Understanding Optimizer Statistics With Oracle Database …

Understanding Optimizer Statistics with Oracle Database 19c Oracle W HITE PAPER / DECEMBER 9, 2019 2 W HIT E PA PER / Understanding Optimizer Statistics with Oracle Database 19c INTRODUCTION When the Oracle Database was first introduced, the decision of how to execute a SQL statement was determined by a Rule Based Optimizer (RBO). The Rule Based Optimizer , as the name implies, followed a set of rules to determine the execution plan for a SQL statement. In Oracle Database 7, the Cost Based Optimizer (CBO) was introduced to deal with the enhanced functionality being added to the Oracle Database at this time, including parallel execution and partitioning, and to take the actual data content and distribution into account. The Cost Based Optimizer examines all of the possible plans for a SQL statement and picks the one with the lowest cost, where cost represents the estimated resource usage for a given plan. The lower the cost, the more efficient an execution plan is expected to be.

6 WHITE PAPER / Understanding Optimizer Statistics with Oracle Database 19c Figure 3 demonstrates how the data distribution of the column “COL1” can be encoded and represented using frequency histograms. Chart 1 (on the left) has a column or “bucket” for each value and the height of the column represents the COUNT of that particular

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