An explain plan is a representation of the access path that is taken when a query is executed within Oracle.
Query processing can be divided into 7 phases:
[1] Syntactic Checks the syntax of the query
[2] Semantic Checks that all objects exist and are accessible
[3] View Merging Rewrites query as join on base tables as opposed to using views
[4] Statement Transformation Rewrites query transforming some complex constructs into simpler ones where appropriate (e.g. subquery merging, in/or transformation)
[5] Optimization Determines the optimal access path for the query to take. With the Rule Based Optimizer (RBO) it uses a set of heuristics to determine access path. With the Cost Based Optimizer (CBO) we use statistics to analyze the relative costs of accessing objects.
[6] QEP Generation QEP = Query Evaluation Plan
[7] QEP Execution QEP = Query Evaluation Plan
Steps [1]-[6] are handled by the parser. Step [7] is the execution of the statement.
The explain plan is produced by the parser. Once the access path has been decided upon it is stored in the library cache together with the statement itself. We store queries in the library cache based upon a hashed representation of that query. When looking for a statement in the library cache, we first apply a hashing algorithm to the statement and then we look for this hash value in the library cache. This access path will be used until the query is reparsed.
Credit: http://www.akadia.com/services/ora_interpreting_explain_plan.html
