SELECT OPEN <cursor> FETCH IF <search condition> THEN UPDATE in Java

Generate Data Matrix in Java SELECT OPEN <cursor> FETCH IF <search condition> THEN UPDATE
SELECT OPEN <cursor> FETCH IF <search condition> THEN UPDATE
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Instead, you want to just do this:
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UPDATE WHERE <search condition>
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To avoid the WHERE CURRENT OF clause and cursor trouble, use a ROWID or serial (auto_increment) column
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The Bottom Line: FETCH and Data Changes
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Don't SELECT/fetch/test/<data change> Do UPDATEWHERE <condition>/test and DELETEWHERE <condition>/test To avoid WHERE CURRENT OF and cursor trouble, use a ROWID or serial column
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COMMIT and ROLLBACK
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It should be true that a COMMIT will actually guarantee that data is written to the disk and flushed That is, it should be safe to pull the power plug as soon as COMMIT is done Unfortunately, any DBMS that makes that guarantee must fight the operating system, because the operating system prefers to do lazy writes and keep its caches until a physical write is convenient Therefore COMMIT is always going to be slow This is true even if the DBMS doesn't wait until the database is updated, because at the very least it must wait until the log file is updated We've mentioned before that all DBMSs have a nonstandard flag called the auto-commit flag If the flag is on, then COMMIT happens automatically after every data-change statement has been executed If the auto-commit flag is off, then you must issue an explicit COMMIT yourself Here are the usual rules:
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If there is only one data-change statement in the transaction, then auto-commit should be on If there are multiple data-change statements in the transaction, then auto-commit should be off The auto-commit flag is on by default if you are using ODBC or JDBC or most other APIs
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Transactions can be difficult, though, because of the following odd behavior by DBMSs:
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IBM, Informix, and Ingres act correctly whether the auto-commit flag is on or off InterBase and Oracle won't allow you to ROLLBACK Data Definition Language (DDL) statements (eg, CREATE TABLE) even if the auto-commit flag is off But they do allow you to ROLLBACK Data Manipulation Language (DML) statements (eg, INSERT) if the auto-commit flag is off Even if the auto-commit flag is off, Microsoft and Sybase will do automatic COMMITs unless you issue an explicit BEGIN TRANSACTION statement Generally, you can't ROLLBACK DDL statements with these DBMSs, but you can ROLLBACK DML statements If your DBMS is MySQL, auto-commit is always on and ROLLBACK is not possible
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The net effect is that it's really rather difficult to use transactions The idea is to reduce the number of network messages because a COMMIT message doesn't have to be sent But you should make what efforts you can to delay COMMIT if (a) auto-commit would compromise data integrity, or (b) you have several statements that will probably change the same pages in the same database files When performing a transaction, the DBMS can make one of two possible assumptions: that COMMIT is likely to happen, or that ROLLBACK is likely to happen If COMMIT is likely, then the DBMS's best strategy is this:
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When a data change happens {
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Put a copy of the original data page in the log file Change the data page immediately } When a COMMIT happens { Just flush the log file } When a ROLLBACK happens { Read the original from the log file Copy it to the data page }
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If ROLLBACK is likely, then this is the DBMS's best strategy:
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When a data change happens { Put a copy of the changed data page in the log file } When a COMMIT happens { Read the change from the log file Copy it to the data page Flush } When a ROLLBACK happens { Mark the log entry as invalid Do nothing else The original data is untouched }
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Most DBMSs assume that COMMIT is likely That's okay, but it has the unfortunate consequence that you can't play "what-if" games in your transactions Start with the belief that your transaction will go through, because if you have to abort it, a ROLLBACK takes longer than a COMMIT Remember also that logging and COMMIT processing happen in separate threads so you won't see this effect by simply timing your transaction But the effect is there and will affect throughput Portability Ingres, InterBase, Oracle, and Sybase assume COMMIT is likely IBM, Informix, and Microsoft assume ROLLBACK is likely MySQL doesn't support transactions
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Usually, the actual event order is that a data-change statement changes an in-memory copy of a page and creates an in-memory log record COMMIT ensures that a separate thread (the log writer) has flushed the log records only Flushing of the changed data pages can happen later Tip If ROLLBACK is expensive, then start a transaction with the operation that's most likely to fail For example, in a library circulation transaction, scan the patron's card before scanning the material the patron wishes to borrow (GAIN: 7/7)
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