mysql联合订单性能

Question

I have a MySQL union of 2 queries, with ORDER BY done on indexed columns. What is better performance wise, order by each of the union queries separately, or order by at the end of the union as I currently have it?

(SELECT pep.P_Surname, pep.P_FirstName, pep.P_ID, adm.Loc_ID AS lid 
FROM lup_people pep
JOIN tr_adm adm ON pep.P_ID=adm.P_ID                    
WHERE (P_FirstName LIKE 'g%' 
OR P_Surname LIKE 'g%')
AND pep.active = 'Y')

 UNION

(SELECT pep.P_Surname, pep.P_FirstName, pep.P_ID, vil.Loc_ID AS lid 
FROM lup_people pep
JOIN tr_village vil ON (pep.P_ID=vil.P1_ID OR pep.P_ID=vil.P2_ID)                    
WHERE (P_FirstName LIKE 'g%' 
OR P_Surname LIKE 'g%') 
AND pep.active = 'Y') 

ORDER BY P_Surname,P_FirstName LIMIT 50

VS

(SELECT pep.P_Surname, pep.P_FirstName, pep.P_ID, adm.Loc_ID AS lid 
FROM lup_people pep
JOIN tr_adm adm ON pep.P_ID=adm.P_ID                    
WHERE (P_FirstName LIKE 'g%' 
OR P_Surname LIKE 'g%')
AND pep.active = 'Y' ORDER BY P_Surname,P_FirstName LIMIT 50)

UNION

(SELECT pep.P_Surname, pep.P_FirstName, pep.P_ID, vil.Loc_ID AS lid 
FROM lup_people pep
JOIN tr_village vil ON (pep.P_ID=vil.P1_ID OR pep.P_ID=vil.P2_ID)                    
WHERE (P_FirstName LIKE 'g%' 
OR P_Surname LIKE 'g%') 
AND pep.active = 'Y' ORDER BY P_Surname,P_FirstName LIMIT 50) 

Answer 1

(1) You should use union all , unless you intend for duplicates to be removed by the union .

(2) The queries are different. The first returns 100 rows. The second returns 50 rows.

(3) You should not depend on the ordering of results from a union query. If you want results in a particular order, then use an order by after the union .

(4) Quite possibly, the fastest approach is to use left outer join . It would look something like:

SELECT pep.P_Surname, pep.P_FirstName, pep.P_ID,
       coalesce(adm.Loc_ID, vil.loc_id, vil2.loc_id) AS lid 
FROM lup_people pep LEFT JOIN
     tr_adm adm
     ON pep.P_ID = adm.P_ID LEFT JOIN
     tr_village vil
     ON pep.P_ID = vil.P1_ID LEFT JOIN
     tr_village vil2
     ON pep.P_ID = vil2.P2_ID                 
WHERE (P_FirstName LIKE 'g%' OR P_Surname LIKE 'g%') AND pep.active = 'Y'
ORDER BY P_Surname, P_FirstName
LIMIT 50;

Answer 2

Based on the volume of data, it could go either way. The issue you are faced with is this. In your first query you are doing a select-from getting ALL POSSIBLE answers that qualify, after which, THEN the order by clause is applied and finally the limit. So, if you have 1000's of qualified records, you are still plowing through them, ordering, then get the top 50.

The second one will AT LEAST, cut their respective result sets down to 50 from each leaving a max of 100 records.