Oracle update performance speed best practices

Oracle update performance speed best practices

The SQL standard for DML UPDATE statements can be complex and convoluted and there are best practices that can help you to write efficient UPDATE statements.

    * Run updates in batch mode
    * Use CTAS in lieu of large updates
    * Include the SET condition in the WHERE clause
    * Simplify the WHERE predicates
    * Have a small, separate data cache for high DML tables

Running updates in batch mode

If you can take your application offline during the monthly update, you use many update performance features to speed up the job performance:

    * Drop indexes/constraints and rebuild after mass update:  Dropping indexes before a mass update and rebuilding them afterwards can improve update performance significantly. Oracle removes index entries without re-balancing the index tree (a "logical delete"), but this is still time-consuming, especially if you have lots of indexes on the target table.  Also note that you can rebuild the dropped indexes nologging mode.

    * Parallelize the updates:  If you have an SMP server you can run Oracle parallel DML.    You can also manually parallelize the update by breaking them into multiple jobs and submit them simultaneously using dbms_scheduler, cron  or nohup.

Use CTAS in lieu of large updates

When you are updating the majority of rows in a table, using Create Table As Select (CTAS) is often more efficient performance than a standard update.  For example, assume that the following update changed 75% of the table rows:

    update
       mytab
    set
       status = 'new'
    where
       status = 'old;

In this case, a parallelized CTAS may perform far faster (Note: Make sure that you have an SMP server before using the parallel degree option):

    create table new_mytab NOLOGGING as
    select  /*+ full parallel(mytab,35)*/
       decode (status,'new','old',status,
       col2, col3, col4
    from mytab;

    -- rebuild indexes, triggers and constraints to new_mytab

    rename mytab to bkup_mytab;
    rename new_mytab to mytab;
    

Include the SET condition in the WHERE clause

This note shows a case where the developer forgot to include the SET condition in the UPDATE WHERE clause, causing high redo waits (log file parallel write waits, log file sync waits).  Simply including the existing state of the SET clause can result in a huger performance improvement for UPDATE statements:

-- zillion row update
UPDATE HISTORY SET FLAG=0 WHERE CLASS='X'

-- hundred row update
UPDATE HISTORY SET FLAG=0 WHERE CLASS='X' AND FLAG!=0
The select for update is not a good locking strategy because there are many things that can go wrong.  Instead of select for update, savvy Oracle developers will adopt alternatives mechanisms like a re-read upon update commit where the transaction re-read the rows and ensure that it has not changed since the original read.
Have a small, separate data cache for high DML tables

As more people adopt 64-bit servers with giant data buffers, we see a delay caused by the database writer process having to scan through giant data buffers seeking dirty blocks.  Many shops are replacing their platter-style disks with solid-state disks, and creating a very small data buffer, just for the updates.  The book Oracle Tuning: The Definitive Reference notes that if you are still using traditional disks, many shops segregate high-updates objects (tables & indexes) into a separate blocksize so that they can have a separate, small data buffer.

    "By segregating high activity tables into a separate, smaller data buffer, Oracle has far less RAM frames to scan for dirty block, improving the throughput and also reducing CPU consumption. This is especially important for high update tables with more than 100 row changes per second."

MOSC Note:223299.1 also embraces the importance of multiple blocksizes, listing the multiple buffer regions as among the most important tuning parameters in Oracle9i.
Simplify the WHERE clause predicates

The most common issue with updates is the requirement to have a complex SELECT statement is the where clause to identify the rows to be updated.  The best techniques for simplifying UPDATE where clauses include:

    * Predicate pushing
    * Rewriting subqueries as outer joins (if possible)
    * Oracle SQL subquery unnesting
    * Partitioning Pruning
    * Avoid IN and NOT IN - It is a good idea to discourage the use of the NOT IN clause (which invokes a sub-query) and to prefer NOT EXISTS (which invokes a correlated sub-query).

11g R2 New Features

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Oracle JDBC encryption

How to configure encrypted connection to encrypt data over the wire, and check summing, which prevents certain classes of network attacks. .

