Optimizations
От | Michael Ansley |
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Тема | Optimizations |
Дата | |
Msg-id | 01BE1EC7.3A7A4790@MANSLEY обсуждение исходный текст |
Ответы |
Re: [SQL] Optimizations
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Список | pgsql-sql |
Please would somebody tell me whether or not the optimizer sufficiently optimizes a cross join like this: SELECT * FROM table1 t1, table2, t2 WHERE t1.field1 = t2.field4 to SELECT * FROM table1 t1 INNER JOIN table2 t2 ON t1.field1 = t2.field4 As I understand it, both queries will always return the same results, however, the second query will execute substantiallyfaster, particularly as the tables involved increase in number of rows. It seems that most people prefer thefirst syntax, as it is probably quite simple logically, however, I suspect that the optimizer does not optimize it absolutely. Also, when LEFT, and OUTER JOINs are required, people forget that the JOIN keyword exists, and try to use the same syntaxas in the first query. If I wanted all records in table1, and only associated records from table2, then the only waythat I know of to retrieve this information would be as follows: SELECT * FROM table1 t1 LEFT JOIN table2 t2 ON t1.field1 = t2.field4 Would somebody please enlighten me (with regard to the optimizer, as well as alternatives for the last query above). I suspectthat a lot of people are inadvertently using cross joins, when that is not what they mean to be doing, and are notimpressed with the speed. Thanks... ---------------------------------------- Michael Ansley Intec (Ireland) Tel : +27 21 430-9000 Cell : +27 82 784-4229 eMail: michael.ansley@intec.co.za ICQ : 23465105 ----------------------------------------
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