Fixing data warehousing queries with group-by

With the standard data warehousing queries we have a fact table and dimension tables and we join them.
For example, the fact table (Table size: 5M rows, ~2G in size) from my previous Loose index scan vs. covered indexes in MySQL post:

    CREATE TABLE `ontime_2010` (
      `YearD` int(11) DEFAULT NULL,
      `MonthD` tinyint(4) DEFAULT NULL,
      `DayofMonth` tinyint(4) DEFAULT NULL,
      `DayOfWeek` tinyint(4) DEFAULT NULL,
      `Carrier` char(2) DEFAULT NULL,
      `Origin` char(5) DEFAULT NULL,
      `DepDelayMinutes` int(11) DEFAULT NULL,
      `AirlineID` int(11) DEFAULT NULL,
      `Cancelled` tinyint(4) DEFAULT NULL,
    ... more fields here ...
    ) ENGINE=InnoDB DEFAULT CHARSET=latin1

(this is not the best possible fact table as the data is not aggregated by I’ll use it for now).

And we have those dimensions tables:

 CREATE TABLE `airlines` (
  `AirlineID` int(11) NOT NULL DEFAULT '0',
  `AirlineName` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`AirlineID`),
  KEY `AirlineName` (`AirlineName`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1

CREATE TABLE `date_dayofweek` (
  `code` int(11) NOT NULL DEFAULT '0',
  `description` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`code`),
  KEY `description` (`description`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1
1 row in set (0.00 sec)

mysql> select * from date_dayofweek order by code;
+------+-------------+
| code | description |
+------+-------------+
|    1 | Monday      |
|    2 | Tuesday     |
|    3 | Wednesday   |
|    4 | Thursday    |
|    5 | Friday      |
|    6 | Saturday    |
|    7 | Sunday      |
|    9 | Unknown     |
+------+-------------+
8 rows in set (0.00 sec)

So here is the example query (find sum of cancelled flights on Sundays for the given airline group by day):

select sum(Cancelled), FlightDate, AirlineName
from ontime_2010 o, date_dayofweek dow, airlines a
where o.dayofweek=dow.code and dow.description = 'Sunday'
and a.AirlineID = o.AirlineID and a.AirlineName = 'Delta Air Lines Inc.: DL'
group by FlightDate order by FlightDate desc limit 10\G

To fix the query we can add a covered index for ontime_2010, so that all fields for ontime_2010 table will be covered:

alter table ontime_2010 add key cov2(AirlineID, dayofweek, FlightDate, Cancelled);

However we will still have “temporary table and filesort”:


mysql> explain select sum(Cancelled), FlightDate
from ontime_2010 o, date_dayofweek dow, airlines a
where o.dayofweek=dow.code and dow.description = 'Sunday'
and a.AirlineID = o.AirlineID and a.AirlineName = 'Delta Air Lines Inc.: DL'
group by FlightDate order by FlightDate desc limit 10\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: dow
         type: ref
possible_keys: PRIMARY,description
          key: description
      key_len: 258
          ref: const
         rows: 1
        Extra: Using where; Using index; Using temporary; Using filesort
*************************** 2. row ***************************
           id: 1
  select_type: SIMPLE
        table: a
         type: ref
possible_keys: PRIMARY,AirlineName
          key: AirlineName
      key_len: 258
          ref: const
         rows: 1
        Extra: Using where; Using index
*************************** 3. row ***************************
           id: 1
  select_type: SIMPLE
        table: o
         type: ref
possible_keys: DayOfWeek,covered,AirlineID,cov2
          key: cov2
      key_len: 7
          ref: ontime.a.AirlineID,ontime.dow.code
         rows: 24417
        Extra: Using where; Using index
3 rows in set (0.00 sec)
To avoid filesort we can re-write this query with "subqueries":
mysql> explain select sum(Cancelled), FlightDate  from ontime_2010 o
where o.dayofweek= (select code from date_dayofweek where description = 'Sunday')
and AirlineID = (select AirlineID from airlines where AirlineName = 'Delta Air Lines Inc.: DL')
group by FlightDate limit 10\G
*************************** 1. row ***************************
           id: 1
  select_type: PRIMARY
        table: o
         type: ref
possible_keys: DayOfWeek,covered,AirlineID,cov2
          key: cov2
      key_len: 7
          ref: const,const
         rows: 152510
        Extra: Using where; Using index
*************************** 2. row ***************************
           id: 3
  select_type: SUBQUERY
        table: airlines
         type: ref
possible_keys: AirlineName
          key: AirlineName
      key_len: 258
          ref:
         rows: 1
        Extra: Using where; Using index
*************************** 3. row ***************************
           id: 2
  select_type: SUBQUERY
        table: date_dayofweek
         type: ref
possible_keys: description
          key: description
      key_len: 258
          ref:
         rows: 1
        Extra: Using where; Using index
3 rows in set (0.00 sec)

As MySQL will use indexes when we have "field = (select .. )" and now all fields in the index belong to the single table, MySQL will use index and avoid filesort. Please note: this will not work with "field in (select ...)" and also make sure that the subselect part will return only 1 row.

Comments are closed.