During my work using the hook "SpecialRecentChangesQuery" (code and detailed analysis see [1]) I found a reproducible problem which arise only under the following conditions:
- if MySQL >= 5.0.12 AND
- if the hook function for SpecialRecentChangesQuery adds table(s) to $table[].
Analysis:
The code in [1] modifies the Recent Changes main SQL statement to this
SELECT * FROM recentchanges FORCE INDEX (rc_timestamp),page LEFT JOIN tag_summary ON ((ts_rc_id=rc_id)) WHERE (rc_timestamp >= '20100211000000') AND rc_bot = '0' ORDER BY rc_timestamp DESC LIMIT 50
This throws an error Unknown column 'rc_id' in 'on clause' (localhost) (MySQL >= 5.0.12 due to new JOIN processing)
This ad-hoc modification works (parentheses added around the table names outside the JOIN)
SELECT * FROM (recentchanges FORCE INDEX (rc_timestamp),page) LEFT JOIN tag_summary ON ((ts_rc_id=rc_id)) WHERE (rc_timestamp >= '20100211000000') AND rc_bot = '0' ORDER BY rc_timestamp DESC LIMIT 50
I add an ad-hoc and very hacky - only experimental - patch, which corrects the problem in a certain case for [1]. The patch is not mentioned for SVN submisson.
Citing [2]: Beginning with MySQL 5.0.12, natural joins and joins with USING, including outer join variants, are processed according to the SQL:2003 standard. The goal was to align the syntax and semantics of MySQL with respect to NATURAL JOIN and JOIN ... USING according to SQL:2003. However, these changes in join processing can result in different output columns for some joins. Also, some queries that appeared to work correctly in older versions must be rewritten to comply with the standard.
Citing [3]:
SELECT * FROM t1, t2 JOIN t3 ON (t1.i1 = t3.i3);
Previously, the SELECT was legal due to the implicit grouping of t1,t2 as (t1,t2). Now the JOIN takes precedence, so the operands for the ON clause are t2 and t3. Because t1.i1 is not a column in either of the operands, the result is an Unknown column 't1.i1' in 'on clause' error. To allow the join to be processed, group the first two tables explicitly with parentheses so that the operands for the ON clause are (t1,t2) and t3:
SELECT * FROM (t1, t2) JOIN t3 ON (t1.i1 = t3.i3);
Alternatively, avoid the use of the comma operator and use JOIN instead:
SELECT * FROM t1 JOIN t2 JOIN t3 ON (t1.i1 = t3.i3);
[1] http://www.mediawiki.org/wiki/Extension:OnlyRecentRecentChanges
[2] MySQL Manual Join Processing Changes in MySQL 5.0.12
http://dev.mysql.com/doc/refman/5.0/en/join.html
[3] Bug #19053 MySQL Unknown column in 'on clause'
http://bugs.mysql.com/bug.php?id=19053
Version: 1.22.0
Severity: normal
URL: http://dev.mysql.com/doc/refman/5.0/en/join.html