How to optimize query with having and group My Task is to optimize a query, which should check for data inconsistency: Return all leads ids with more than one entries and multiple is_first flag.

| id | lead_id | is_first| (other data) |
|----|---------|---------|-------- ...
|   1|       20|        1|...
|   2|       20|        0|
|   3|       21|        1|
|   4|       21|        0|
|   5|       21|        1|
|   6|       22|        1|


So this dataset should return 21. (20 ok, because is_first is only true/1 once)

Old Query:

SELECT DISTINCT s1.lead_id as lead_id
FROM history as s1
INNER JOIN history as s2 ON
  s1.is_first = 1 AND
  s2.is_first = 1 AND
    s1.id != s2.id AND
    s1.lead_id = s2.lead_id;


New Query: I already increased performance somewhat with this query

SELECT history.lead_id AS lead_id
FROM history
WHERE is_first = 1 GROUP BY lead_id
HAVING count(lead_id) != 1;


But I feel it's not end of the line. Any recommendations?
If you dont have it already, you could create a index on both fields: is_first and lead_id; 



If it's a table with few record you won't see much difference, so I always recommend to check the statistics from the query.