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copied from this: http://www.dba-oracle.com/t_hard_vs_soft_parse_parsing.htm

Oracle hard-parse vs. soft parse

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