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Oracle - How a SQL Statement is Parsed.Hard and Soft Parsing




















SQL statement is parsed in a sequence of steps. 

Syntax Check: is the statement a valid one? Does it make sense given the SQL grammar documented in the SQL Reference Manual. Does it follow all of the rules for SQL.

Semantic Check: Going beyond the syntax ? is the statement valid in light of the objects in the database (do the tables and columns referenced exist). Do you have access to the objects ? are the proper privileges in place? Are there ambiguities in the statement ?

The next step in the parse operation is to see if the statement we are currently parsing has already in fact been processed by some other session. If it has ? we may be in luck here, we can skip the next two steps in the process. If we can skip these next two steps in the process, we have done what is known as a Soft Parse.

If we cannot, if we must do all of the steps, we are performing what is known as a Hard Parse.

Below are only done during Hard Parsing:

Transformation: Oracle will transform complex SQL into simpler, equivalent forms


Optimization: Oracle then creates an execution plan, based on your schema statistics Oracle build the decision tree of costs during this period, choosing the path with the lowest perceived cost.

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