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Oracle hard-parse vs. soft parse

Oracle Tips by Burleson Consulting

Oracle SQL is parsed before execution, and a hard parse includes these steps:

  1. Loading into shared pool - The SQL source code is loaded into RAM for parsing. (the "hard" parse step)

  2. Syntax parse - Oracle parses the syntax to check for misspelled SQL keywords.

  3. Semantic parse - Oracle verifies all table & column names from the dictionary and checks to see if you are authorized to see the data.

  4. Query Transformation - If enabled (query_rewrite=true), Oracle will transform complex SQL into simpler, equivalent forms and replace aggregations with materialized views, as appropriate.

  5. Optimization - Oracle then creates an execution plan, based on your schema statistics (or maybe with statistics from dynamic sampling in 10g).

  6. Create executable - Oracle builds an executable file with native file calls to service the SQL query.

Oracle gives us the shared_pool_size parm to cache SQL so that we don't have to parse, over-and-over again.  However, SQL can age-out if the shared_pool_size is too small or if it is cluttered with non-reusable SQL (i.e. SQL that has literals "where name = "fred") in the source.

What the difference between a hard parse and a soft parse in Oracle?  Just the first step, step 1 as shown in red, above.  In other words, a soft parse does not require a shared pool reload (and the associated RAM memory allocation).

A general high "parse call" (> 10/sec.) indicates that your system has many incoming unique SQL statements, or that your SQL is not reentrant (i.e. not using bind variables).

A hard parse is when your SQL must be re-loaded into the shared pool.  A hard parse is worse than a soft parse because of the overhead involved in shared pool RAM allocation and memory management.  Once loaded, the SQL must then be completely re-checked for syntax & semantics and an executable generated. 

Excessive hard parsing can occur when your shared_pool_size is too small (and reentrant SQL is paged out), or when you have non-reusable SQL statements without host variables.

See the cursor_sharing parameter for a easy way to make SQL reentrant and remember that you should always use host variables in you SQL so that they can be reentrant.

 

If you like Oracle tuning, you may enjoy my new book "Oracle Tuning: The Definitive Reference", 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.

Verify experience! Anyone considering using the services of an Oracle support expert should independently investigate their credentials and experience, and not rely on advertisements and self-proclaimed expertise. All legitimate Oracle experts publish their Oracle qualifications.

Errata?  Oracle technology is changing and we strive to update our BC Oracle support information.  If you find an error or have a suggestion for improving our content, we would appreciate your feedback.  Just  e-mail:  and include the URL for the page.


                    









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