Mysql GROUP BY really slow on a view - mysql

Question

I have these tables.
CREATE TABLE `movements` (
`movementId` mediumint(8) UNSIGNED NOT NULL,
`movementType` tinyint(3) UNSIGNED NOT NULL,
`deleted` tinyint(1) UNSIGNED NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
ALTER TABLE `movements`
ADD PRIMARY KEY (`movementId`),
ADD KEY `movementType` (`movementType`) USING BTREE,
ADD KEY `deleted` (`deleted`),
ADD KEY `movementId` (`movementId`,`deleted`);
CREATE TABLE `movements_items` (
`movementId` mediumint(8) UNSIGNED NOT NULL,
`itemId` mediumint(8) UNSIGNED NOT NULL,
`qty` decimal(10,3) UNSIGNED NOT NULL,
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
ALTER TABLE `movements_items`
ADD KEY `movementId` (`movementId`),
ADD KEY `itemId` (`itemId`),
ADD KEY `movementId_2` (`movementId`,`itemId`);
and this view called "movements_items_view".
SELECT
movements_items.itemId, movements_items.qty,
movements.movementId, movements.movementType
FROM movements_items
JOIN movements ON (movements.movementId=movements_items.movementId
AND movements.deleted=0)
The first table has 5913 rows, the second one has 144992.
The view is very fast, it loads 20 result in PhpMyAdmin in 0.0011s but as soon as I ask for a GROUP BY on it (I need it to do statistics with SUM()) es:
SELECT * FROM movements_items_view GROUP BY itemId LIMIT 0,20
time jumps to 0.2s or more and it causes "Using where; Using temporary; Using filesort" on movements join.
Any help appreciated, thanks.
EDIT:
I also run via phpMyAdmin this query to try to not use the view:
SELECT movements.movementId, movements.movementType, movements_items.qty
FROM movements_items
JOIN movements ON movements.movementId=movements_items.movementId
GROUP BY itemId LIMIT 0,20
And the performance is the same.
Edit. Here is the EXPLAIN
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE movements index PRIMARY,movementid movement_type 1 NULL 5913 Using index; Using temporary; Using filesort
1 SIMPLE movements_items ref movementId,itemId,movementId_2 movementId_2 3 movements.movementId 12 Using index

Answer 1

Turn it inside out. See if this works:
SELECT movementId, m.movementType, mi.qty
FROM
( SELECT movementId, qty
FROM movements_items
GROUP BY itemId
ORDER BY itemId
LIMIT 20
) AS mi
JOIN movements AS m USING(movementId)
The trick is to do the LIMIT sooner. The original way had all the data being hauled around, not just 20 rows.
In movements_items, is no column or combination of columns 'unique'? If so, make it the PRIMARY KEY.
In movement, KEY movementId (movementId,deleted) is redundant and should be dropped.
In movement_items, KEY movementId (movementId) is redundant and should be dropped.