如何避免sql查询中的临时表？

Question

I am running a query (taking 3 minutes to execute)-

SELECT c.eveDate,c.hour,SUM(c.dataVolumeDownLink)+SUM(c.dataVolumeUpLink) 
FROM cdr c 
WHERE c.evedate>='2013-10-19'
GROUP BY c.hour;

with explain plan -

id  select_type  table  type  possible_keys                 key  key_len  ref  rows      Extra                                         
1   SIMPLE       c      ALL   evedate_index,eve_hour_index                     31200000  Using where; Using temporary; Using filesort  

I am using table(Myisam)-

Primary key(id,evedate),

with weekly 8 partitions with key evedate,

index key- on evedate,

composite index on evedate,hour.

I have changed the mysql tuning parameters from my.ini as (4GB RAM)-

tmp_table_size=200M

key_buffer_size=400M

read_rnd_buffer_size=2M

But still its using temporary table and file sort. Please let me know what should I do to exclude this.

After adding new composite index(evedate,msisdn)

I have found some changes in few queries they were not using any temporary case, even in above query if I omit group by clause, its not using temporary table.

Answer 1

You cannot do anything. MySql is not able to optimize this query and avoid temporaty table.

According to this link: http://dev.mysql.com/doc/refman/5.7/en/group-by-optimization.html
there are two methods MySql is using to optimize GROUP BY.

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The first method - Loose Index Scan - cannot be used for your query because this condition is not meet:

The only aggregate functions used in the select list (if any) are MIN() and MAX() ....

Your query contains SUM, therefore MySql cannot use the above optimalization method.

---

The second method - Tight Index Scan - cannot be used for your query, because this condition is not meet:

For this method to work, it is sufficient that there is a constant equality condition for all columns in a query referring to parts of the key coming before or in between parts of the GROUP BY key.

Your query is using only a range operator : WHERE c.evedate>='2013-10-19' , there is no any equality condition in the WHERE clause, therefore this method cannot be used to optimize the query.