Manual:$wgShowSQLErrors/en

When set to  (default) any database error message will also say "(SQL query hidden)" rather than showing the SQL which caused the error. You will then only see "". Set $wgShowSQLErrors to  to see the actual query, which caused the error.

Details
Clearly showing the SQL will aid in debugging in the event of a database error, however...


 * Showing SQL to all users, will give crackers some clues about the database structure of your website, and might help them find security weaknesses.
 * SQL database errors are not supposed to occur during any normal MediaWiki operation, and so with luck you will never need to improve the error information
 * If you receive the error  from the API, enabling this setting will display a more descriptive error message in the info field than the default.

If you do see an error including "(SQL query hidden)", it can be useful to set this to $wgShowSQLErrors = true; temporarily.

This variable replaces, which should no longer be used.