Re: Index Not being used.

[Rick James]

    SELECT  l.ORD_Item, l.ORD_Desc, l.ORD_Item_Class, SUM(l.ORD_Qty),
            SUM(ROUND(l.ORD_Qty * l.ORD_Price, 2))
        FROM  pos_ord_headers AS h
        JOIN  pos_ord_lines AS l  ON l.ORD_ID = h.ORD_ID
        WHERE  h.ORD_Date>='2015-01-01'
          AND  h.ORD_DATE <'2015-01-01' + INTERVAL 1 YEAR;

    headers: INDEX(ORD_DATE, ORD_ID) -- 'covering index' is much smaller and faster

Changing to InnoDB would speed things up, too. This is because of the PRIMARY KEY(ORD_ID, ...) being 'clustered'. Caveat: Changing to InnoDB would double or triple the disk footprint of the tables. See http://mysql.rjweb.org/doc.php/myisam2innodb for notes on conversion.

You have an accuracy issue... FLOAT can contain only 6-7 significant digits. If you had the monetary value of 987654.32, the 0.32 would be messed up. Also, FLOAT incurs an extra rounding when reading and writing; this can lead to other errors. Recommend DECIMAL instead of FLOAT. (Note: FLOAT occupies 4 bytes; DECIMAL(15,4) takes 7; DECIMAL(9,4) takes 5.)

Another solution is to build "summary tables". You could summarize each day (or week or month) into another table, then do this 'report' from that table. The speedup might be 10x. More discussion: http://mysql.rjweb.org/doc.php/summarytables