Notes when an index cannot be used

Warning about incompatible index comparison

A frequent mistake database developers do 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 type different types:

If one compares an indexed columnc with a value of a different type one will get a warning like the following:

Note   1105    Cannot use key `PRIMARY` part[0] for lookup: `test`.`t1`.`id` of type `char` = "1" of type `bigint`

If one compares indexed character columns with a value of a a not compatible collation one will get a warning like the following:

Note   1105    Cannot use key `s2` part[0] 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 -> 11.3 we will use the error 1105 (Unknown error) as we cannot add an new error code in a GA version. In 11.4 we will change this to be an unique error code.

Enabling the note

By default, the warning is only shown when on executes EXPLAIN on a query. If one wants to enable it for all queries, one can do with the option/server variable:

In config file:
--note-verbosity=all

As a server variable:
@@note_verbosity="all";

note_verbosity is a set variable that describes with note categories one want to get notes for. It can have one or many of the following options:

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 log_slow_verbosity=all.

In config file:
--log-slow-verbosity=warnings

As a server variable:
@@log_slow_verbosity="all";

See Also

• Type conversions