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Don Burleson Blog 







Oracle select for update tips

Oracle Database Tips by Donald BurlesonJanuary 1, 2015

  I am having trouble with locking using select for update, and I want to know how to prevent deadlocks while holding row locks.  Are there alternatives to select for update?

Answer:  The select for update has many issues, and select for update is especially dangerous when a transaction aborts and a "zombie" process continues to hold rows locks.

Oracle allows you to choose the strategy for locking, either pessimistic or optimistic, depending on your needs.  This is the issue of pessimistic vs. optimistic locking:

Pessimistic locking:  The developer must declare their intent to update the row set.  This is done with the SELECT xxx FOR UPDATE clause.
Optimistic locking:  You re-read data and only update it if it did not change since the initial fetch.

The select for update is the bane of the DBA, and there are many better alternatives to select for update.

The select for update is not a good locking strategy because there are many things that can go wrong. 

When managing web-based Oracle databases, the traditional "select for update" locking is inappropriate, and Oracle professionals have struggled with alternative mechanisms to maintain data integrity using an "optimistic" coding strategy:

Re-read - When an update comes in from the internet, the Oracle code re-reads the data to ensure that there are no changes since the data was originally delivered.
Timestamp - Some Oracle shops add a timestamp column to enforce serial updates and prevent accidental overlaying of data.  Now in Oracle 10g and beyond we see the new rowscn pseudo-column and the rowdependencies clause.

Instead of select for update, savvy Oracle developers will adopt alternatives mechanisms like:

1 - On initial read, save the row contents in RAM.

2 - Re-read upon update commit time.  If the rows has not changed since the initial read, update the row.

3 - If row has changed re-read again, process and update the row

See my related notes on select for update clause:

If you like Oracle tuning, you may enjoy my bestselling book Oracle Tuning: The Definitive Reference", with over 900 pages of BC's favorite tuning tips & scripts. 

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts.



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Note: This Oracle documentation was created as a support and Oracle training reference for use by our DBA performance tuning consulting professionals.  Feel free to ask questions on our Oracle forum.

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