优化连接中的 MySQL“OR”查询

Question

I am facing some difficulty when using 'OR' queries with multi-table joins with MySql 5.7.

For example, let's say I want a list of Animals that are either currently at a location or are being shipped there.

Select * from animals
left outer join animal_locations on animals.id = animal_locations.animal_id
left outer join animal_shipments on animals.id = animal_shipments.animal_id
where animal_locations.location_id = 5 OR animal_shipments.location_id = 5

The queries are very fast when using "AND" as well as using individual columns without any operator. But when "OR" queries are used, there is a big performance hit, the right indexes are not being hit.

Any way I can achieve this without splitting it up into multiple queries?

Answer 1

SELECT * 
FROM animals
LEFT OUTER JOIN animal_locations ON animals.id = animal_locations.animal_id
                                AND animal_locations.location_id = 5
LEFT OUTER JOIN animal_shipments ON animals.id = animal_shipments.animal_id
                                AND animal_shipments.location_id = 5
WHERE COALESCE(animal_locations.animal_id, animal_shipments.animal_id) IS NOT NULL

The logic - previously the rows in slave tables with location_id = 5 are filtered, and then filtered rows are joined to main table.