加入一个外键时如何避免全表扫描

Question

how to avoid full table scan when joining on one foreign in key below is my sql query when i use explain select it show the query is scanning all the table even with a where clause

SELECT  message_recipients.id, message_recipients.user_type,
COALESCE(guardians.firstname, students.firstname)
    FROM  message_recipients
    LEFT JOIN  students  ON message_recipients.user_id = students.student_id
    LEFT JOIN  guardians  ON message_recipients.user_id = guardians.guardian_id
    WHERE  message_recipients.message_id = 2 

Also i added index on the message_id column still the same here below is the image of the explain select may be am reading it wrong

the total rows in the table is 8 but the message_id = 2 is just 6 rows and if you check the image you can see its scanning all the 8 rows which is not suppose to be the big question is how do i optimize this to avoid full table scan thanks

Answer 1

One way to avoid a full scan is to use clustered queries with a "with" clause like this:

WITH TB_A AS (
    SELEC
        A.*
    FROM 
        some_table A
    WHERE 
        A.field_1 = 'some condition to filter first table'
)
, TB_B AS (
    SELECT 
        B.*
    FROM 
        other_table B
    WHERE 
        B.field_2 = 'some condition to filter second table if you need that too'
)
SELECT 
    A.*
    , B.*
FROM 
    TB_A A 
INNER JOIN 
    TB_B B
    ON A.field_1 = B.field_1 --Running without full scan on join and much faster
;

;D

Answer 2

A FOREIGN KEY creates an INDEX , but that INDEX may not be optimal. See if these 'composit' indexes are better:

message_recipients:  INDEX(message_id, user_id,  id, user_type)
guardians:  INDEX(guardian_id,  firstname)
students:   INDEX(student_id,   firstname)

When adding a composite index, DROP index(es) with the same leading columns. That is, when you have both INDEX(a) and INDEX(a,b), toss the former.