Index on SUM function in MYSQL

Question

I have a query in MYSQL like this (simplified) :

SELECT col1, SUM(DISTINCT col2) AS S
FROM tbl1
WHERE col1='abbc'
GROUP BY col1
ORDER BY S ASC

I know that an index on col1 would be useful for that kind of query. I would like to know if a covering index on (col1, col2) would be more usefull or if it doesnt make any difference.

Answer 1

i try it , it seems different and more usefull

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Index Version Execution Plan :

without distinct

SELECT col1, SUM(col2) AS S
FROM tbl1
WHERE col1='abbc'
GROUP BY col1
ORDER BY S ASC;

distinct

SELECT col1, SUM(distinct col2) AS S
FROM tbl1
WHERE col1='abbc'
GROUP BY col1
ORDER BY S ASC;

SQL Fiddle

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without Index Version Execution Plan :

it's no different

SQL Fiddle