Interpreting Explain Plan
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Perfomance Tuning
What's an explain plan?
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 |
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February 4, 2009 12:20 PM
This is great stuff! Thanks for posting.
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