bester Wed, 16 Jun 2010 08:46:13 +0000 http://www.mysqlperformanceblog.com/?p=2508#comment-767141 My favourite use of explain extended is to check if optimizer doesn't use indexes due to conversions of charsets. For example it'll show something like this: SELECT ... where (`b`.`t1`.`a` = convert(`b`.`t2`.`b` using utf8)) Sometimes when tables are temporary or created as result of some queries, it's not always obvious what their schemas are :) My favourite use of explain extended is to check if optimizer doesn’t use indexes due to conversions of charsets. For example it’ll show something like this:

SELECT … where (`b`.`t1`.`a` = convert(`b`.`t2`.`b` using utf8))

Sometimes when tables are temporary or created as result of some queries, it’s
not always obvious what their schemas are :)

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