优化我的查询（索引，解释）Mysql

Question

With the help of developers here on stackoverflow, I have been able to optimize my query to perform better than it was initially doing. Execution time dropped to: 1.2s

However, after doing EXPLAIN on the query, it seemed like the loan_applications_tbl ( date_updated, loan_status, current_loan, ippis ) indexes were not being used, as the type column says ALL for table a . What might be the cause for this please? Why is the index for this table being ignored and how do I fix this?

See my query below:

 EXPLAIN 
SELECT 
        a.id, 
        a.user_unique_id, 
        a.loan_location, 
        a.ippis, 
        a.tel_no,
        a.organisation, 
        a.branch, 
        a.loan_agree, 
        a.loan_type, 
        a.appr, 
        a.sold, 
        a.loan_status, 
        a.top_up, 
        a.current_loan, 
        a.date_created, 
        a.date_updated, 
        c.loan_id, 
        c.user_unique_id tu_user_unique_id, 
        c.ippis tu_ippis, 
        c.top_up_approved,
        c.loan_type tu_loan_type, 
        c.dse, 
        c.status, 
        c.current_loan tu_current_loan,
        c.record_category, 
        c.date_created tu_date_created,
        c.date_updated tu_date_updated 
    FROM 
        -- this creates inline mySQL variables I can use for the WHERE condition
        -- by doing comma after with no explicit join, it is a single row
        -- and thus no Cartesian result, just @variables available now
        ( select 
                -- first truncating any TIME portion by casting to DATE()
                @myToday := date(curdate()),
                -- now subtract day of month -1 to get first of THIS month
                @beginOfMonth := date_sub( @myToday, interval dayOfMonth( @myToday ) -1 day ),
                -- and now, add 1 month for beginning of next
                @beginNextMonth := date_add( @beginOfMonth, interval 1 month ) ) SqlVars,

        loan_applications_tbl a
    
            LEFT JOIN topup_or_reapplication_tbl c
                ON  a.ippis = c.ippis   
                AND c.current_loan='1'
                AND c.status IN ('pending', 'corrected', 'Rejected', 
                                'Processing', 'Captured', 'Reviewed', 'top up') 

                
                AND @beginOfMonth <= c.date_updated
                AND c.date_updated < @beginNextMonth
    WHERE
            -- forces only activity for the single month in question
            -- since the "a" table knows of any "updates" to the "C",
            -- its updated basis will keep overall restriction to any accounts
            -- updated within this month in question only
            @beginOfMonth <= a.date_updated 
        AND a.date_updated < @beginNextMonth
        
        -- and NOW we can easily apply the OR without requiring
        -- to run against the ENTIRE set of BOTH tables.
        AND (
                    c.ippis IS NOT NULL
                OR 
                    ( a.loan_status IN (  'pending', 'corrected', 'Rejected', 'Processing', 
                            'Captured', 'Reviewed', 'top up')
                    AND (   
                            a.current_loan = '1' 
                        OR  (   a.current_loan = '0' 
                            AND a.loan_status IN ('Approved', 'Closed')
                            )
                        )
                    )
            )

Indexes used:

loan_applications_tbl ( date_updated, loan_status, current_loan, ippis )
topup_or_reapplication_tbl ( ippis, status, current_loan, date_updated )

EXPLAIN on the query gives the screenshot below:

Answer 1

Try https://dev.mysql.com/doc/refman/8.0/en/index-hints.html#:~:text=The%20USE%20INDEX%20(%20index_list%20)%20hint,some%20particular%20index%20or%20indexes .

eg

SELECT * FROM table1 USE INDEX (col1_index,col2_index) WHERE col1=1 AND col2=2 AND col3=3;