Is having faster than where? How do you optimize a query? Avoid using functions in predicates. Use inner join, instead of outer join if possible. How many types of subqueries are there in SQL Server? In this chapter, learn about the three broad divisions of a subquery in SQL: Single-row, multiple-row and correlated subqueries. There are three broad types of a subquery in SQL. What is self join? What is an inner join SQL? What is the difference between select distinct and group by?
The functional difference is thus obvious. But there are times when it can be useful for performance reasons when using views. If everything works as it should it shouldn't, BUT we all know everything doesn't work the way it should especially when it comes to the query optimizer, query plan caching and statistics. First I would suggest rebuilding index and statistics, then clearing the query plan cache just to make sure that's not screwing things up. However I've experienced problems even when that's done.
The underlying reason is this: If you have two tables and you join on a column with an index on both tables. The inner join will produce the same result no matter if you loop over the entries in the index on table one and match with index on table two as if you would do the reverse: Loop over entries in the index on table two and match with index in table one.
The problem is when you have misleading statistics, the query optimizer will use the statistics of the index to find the table with least matching entries based on your other criteria. If you have two tables with 1 million in each, in table one you have 10 rows matching and in table two you have rows matching. The best way would be to do an index scan on table one and matching 10 times in table two.
The reverse would be an index scan that loops over rows and tries to match times and only 10 succeed. So if the statistics isn't correct the optimizer might choose the wrong table and index to loop over. The retrieval time of the query using joins almost always will be faster than that of a subquery. By using joins, you can maximize the calculation burden on the database i.
The query optimizer should be able to generate an identical execution plan. However, some database engines can produce better execution plans for one of them not likely to happen for such a simple query but for complex enough ones.
JOIN combines data from many tables based on a matched condition between them. How fast is SQL? Why are Joins expensive? Do Joins slow down query? When to use a subquery VS join? Why do we use subquery? Do subqueries improve performance? What are views in SQL? What is the difference between left join and left outer join?
What is outer join? Which is faster join or subquery in Oracle? How do you optimize a query? Similar Asks. Popular Asks.
0コメント