Query Optimization in Oracle - How statistics are used
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The cost-based optimizer finds the optimal execution plan by assigning an optimization score for each of the potential execution plans using its own internal rules and logic along with statistics that reflect the state of the data structures in the database. These statistics relate to the tables, columns, and indexes involved in the execution plan. The statistics for each type of data structure are listed in Table 4-1.
Table 4-1. Database statistics
Type of statistics
Number of rows
Number of blocks
Number of unused blocks
Average available free space per block
Number of chained rows
Average row length
Number of distinct values per column
Second-lowest column value
Second-highest column value
Column density factor
Depth of index B*-tree structure
Number of leaf blocks
Number of distinct values
Average number of leaf blocks per key
Average number of data blocks per key
Oracle Database 10g and more current database releases also collect overall system statistics, including I/O and CPU performance and utilization. These statistics are stored in the data dictionary, described in this chapterís final section, ďData Dictionary Tables .Ē
You can see that these statistics can be used individually and in combination to determine the overall cost of the I/O required by an execution plan. The statistics reflect both the size of a table and the amount of unused space within the blocks; this space can, in turn, affect how many I/O operations are needed to retrieve rows. The index statistics reflect not only the depth and breadth of the index tree, but also the uniqueness of the values in the tree, which can affect the ease with which values can be selected using the index.
The accuracy of the cost-based optimizer depends on the accuracy of the statistics it uses, so updating statistics has always been a must. Formerly, you would have used the SQL statement ANALYZE to compute or estimate these statistics. When managing an older release, many database administrators also used a built-in PL/SQL package, DBMS_STATS, that contains a number of procedures that helped automate the process of collecting statistics.
Stale statistics can lead to database performance problems, which is why database statistics gathering has been automated by Oracle. This statistics gathering can be quite granular. For example, as of Oracle Database 10g, you can enable automatic statistics collection for a table, which can be based on whether a table is either stale (which means that more than 10 percent of the objects in the table have changed) or empty.
The use of statistics makes it possible for the cost-based optimizer to make a much more well-informed choice of the optimal execution plan. For instance, the optimizer could be trying to decide between two indexes to use in an execution plan that involves a selection based on a value in either index. The rule-based optimizer might very well rate both indexes equally and resort to the order in which they appear in the WHERE clause to choose an execution plan. The cost-based optimizer, however, knows that one index contains 1,000 entries while the other contains 10,000 entries. It even knows that the index that contains 1,000 values contains only 20 unique values, while the index that contains 10,000 values has 5,000 unique values. The selectivity offered by the larger index is much greater, so that index will be assigned a better optimization score and used for the query.
Testing the Effect of New Statistics
There may be times when you donít want to update your statistics, such as when the distribution of data in your database has reached a steady state or when your queries are already performing optimally (or at least deliver adequate, consistent performance). Oracle gives you a way you can try out a new set of statistics to see if they might make things better while still maintaining the option of returning to the old set: you can save your statistics in a separate table and then collect new ones. If, after testing your application with these new statistics, you decide you preferred the way the old statistics worked, you can simply reload the saved statistics.
In Oracle9i, you have the option of allowing the cost-based optimizer to use CPU speed as one of the factors in determining the optimal execution plan. An initialization parameter turns this feature on and off. As of Oracle Database 10g, the default cost basis is calculated on the CPU cost plus the I/O cost for a plan.
Even with all the information available to it, the cost-based optimizer did have some noticeable initial flaws. Aside from the fact that it (like all software) occasionally had bugs, the cost-based optimizer used statistics that didnít provide a complete picture of the data structures. In the previous example, the only thing the statistics tell the optimizer about the indexes is the number of distinct values in each index. They donít reveal anything about the distribution of those values. For instance, the larger index can contain 5,000 unique values, but these values can each represent two rows in the associated table, or one index value can represent 5,001 rows while the rest of the index values represent a single row. The selectivity of the index can vary wildly, depending on the value used in the selection criteria of the SQL statement. Fortunately, Oracle 7.3 introduced support for collecting histogram statistics for indexes to address this exact problem. You could create histograms using syntax within the ANALYZE INDEX command when you gathered statistics yourself in Oracle versions prior to Oracle Database 10g. This syntax is described in your Oracle SQL reference documentation.
Please check back next week for the conclusion to this series.
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