Zde je řešení, které, jak se zdá, funguje:
SELECT t.*, DATEDIFF(t.LatestAvailable, t.EarliestAvailable) AS LengthAvailable
FROM
(SELECT u.*,
COALESCE(b1.End, @StartOfWindow) AS EarliestAvailable,
COALESCE(b2.Start, @EndOfWindow) AS LatestAvailable
FROM LettingUnits u
LEFT OUTER JOIN LettingUnitBookings b1
ON (u.ID = b1.F_LU_ID AND b1.End BETWEEN @StartOfWindow AND @EndOfWindow)
LEFT OUTER JOIN LettingUnitBookings b2
ON (u.ID = b2.F_LU_ID AND b2.Start BETWEEN @StartOfWindow AND @EndOfWindow
AND b2.Start >= b1.End) -- edit: new term
) AS t
LEFT OUTER JOIN LettingUnitBookings x
ON (t.ID = x.F_LU_ID AND x.Start < t.LatestAvailable AND x.End > t.EarliestAvailable)
WHERE x.ID IS NULL AND DATEDIFF(t.LatestAvailable, t.EarliestAvailable) >= @WindowSize;
Výstup je:
+-----+-------------+-------------------+-----------------+-----------------+
| ID | Name | EarliestAvailable | LatestAvailable | LengthAvailable |
+-----+-------------+-------------------+-----------------+-----------------+
| 123 | Foo Cottage | 2009-01-05 | 2009-01-10 | 5 |
| 123 | Foo Cottage | 2009-01-20 | 2009-01-25 | 5 |
| 456 | Bar Cottage | 2009-01-20 | 2009-01-31 | 11 |
+-----+-------------+-------------------+-----------------+-----------------+
Analyzujte to pomocí EXPLAIN ukazuje, že docela dobře využívá indexy:
+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------------------------+
| 1 | PRIMARY | <derived2> | ALL | NULL | NULL | NULL | NULL | 9 | Using where |
| 1 | PRIMARY | x | ref | F_LU_ID | F_LU_ID | 8 | t.ID | 2 | Using where; Not exists |
| 2 | DERIVED | u | system | NULL | NULL | NULL | NULL | 1 | |
| 2 | DERIVED | b1 | ref | F_LU_ID | F_LU_ID | 8 | const | 0 | |
| 2 | DERIVED | b2 | ref | F_LU_ID | F_LU_ID | 8 | const | 0 | |
+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------------------------+
Porovnejte s EXPLAIN nahlásit řešení dané od @martin clayton:
+----+--------------+---------------------+--------+---------------+---------+---------+------+------+---------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------+---------------------+--------+---------------+---------+---------+------+------+---------------------------------+
| 1 | PRIMARY | lu | system | PRIMARY,ID | NULL | NULL | NULL | 1 | |
| 1 | PRIMARY | <derived2> | ALL | NULL | NULL | NULL | NULL | 4 | Using where |
| 2 | DERIVED | <derived3> | ALL | NULL | NULL | NULL | NULL | 4 | Using temporary; Using filesort |
| 2 | DERIVED | <derived5> | ALL | NULL | NULL | NULL | NULL | 4 | Using where; Using join buffer |
| 5 | DERIVED | LettingUnitBookings | ALL | NULL | NULL | NULL | NULL | 3 | |
| 6 | UNION | LettingUnitBookings | index | NULL | F_LU_ID | 8 | NULL | 3 | Using index |
| NULL | UNION RESULT | <union5,6> | ALL | NULL | NULL | NULL | NULL | NULL | |
| 3 | DERIVED | LettingUnitBookings | ALL | NULL | NULL | NULL | NULL | 3 | |
| 4 | UNION | LettingUnitBookings | index | NULL | F_LU_ID | 8 | NULL | 3 | Using index |
| NULL | UNION RESULT | <union3,4> | ALL | NULL | NULL | NULL | NULL | NULL | |
+----+--------------+---------------------+--------+---------------+---------+---------+------+------+---------------------------------+
Obecně se chcete vyhnout plánům optimalizace, které vynucují Using filesort nebo Using temporary protože to jsou zabijáci výkonu. Dotaz pomocí GROUP BY je téměř jisté, že způsobí tento druh optimalizace, alespoň v MySQL.