Notes When an Index Cannot Be Used
MariaDB starting with 10.6.16
This is a new note added in 10.6.16.
Warning About Incompatible Index Comparison
A frequent mistake database developers make is to compare an indexed column with another column that is not compatible with the indexed column. For example, comparing string columns with number columns, or using incompatible character sets or collations.
Because of this we have introduced notes (low severity warnings) when an indexed column cannot use the index to lookup rows.
The warnings are of different types:
If one compares an indexed column with a value of a different type one, will get a warning like the following:
Note 1105 Cannot use key `PRIMARY` part for lookup: `test`.`t1`.`id` of type `char` = "1" of type `bigint`
If one compares indexed character columns with a value of an incompatible collation, one will get a warning like the following:
Note 1105 Cannot use key `s2` part for lookup: `test`.`t1`.`s2` of collation `latin1_swedish_ci` = "'a' collate latin1_german1_ci" of collation `latin1_german1_ci`
Note that in MariaDB 10.6 to MariaDB 11.3 we will use the error 1105 (Unknown error), as we cannot add an new error code in a GA version. In MariaDB 11.4 we will change this to be a unique error code.
Enabling the Note
By default, the warning is only shown when executing EXPLAIN on a query. To enable for all queries, use the option/server variable:
In config file: --note-verbosity=all As a server variable: @@note_verbosity="all";
It can have one or many of the following options:
|All old notes.|
|Give warnings for unusable keys for SELECT, DELETE and UPDATE.|
|Give warnings about unusable keys for EXPLAIN.|
One can also set
note_verbosity to the value of
all to set all options.
Enabling Warnings and Notes for the Slow Query Log
One can get the note about incompatible keys also in the slow query log by adding the option
warnings in the
log_slow_verbosity option/variable. It will automatically be enabled if one uses
In config file: --log-slow-verbosity=warnings As a server variable: @@log_slow_verbosity="all";