Question

I have this query:

SELECT 
    COUNT(*) AS `numrows` 
FROM (`tbl_A`) 
JOIN `tbl_B` ON `tbl_A`.`B_id` = `tbl_B`.`id` 
WHERE 
    `tbl_B`.`boolean_value` <> 1;

I added three indexes for tbl_A.B_id, tbl_B.id and tbl_B.boolean_value but mysql still says it doesn’t use indexes (in queries not using indexes log) and it examine whole of tables to retrieve the result.

I need to know what I should do to optimize this query.

EDIT:

Explain output:

id  select_type table  type  possible_keys          key   key_len  ref      rows  Extra 
1    SIMPLE     tbl_B  ALL   PRIMARY,boolean_value  NULL  NULL     NULL     5049  Using where
1    SIMPLE     tbl_A  ref   B_id                   B_id  9        tbl_B.id 9     Using where; Using index

Answer 1

The explain show us that an index is used to make the join to tbl_B but no index is used to filter tbl_A on the boolean value.

An index was available but the engine choose not to use it. Why it happen:

• maybe 5049 rows is not a big deal and the engine saw that using the index to filter something like 10% of the rows using the index would be as fast as doing it without using it.
• Booleans takes only 3 values: 1, 0 or NULL. So the cardinality of the index will always be very low (3 max). Low cardinality index are usually dropped by the query analyser (which is quite right usually thinking this index won’t help him a lot)

It would be interesting to see if the query analyser behaves the same way when you have a 50/50 repartition of true and false value for this boolean, or when you have just a few False.

Now usually boolean fields are useful only on indexes containing multiple keys, so that if your queries use all the fields of the index (in where or order by) the query analyser will trust that index to be really a good tool.

Note that indexes are slowing down your writes and takes extra-spaces, do not add useless indexes. Using logt-query-not-using-indexes is a good thing, but you should compensate that log information with the slow queries log.If the query is fast it’s not a problem.