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Update Seller s SET s.status = 'Gold' WHERE s.createDate <= 1
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This JPQL statement will update all Sellers to a Gold status with the creation date specified in one fell swoop. If 10,000 sellers meet this criterion, executing this in one SQL statement instead of 10,000 is a huge performance improvement. Avoiding association tables in one-to-many relationships Association tables are commonly used to store unidirectional, one-to-many relationships. This is supported in EJB 3 with the @JoinTable annotation. Using an association table will require extra SQL statements to manage the relationship, as well as unnecessary JOINs between the base tables and association tables. You can gain some performance benefit by avoiding association tables.
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Taming wild EJBs: performance and scalability
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13.2.4 Improving query performance Even trivial applications can make extensive use of queries. When using EJB 3, you write queries in JPQL and they are translated to SQL. Although you develop in JPQL, you can take certain actions to ensure that the corresponding queries will perform well. DBAs can certainly play a big part in helping improve queries. You may also be interested in enabling a higher level of logging in your persistence provider to expose and capture the generated SQL statements, and run them through a tuning utility provided by your database vendor. This can help you determine whether the SQL can be improved. Certain databases provide an automatic SQL tuning utility that provides suggestions for improving the SQL executed by an application. You can work with your DBA to use such tools and get their recommendations on how to improve query performance. There is no magic sequence of steps that address all query issues, but we ll discuss some of the more common scenarios.
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Avoiding full-table scans Unless your entity is mapped to a very small table, you must avoid using SELECT statements that perform full-table scans. For example, you can retrieve all items in a query like this:
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SELECT FROM Item I
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Next, you retrieve the returned collection, iterate through the collection, and perform one or more operations on the resulting data. The persistence provider will generate the following SQL:
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SELECT * FROM ITEMS
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There are two problems here. First, this code will retrieve and bring in a lot of rows into the middle tier and consume a lot of memory. Second, it will cause a FULL TABLE SCAN in your database and the query will be very slow. Your DBA will advise you to avoid such SQL. Realistically, the number of available items you want is much less than the total number of items in your database. You must utilize the full potential of database filtering by changing your query to limit the number of rows retrieved as follows:
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SELECT i FROM Item i WHERE i.status = "Available"
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The query will be much faster and you don t have to do any extra filtering work in the middle tier.
Improving entity performance
Using indexes to make queries faster Indexes make your query faster. Your DBAs are probably responsible for building the indexes on tables, but there s a good chance that they don t know the details of how your application works. You should work with them so that they understand the queries used by your application. Only then can they build appropriate indexes for your application. Queries that include the primary key always use an indexed scan, meaning that no additional indexes are required. In spite of this, here are some additional cases where you ll want to use an index to improve performance. Filtering based on a nonidentity field This is very prevalent in applications. For example, suppose you want to retrieve your Item entities by itemTitle as follows:
SELECT i FROM Item i WHERE i.itemTitle = 1
This JPQL statement will be translated to SQL as follows:
SELECT * FROM ITEMS WHERE ITEMS.ITEM_TITLE = 1
If you do not have an index on ITEM_TITLE, the query will include a FULL TABLE SCAN. Therefore, we recommend you create an index in these situations. In this case, the index would be created on ITEM_TITLE. Using indexes for relationship fields Relationships are implemented in the database by using foreign key constraints. However, you may not have indexes on the foreign key column(s). When you retrieve an associated entity, a JOIN between the two underlying tables is performed. But this is slow because a FULL TABLE SCAN will be executed on the associated entities. Creating an index on the underlying table will allow the database to use that index while joining the tables, which is must faster than joining two large tables without an index. In ActionBazaar, Item and Bid have a one-to-many relationship due to eager loading. If you have a JPQL query that uses a JOIN clause, the persistence provider could generate the following SQL statement:
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