Preliminary - Gather Information

DBA must have installed the Oracle server software with the Advanced Security option, and the DBA must have configured the database listener to at least allow encrypted sessions and check summing.

Install Oracle Client

The standard Oracle client software is sufficient on the client side. There are no special client editions of the software to consider. Install the client software and test it as per usual.


Configuring the Client for Encryption

Edit your $ORACLE_HOME/network/admin/sqlnet.ora file on the client machine. For this example, we'll assume DBA has setup your instance to use RC4_256 for encryption and MD5 for checksums. We'll also assume you want your connection to be encrypted, or else let the connection attempt fail.

To configure this, add the following lines to your sqlnet.ora :

SQLNET.CRYPTO_CHECKSUM_TYPES_CLIENT = (MD5)
SQLNET.ENCRYPTION_TYPES_CLIENT = (RC4_256)
SQLNET.ENCRYPTION_CLIENT = required
SQLNET.CRYPTO_CHECKSUM_CLIENT = required
SQLNET.CRYPTO_SEED = 'insert a random string from 10-70 characters here'

The full set of options for both ENCRYPTION_CLIENT and CRYPTO_CHECKSUM_CLIENT are: accepted, rejected, requested, required

The full set of options for ENCRYPTION_TYPES_CLIENT are: RC4_256, RC4_128, RC4_56, RC4_40, AES256, AES192, AES128, 3DES168, 3DES112, DES, DES40 (Note: Oracle recommends RC4_256 for performance reasons.)

The full set of options for CRYPTO_CHECKSUM_TYPES_CLIENT are: MD5, SHA1

The CRYPTO_SEED must be quoted in single or double quotes.


It's possible to specify more than one encryption or checksum algorithm. To do so, separate them by commas like so:

SQLNET.ENCRYPTION_TYPES_CLIENT = (AES256,RC4_256,DES)

They will be tried in order until a match is found. If no match is found, then the connection is refused (if either client or server require encryption) or allowed but is unencrypted (all other cases). This may be useful if two different instances have a disjoint set of accepted encryption algorithms.

Note: There is no plausible reason we can think of to configure the client or server to always reject encrypted connections. However, if one side is set to reject, and the other to require, the result is what you'd expect: a connection can never be established.

Verifying encryption is working

To verify the encryption is working, it's both tedious and error prone to use a packet sniffer to verify the connect to the Oracle instance is encrypted. It is not trivial to distinguish compressed data streams from encrypted ones.

A more straightforward and reasonably solid approach is to enable client-side tracing and let Oracle's diagnostic information verify if encryption is working. To do this, edit your sqlnet.ora file, and temporarily add the following lines:

TRACE_LEVEL_CLIENT=10
TRACE_DIRECTORY_CLIENT=(a fully qualified directory to which you have write permissions)

NOTE: On Windows, TRACE_DIRECTORY_CLIENT is reported to be ignored. Instead, look under:

"C:\Documents and Settings\\Oracle\oradiag_\diag\clients\user_\host_<...>\trace\"

Use sqlplus to login to your instance. We recommend not using more elaborate or GUI tools for this test, as many such tools tend to automatically issue a number of queries on login, and will make the trace logs extremely verbose. After you have logged in successfully, logout.

In the TRACE_DIRECTORY_CLIENT, you should see a file named cli_NNNNN.trc, where NNNNN is an integer. (Note for Windows: the trace files are reported to begin with ora_ rather than cli_.) Grep or visually inspect this file, looking for the string 'encryption'. For example, a session that has successfully set up an encrypted connection will contain a line such as:

[24-OCT-2006 10:00:32:087] na_tns: encryption is active, using RC4_256

A cleartext connection would instead look like:

[24-OCT-2006 10:07:34:707] na_tns: encryption is not active

If you are familiar with Oracle client traces, please note encrypted session negotiation will not turn up at trace levels below 10. You must use an ADMIN or SUPPORT trace level.

Once testing is complete, you will probably want to remove TRACE_LEVEL_CLIENT and TRACE_DIRECTORY_CLIENT from your sqlnet.ora file.

Reference:
http://download.oracle.com/docs/cd/B28359_01/java.111/b31224/clntsec.htm#insertedID5