The Bottom Line: Data Changes in Java

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The Bottom Line: Data Changes
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The correct way to do an ordinary data change is still the same Use SQL's INSERT, UPDATE, or DELETE statements, or use stored procedures Don't bother with cursors Updating in batches is a good idea because it reduces network messaging A stored procedure can be called directly with RPC, thus bypassing the DBMS parsing layer The trick is to use parameters so that you allow RPC to happen if the DBMS can handle it
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If you're using JDBC with applets, define methods with the keyword synchronized That prevents other synchronized methods from getting in while an object is being accessed You can put Java/JDBC routines in JAR files that the DBMS can read The SQL procedure declaration would look something like this:
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CREATE PROCEDURE Sp_proc READS SQL DATA DYNAMIC RESULT SETS 1 EXTERNAL NAME <jar:name> LANGUAGE JAVA PARAMETER STYLE JAVA
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The assumption here is that the DBMS supports SQL Java (SQLJ) routines, can load JARs, and can invoke a Java Virtual Machine (JVM) Some DBMSs (Informix and Sybase are examples) supply copies of Apache Web Server with their installation CD-ROMS Oracle takes a different approach and
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loads its own JVM With a standard Web server installed, it should be easy for the DBMS to access Java routines as servlets
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14 Data Changes
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The term data change appears in the SQL Standard document It means any change to the database that is caused by INSERT, UPDATE, or DELETE statements We often refer to such changes as updates (lowercase), but that's a bit confusing because usually UPDATE (uppercase) refers to the UPDATE statement alone So we'll use "data change" the way the SQL Standard does: to mean an INSERT, an UPDATE, or a DELETE Data-change statements occur less frequently than SELECT statements, but a typical data-change statement is slow For example, compare these two SQL statements:
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SELECT column1 FROM Table1 WHERE column1 = 12345 UPDATE Table1 SET column2 = 10 WHERE column1 = 12345
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Both statements must go through the same retrieval steps to find the matching rows, but the UPDATE will be slower, mainly for these reasons:
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The UPDATE must get an exclusive lock instead of a more-permissive shared lock; see 15, "Locks" The UPDATE must check any constraints or triggers The UPDATE must add one, and possibly two, new entries in the log file The UPDATE must shift following rows in the page up or down if the size of the updated row changes An expanded row might also have to go on a different page The UPDATE must delete and replace any index keys for the changed column If a later COMMIT happens, some writing to disk will occur
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Changing a row always takes at least 3 times longer than retrieving it, and can take 100 times longer in worst-case scenarios So it's worthwhile to examine the main performance difficulties It's impossible to deal with data-change statements in isolation There are all sorts of effects that we discuss in other chapters, notably 8, "Tables," 10, "Constraints," and 15, "Locks" In this chapter, though, we'll deal only with matters that relate directly to data change: the data-change statements themselves (INSERT, UPDATE, DELETE) and the transaction-end statements (COMMIT, ROLLBACK)
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For some DBMSs, logging is an optional step Nevertheless it's very rare that database administrators will decide to turn logging off for performance reasons So it's safe to assume that logging will happen when you do a data change When will logging happen Before data changes become permanent The reasoning is that, if a database gets smashed, then one can recover from the log If the data change were written before the log entry though, then the last entry in the log would be absent So the DBMS always logs first
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What goes in the log A copy of the new data, or a copy of the old data, or both This point requires emphasis You might think that the only necessary data in the log is the data-change statement itself for example, the DBMS could just write this SQL statement into the log file:
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INSERT INTO Table1 (column1) VALUES (1)
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Well, it could for instance, Oracle does just that (Oracle calls the SQL-statement log a journal) However, Oracle also writes a complete "before image" data copy into the log file, along with a complete "before image" index page if a data change also changes an index And so will most other DBMSs[1]
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